Ecology of the European Otter Lutra lutra Conserving Natura 2000 Rivers Ecology Series No 10
Ecology of the European Otter Conserving Natura 2000 Rivers Ecology Series No 10 Paul Chanin For more information on this document contact English Nature Northminster House Peterborough PE1 1UA Tel 44 0 1733 455100 Fax 44 0 1733 455103 This document was produced with the support of the European Commission s LIFE Nature programme It was published by Life in UK Rivers a joint venture involving English Nature EN the Countryside Council for Wales CCW the Environment Agency EA the Scottish Environment Protection Agency SEPA Scottish Natural Heritage SNH and the Scotland and Northern Ireland Forum for Environmental Research SNIFFER Text only EN CCW EA SEPA SNH SNIFFER 2003 ISBN 185716 716 3 Catalogue code IN11 1 A full range of Life in UK Rivers publications can be ordered from The Enquiry Service English Nature Northminster House Peterborough PE1 1UA Email enquiries english nature org uk Tel 44 0 1733 455100 Fax 44 0 1733 455103 This document should be cited as Chanin P 2003 Ecology of the European Otter Conserving Natura 2000 Rivers Ecology Series No 10 English Nature Peterborough Technical Editor Lynn Parr Series Ecological Coordinator Ann Skinner Cover design Coral Design Management Peterborough Printed by Astron Document Services Norwich on Revive 75 recycled post consumer waste paper Elemental Chlorine Free 1M Cover Paul Glendell English Nature
Ecology of the European Otter Conserving Natura 2000 Rivers This account of the ecological requirements of the European otter Lutra lutra has been produced as part of Life in UK Rivers a project to develop methods for conserving the wildlife and habitats of rivers within the Natura 2000 network of protected European sites The project s focus has been the conservation of rivers identified as Special Areas of Conservation SACs and of relevant habitats and species listed in annexes I and II of the European Union Directive on the Conservation of Natural Habitats and of Wild Fauna and Flora 92 43 EEC the Habitats Directive One of the main products is a set of reports collating the best available information on the ecological requirements of each species and habitat while a complementary series contains advice on monitoring and assessment techniques Each report has been compiled by ecologists who are studying these species and habitats in the UK and has been subject to peer review including scrutiny by a Technical Advisory Group established by the project partners In the case of the monitoring techniques further refinement has been accomplished by field testing and by workshops involving experts and conservation practitioners Life in UK Rivers is very much a demonstration project and although the reports have no official status in the implementation of the directive they are intended as a helpful source of information for organisations trying to set conservation objectives and to monitor for favourable conservation status for these habitats and species They can also be used to help assess plans and projects affecting Natura 2000 sites as required by Article 6 3 of the directive As part of the project conservation strategies have been produced for seven different SAC rivers in the UK In these you can see how the statutory conservation and environment agencies have developed objectives for the conservation of the habitats and species and drawn up action plans with their local partners for achieving favourable conservation status Understanding the ecological requirements of river plants and animals is a prerequisite for setting conservation objectives and for generating conservation strategies for SAC rivers under Article 6 1 of the European Habitats Directive Thus the questions these ecology reports try to answer include z z z What water quality does the species need to survive and reproduce successfully Are there other physical conditions such as substrate or flow that favour these species or cause them to decline What is the extent of interdependence with other species for food or breeding success For each of the 13 riverine species and for the Ranunculus habitat the project has also published tables setting out what can be considered as favourable condition for attributes such as water quality and nutrient levels flow conditions river channel and riparian habitat substrate access for migratory fish and level of disturbance Favourable condition is taken to be the status required of Annex I habitats and Annex II species on each Natura 2000 site to contribute adequately to favourable conservation status across their natural range Titles in the Conserving Natura 2000 Rivers ecology and monitoring series are listed inside the back cover of this report and copies of these together with other project publications are available via the project website www riverlife org uk 1
Conserving Natura 2000 Rivers 2
Ecology of the European Otter Contents Introduction 5 Status and distribution UK distribution Predicting the future Summary of population changes in the UK 6 6 10 10 Life history Births Litter size and frequency Cub development and dispersal 10 10 11 12 Population parameters Life expectancy Age structure Sex ratio Population density Genetic diversity 13 14 14 15 15 16 Habitat requirements Nature of waterway Riparian habitat Potential benefits of trees and woodland Potential harm Dens resting and breeding sites Natal dens 17 17 17 19 19 20 23 Food availability Freshwater prey Fish species Size of fish taken Summary Marine prey Food availability and limits to otter populations Productivity Biomass Fish availability in seven rivers in England Declines in fish populations 24 25 26 26 26 27 27 27 28 28 29 Physical and chemical attributes Substrate Channel structure and management Water quality requirements 29 31 31 31 Toxic chemicals Overview Types of pollutant Significance of PCBs Pesticides Heavy metals 31 31 32 34 34 34 3
Conserving Natura 2000 Rivers Local incidents Standards PCBs Organochlorine pesticides Heavy metals Monitoring Conclusions Summary of the effects of physical and chemical attributes 35 35 35 36 36 37 37 38 Disturbance Anthropogenic disturbance Introduced species 38 38 39 Substantive threats in different areas of the UK Disease Road casualties Investigations Impact on otter populations Acidification 39 39 40 40 40 41 Recommendations for further work Population biology Population size Population ecology in lowland areas Diet in lowland areas Review of toxic chemicals in otters Monitoring National Surveys Road casualties and postmortems Priorities for action 41 41 42 42 42 42 43 43 43 43 Conclusions 44 Acknowledgements 45 References 45 Appendix A The diet of the European otter Appendix B Fish population changes Appendix C Changes in water quality in rivers Appendix D The Otters and Rivers Project 52 55 61 63 4
Ecology of the European Otter Introduction Over the last two decades the literature on the European or Eurasian otter appears to have increased at an exponential rate There is a wealth of information on diet distribution and levels of toxic chemicals in otters their faeces and their prey While these aspects of biology are relatively easy to study though not necessarily to interpret others such as population dynamics density home ranges and movements are more difficult to investigate in such a rare and elusive animal Thus a review of existing data reveals an abundance of information in some areas and a paucity in others Paul Glendell English Nature Although there is a lot of information available about the biology and distribution of the otter a great deal still remains to be discovered about its general ecology such as home range density and population dynamics The principles of conservation management for otters were first established by the reports of the Joint Otter Group O Connor et al 1977 1979 and focused very strongly on perceived detrimental factors such as habitat destruction disturbance by humans and lack of resting or breeding sites The reports emphasised the experimental nature of these practices and it has become clear as more has been learned about otters that the animals are more resilient than had been previously supposed Many people working in the field of otter conservation are aware of this fact but there is still a need to elucidate more clearly factors that are truly detrimental to otters so that activities can be focused most effectively As large warm blooded top predators otters are insulated against many of the small scale environmental factors that can have a marked impact on the survival of riparian invertebrates and plants temperature rates of flow water chemistry etc Since individual otters can have home ranges extending over tens of kilometres populations depend on the conditions in catchments or even groups of catchments which may encompass a wide range of physical states in the riparian environment For these reasons determining favourable conditions and setting targets for conservation led management is not feasible in the way that it might be for smaller organisms This report consists of two sections 1 A review of current knowledge about otters The report attempts to evaluate such data to highlight areas where there are disagreements and to indicate where long held beliefs about the needs of otters are not supported by recent studies 2 Four appendices reporting on short investigations into matters related to the review 5
Conserving Natura 2000 Rivers a A summary of dietary studies b Changes in fish populations in a number of rivers in the Midlands and southern England over the last 30 years c Changes in water quality in some of these rivers d The work of the Otters and Rivers Project Status and distribution The otter population of western Europe underwent a widespread decline during the 20th Century The decline and subsequent recovery has been well documented in Britain Chanin Jefferies 1978 Andrews Howell Johnson 1993 Strachan Jefferies 1996 Green Green 1997 although less historical information is available for most other countries Macdonald Mason 1994 reviewed the situation in western Europe at that time showing that otters were rare or extinct in much of central Europe in a broad band extending from Italy across to central Spain in the south up to Sweden and southern Norway Widespread populations existed mainly in western areas Portugal Ireland Scotland and parts of Spain France Wales and England or eastern areas from Finland through to Greece JNCC Although the otter is rare or extinct in many European countries it has recovered in others A recent review Conroy Chanin 2001 found evidence of a recovery when comparing contemporary reports with those used by Macdonald Mason on a country by country basis It showed that although European populations were still considered healthy and widespread in only a third of the 37 countries for which data were available the number where they were believed to be increasing had gone up from 28 to 38 The proportion where otters were believed to be threatened declining very rare or extinct had gone down from 40 to 22 UK distribution Chanin Jefferies 1978 identified a sudden and widespread decline in the success of otter hunts throughout much of England and Wales which corresponded closely with a perceived decline in the otter population from the mid 1950s This was similar to changes observed in populations of various species of predatory birds and mammals and led the authors to the conclusion that the decline in otters probably had the same cause the introduction of cyclodiene pesticides dieldrin and related compounds in the mid 1950s Since 1977 as a result of a series of national otter surveys substantial parts of England Wales and Scotland have been surveyed three times the whole of Ireland once and parts of it twice In England and Ireland alternate 50km squares were searched in Wales and Scotland the whole land area was covered These surveys involved recording the presence or absence of otter signs usually their faeces known as spraints according to a protocol that has been widely used in Europe Reuther et al 2000 In addition the coast of Shetland has been surveyed twice by a different method involving the counting of active otter holts Spraint surveys only provide information on distribution while the holt surveys which can only be used in certain coastal areas provided estimates of the population The results of the spraint surveys are shown in Table 1 where it can be seen that the Irish otter 6
Ecology of the European Otter population was widespread and abundant in the early 1980s when in Britain the distribution was much more patchy Table 1 Results of National Surveys expressed as percentage of sites where signs were found Ireland Scotland Wales England No Sites1 2373 2650 1102 2940 1977 812 92 57 20 6 1984 6 1991 4 2000 2 65 38 10 83 53 24 34 Data from Chapman Chapman 1982 Green Green 1997 Andrews Howell Johnson 1993 Strachan Jefferies 1996 Crawford 2003 1Data selected only from sites which were surveyed in all three surveys 2Dates within which all surveys were completed Irish survey was carried out in 1980 81 others in 1977 1979 In the first survey of Scotland otters were widespread throughout the islands northern Strathclyde the Highland Region north of the Great Glen fault and Dumfries and Galloway Here for the most part signs of otters were found at 90 or more of the sites searched However signs were much more difficult to find in the Central Region between the firths of Clyde and Forth or in the Borders Region under 25 of sites while other areas were intermediate By the time of the third survey there had been a substantial increase in the number of positive sites in most regions it exceeded 90 and was only below 80 in the Fife Lothian and Borders regions 59 63 and 28 respectively The first survey of Wales revealed a more patchy distribution with otters present in most counties north or west of Glamorgan and Gwent though at a much lower levels than in Scotland Rather low numbers of positive sites were found in the Cambrian mountains and in Clwyd By the third survey the total number of positive sites had increased by more than two and a half times and otters were present in every county and hydrometric area except Anglesey where the population appeared to have become extinct between the first and second surveys Otters have returned to Anglesey since the third survey Ruth Warren pers comm The highest frequency of signs in this survey were in the Tywi 70 Severn 74 Cleddau 78 and Wye 82 catchments The first survey in England revealed a very low proportion of sites with signs of otters which were found mainly in peripheral areas the Southwest the Welsh borders north Norfolk and more sparsely in northern England At this time otters appeared to be close to extinction throughout much of central England By the time of the third survey the number of positive sites had quadrupled and in two areas Welsh borders and the Southwest more than 60 of sites were positive The greatest increase between the first and third surveys was in western parts of England but signs of otters were recorded in all regions including Kent and on all major catchments including the Thames Trent and Great Ouse The increase between the first and third surveys was not universal and in East Anglia the number of positive sites decreased from 20 in the first survey to eight in the second out of a total of 623 The increase from this to 52 sites by the time of the third survey is believed to be due at least in part to the reintroduction of otters by the Otter Trust A fourth survey of England was carried out between 2000 and 2002 The number of positives sites had increased to 34 over the whole country Crawford 2003 and increases were recorded in all Environment Agency regions Otters were recorded in 73 of the 105 Local Environment Agency Plan LEAP areas that correspond to individual river catchments sub catchments in the case of very large rivers or groups of catchments for very small ones In some regions the increase in positive sites was less than expected In the Severn Trent Region re colonisation of the lower reaches of the Severn and of the Warwickshire Avon was slow in contrast with the Trent catchment where the number of positive sites had tripled from 6 to 24 In the Northwest a modest increase from 29 to 34 included a substantial increase in the north on the Eden catchment from 43 to 71 as well as a slight increase in central areas west Cumbria and little change in the southern half of the region The fourth Welsh survey and a survey of Northern Ireland have also been completed and the results will be published later in 2003 The fourth Scottish survey will be completed by the end of 2004 and 7
Conserving Natura 2000 Rivers Figure 1 Distribution of otters in Britain based on the National Surveys of 1991 1994 1980 81 for Ireland English distribution based on Water Authority areas with Severn and Trent catchments divided Welsh distribution based on hydrometric areas Scottish distribution based on administrative regions Ireland by 100km square Results for rivers on the border between England and Wales are combined for the whole catchment Dee Severn and Wye Intensity of shading is directly proportional to percentage of sites with signs of otters 8
Ecology of the European Otter Figure 2 Distribution of otters in England third National Survey 1991 94 Each circle represents a 5km square where surveys were carried out Closed circles indicate those where signs were found 9
Conserving Natura 2000 Rivers the results published during 2005 Figure 1 summarises the status of otters in Britain at the time of the third national survey while Figure 2 shows the distribution in England at that time in more detail In the absence of recent data for Scotland and Wales it is not possible to provide a comparable contemporary map but shaded maps for all four English surveys are provided in Crawford 2003 which also includes a dot map at 10 km scale together with detailed maps for each region Predicting the future Strachan Jefferies 1996 used data from the first three national surveys to calculate the rate of recovery for each region of England and for Wales and predicted that by the time of the fourth survey 43 of sites in England and 70 of sites in Wales would have signs of otters The results for England proved to be lower than Strachan and Jeffries predictions 36 for comparable sites On a regional basis predictions for Northumbria and the Wessex and Thames regions were close to the actual result while the results for Severn Trent Anglia and the Southwest were higher than predicted by approximately 33 25 and 14 respectively In the North West and Yorkshire regions the results were approximately 30 lower than predicted Crawford 2003 did not discuss the discrepancy between predicted and actual results in Yorkshire but did comment on a perceived slowing down in the rate of recolonisation in the North West Region pointing out that the recovery appeared to be proceeding better in the north than in the south of the region Summary of population changes in the UK The otter population of the UK underwent a dramatic decline from the mid 1950s until at least the mid 1970s when systematic surveys first began The greatest declines were in England and Wales and although otters were generally widespread in Scotland in the 1970s very low populations were recorded in lowland areas in south central Scotland Since the first surveys the population has increased in most areas but there have been some regional variations from this Thus otters appear to have declined to extinction in Anglesey by the time of the second survey though they have returned since the third survey In East Anglia the population continued to decline into the 1980s though it has begun to recover since probably as a result of the introduction of captive bred animals The most recent survey shows a continuation of the recovery throughout England although it appears to be slower than expected in some northern areas Life history Births Births in Britain have been recorded throughout the year and breeding has been considered to be aseasonal at least on the mainland Chanin 1991 However in a recent study in Scotland found that otters from the mainland tended to have their young in the winter November to January and a number of studies elsewhere in Europe have demonstrated that seasonal breeding can occur The lack of evidence for it may in some places reflect insufficient data or the pooling of data from large areas In many of these areas it has been possible to demonstrate seasonal changes in the availability of prey Table 2 and that births occur at such a time as to maximise the abundance of prey when females are lactating Beja 1996a Kruuk 1995 Pikulik Sidorovich 1996 Heggberget Christensen 1994 The evidence for a lack of seasonal breeding in mainland UK comes from data gathered by Harris 1968 which include a substantial dataset from Stephens 1957 This comprises 134 instances where the age of cubs was estimated by otter hunters or naturalists and the date of birth calculated To this Harris has added a further 30 records from the literature Curiously while Stephens data show approximately equal numbers of young born in each month Harris additional data are highly biased towards the colder months 25 births estimated for the period October to March five from April to 10
Ecology of the European Otter Table 2 Records of seasonal breeding Area Sweden Portugal Belarus Shetland UK Denmark Norway Portugal Scotland Habitat Inland Inland Inland Coastal Both Coastal Coastal Inland Births Late winter January March April and May May and June Summer Summer and autumn October December Winter Authors Erlinge 1967a Beja 1996a Sidorovich 1991 Kruuk 1995 Elmeros Madsen 1999 Heggberget Christensen 1994 Beja 1996a Conroy Bacon in prep September Whether this represents a biologically significant difference is difficult to determine It is clear that many perhaps most of Stephens records were based on sightings in the hunting field where the animals may not have been observed closely while a high proportion of Harris came from animals that had been handled and in some cases weighed A very limited set of data from southwest England from which more accurate estimates of birth could be determined at autopsy yielded information from six breeding females each of which probably gave birth between September and February Simpson 1998 Unfortunately Bradshaw 1999 while reporting that 16 of the females in her sample were pregnant or lactating provides no information on dates On the evidence currently available it would appear that while births have been recorded throughout the year in England and Wales the possibility of seasonality should not yet be excluded Litter size and frequency Kruuk Conroy Moorhouse 1987 have suggested that litters of otters recorded in fresh water are larger than those on the coast Mean litter size ranged between 2 3 and 2 8 for four inland studies Geoff Liles An aerial view of a woodland and scrub otter breeding site in west Wales The woodland and scrub is totally undisturbed and largely impenetrable because of dense brambles 11
Conserving Natura 2000 Rivers Mason Macdonald 1986 and between 1 55 and 1 95 for three coastal studies Kruuk et al 1987 These discrepancies may be partly explained by the ways in which observations were obtained Heggberget Christensen 1994 found that litter sizes of coastal otters at birth averaged 2 5 based on autopsies whereas litters that were observed when the young had become mobile averaged 2 0 cubs Geoff Liles If as seems likely the coastal data referred to A greater tussock sedge Carex paniculata natal den site within woodland on a small stream in Pembrokeshire Wales above are based on counting cubs and the inland observations on data from corpses part of the difference may simply reflect mortality in the early stages of the otters lives Other places where litter sizes have been estimated for the same population at different stages include Belarus Sidorovich 1991 and Upper Lusatia in Germany Ansorge Schipke Zinke 1997 Sidorovich recorded mean litter sizes of 2 7 from embryos n 7 2 6 at one month old n 42 and 2 1 after leaving the den at about three months n 133 Ansorge et al recorded an average of 2 7 for counts of embryos and placental scars n 14 and 2 1 for observations of cubs n 48 Kruuk 1995 reported annual births for some females but that not all adult females produced cubs each year It was not possible to distinguish between females that failed to conceive and those that lost a litter of cubs at a very young age before emergence from the natal den Cub development and dispersal Watt 1993 studied otters living on the coast of Mull and recorded the development of hunting abilities in cubs He found that as they grew not only did their reliance on their mother for food decrease but their hunting ability increased and the proportion of the diet formed by unprofitable crabs declined By 13 months old young otters were entirely self sufficient at foraging and became independent from their mothers but continued to catch a higher proportion of crabs than experienced adults Watt concluded that this long period of dependence in his study area about three months more than for otters in Shetland reflected a lower abundance of prey Geoff Liles Orphaned otter cub being hand raised The litters of freshwater otters tend to be larger than those of otters that live in coastal areas 12 In Sweden Erlinge 1968 found that the distances covered by family groups of otters increased as the young developed until by the age of one year groups travelled up to 7 km in one night He found that the young otters dispersed when they were about one year old
Ecology of the European Otter Geoff Liles High sloping wooded banks above flood level often provide good root system holts for otters This one has a well used otter path from the stream up to the holt and was used by a female with cubs Depending on area and conditions cubs become independent from their mothers at around one year old Jenkins 1980 followed the movements of one young male otter by injecting it with radioactive zinc as Zn65Cl and testing spraints with a Geiger counter Up to the age of around eight months the animal s activities were confined to the loch on which it was raised together with the inlet streams It then began to exploit the nearby river Dee and extended its range on this river over the succeeding months At the age of one year it had been recorded along 68 km of the river travelling distances of more than 20 km in one night A similar pattern of range expansion was reported for the North American river otter L canadensis by Melquist Hornocker 1983 who radio tracked several young animals some of which dispersed at 12 13 months of age One young male travelled 104 km in 30 days and established a home range 32 km from its natal area One female travelled 195 km in 50 days but then settled into a range adjacent to its mother s and partly overlapping it Two other males had not dispersed from the natal range when contact was lost at 16 and 25 months old Population parameters Reliable information on the demographic characteristics of otter populations is only available from the study of dead otters Most of these have died as a result of human activity road kills and those drowned in fishing apparatus and in most studies relatively few died a natural death Most authors combine these data although the two types of sample represent different parts of the population see below The age of otters is usually determined from growth lines in teeth supplemented by cranial development in animals under two years of age Heggberget 1984 Life expectancy Gorman et al 1998 analysed data from 391 otters from three areas in the UK England and Wales 13
Conserving Natura 2000 Rivers n 97 mainland Scotland n 148 Shetland n 146 separating the samples into violent deaths and natural or non violent deaths Life tables were constructed from each of these taking the violent death sample as an indicator of the age structure of the population and non violent deaths as an indicator of the normal age at death Estimates of the numbers of otters born were based on age specific fecundity rates of animals in the sample No difference could be detected between males and females so the data were combined There was however a significant difference in the median age at death between otters from Shetland four years and those from England and Wales three years The median age for mainland Scottish otters was four not significantly different from either of the other samples The maximum age at death in this sample was 16 years Age specific mortality for Scottish otters fluctuated between 0 1 and 0 4 up to the age of six and then increased rapidly Otters from England and Wales showed a higher mortality than Scotland in all age classes greater than two and mortality was higher than 0 5 at all ages greater than three A smaller sample of otters from southwest England studied by Simpson 1998 also suggested a low life expectancy for otters in this region Age was determined for 28 otters submitted between 1990 and 1993 and none was more than four years old Elsewhere in Europe substantial samples of otters have been aged from three areas the Iberian Peninsula Ruiz Olmo Delibes Zapata 1998 n 106 Norway Heggberget 1991 n 410 and eastern Germany Hauer Ansorge Zinke 2000 n 1027 These authors all estimated age from growth lines in teeth and their results are broadly similar with median ages at death of three to four years and the maximum age ranging between 12 and 16 years Age structure Although the authors of the three studies referred to above estimated exact ages for the otters they sampled they did not present their data in comparable ways grouping them into differing categories Hauer et al used five categories while the other authors used fewer In order to compare data from the three areas Figure 3 the data presented by the authors have been modified to fit in the following categories Juvenile less than one year Sub adult animals in their second year Adult all others There are no large datasets of a comparable nature from the UK Hauer et al noted the very low proportion of animals less than two years old in their sample and suggested that this is probably an underestimate of the proportion of animals in these age classes An analysis of a similar though smaller set of data from Germany by Ansorge et al 1997 suggested that Figure 3 Age structure of otter casualties from three European areas Proportion of otters in each age class 80 70 60 50 Germany 40 Norway 30 Iberia 20 10 0 Juvenile Sub adult Adult Age Class Sources Germany Hauer et al 2000 Norway Heggberget 1991 Iberia Ruiz Olmo et al 1998 14
Ecology of the European Otter the proportion of juveniles in this sample was underestimated by about 50 The only substantial set of data for England and Wales is provided by Bradshaw 1999 who estimated age on the basis of the animals weight and reproductive condition Bradshaw used different age classes to those illustrated in Figure 3 and her assessment of juveniles was based on growth curves for two captive reared otter cubs and should therefore be treated with some caution Otters calculated to be under five months males 3 0 kg females 2 1 kg were described as juveniles Females larger than this were assumed to be sub adults 6 18 months if they showed no signs of reproduction immature uterus and absence of nipples Adult males were discriminated by possessing a baculum penis bone length 60mm Only 3 of this sample consisted of juveniles 23 were classified as sub adult and 74 as adult Sex ratio The proportion of male otters in the samples studied is normally in the range 50 60 Bradshaw s study is at the extreme with 61 of the sample being male otters In her case the disparity was greatest in adult otters 65 male whereas Hauer et al found a greater disparity in the juvenile and sub adult age classes 66 and 71 male respectively Male biased sex ratios in mortality samples from mustelid populations are not unusual and are frequently ascribed to differences in behaviours such as dispersal territory size and distances travelled Population density There are considerable difficulties in determining the density of otters In part this relates to the difficulties of monitoring such rare and elusive animals but even if the number is accurately known there is no obvious solution to the problem of which spatial units to use Otter territory sizes have most frequently been measured in terms of length of waterway but this is not particularly useful in places where part or all of the range consists of lakes or ponds or when comparing very large waterways with small ones This problem is clearly exemplified in data recorded by Durbin 1996 who found that in the case of a male and female otter with overlapping territories the male had a home range encompassing 50 km of river compared to 24 km for the female However when he calculated the area of water within each territory the female otter had a larger range than the male 34 ha compared to 29 ha A small number of values for population density have been calculated in Britain in most cases on the basis of very small sample sizes Harris et al 1995 calculated otter densities in East Anglia and Perthshire from estimates of land area using data supplied by DJ Jefferies They reported that an area of 75 km2 in East Anglia contained three otters while in Perthshire 57 km2 contained four otters These values correspond to densities of one otter per 25 km2 and one per 14 km2 respectively From this together with average values for the lengths of waterway in England Wales and Scotland respectively Harris et al 1995 calculated densities of one adult otter per 24 km of water in Scotland and one individual per 27 km in England and Wales Clearly the density of otters per unit of land area will vary with the proportion of it that is open water and these average values conceal considerable local variation Kruuk et al 1993 calculated otter density as ranging from 2 50 ha of water per otter which was equivalent to one individual every 3 50 km of stream median value of one otter per 15 km of stream A study of otters using DNA fingerprinting Coxon et al 1999 revealed minimum numbers of otters present on the Tone and Itchen catchments during one year to be 23 and 13 otters respectively The area of the Itchen catchment is 473 km2 while the Tone is approximately 400 km2 corresponding to one otter per 36 km2 and one individual per 17 km2 respectively These data include sub adult and possibly juvenile animals which were excluded by Harris and also transient as well as resident animals They are also minimum estimates since it is likely that not every otter present in the catchment during the year of the study was recorded These data not only demonstrate the difficulty of obtaining values for otter densities but also that they 15
Conserving Natura 2000 Rivers can vary widely The data from Kruuk et al in particular varies by a factor of 17 from the lowest to the highest It is evident that there is no basis at the moment for determining a normal or desirable density of otters to set as a target One indication that the otter population may have reached a high level comes from observations by Simpson 1998 who found that 16 of otters received for post mortem analysis in southwest England suffered from bite wounds inflicted by other otters These are found in both sexes though more frequently in males In this case they had led to severe injury infection and in some cases probably death The pattern of wounding lesions around the face and head or around the perineal area suggested a stylised form of fighting which suggests that this is the result of territorial behaviour and that it was exacerbated by high densities of animals in the area Although such fighting has been reported elsewhere e g Erlinge 1968 it has not previously been recorded at this high a frequency during post mortems Bradshaw 2001 has also observed wounds in the groins of male otters including damaged bacula and one case of castration Genetic diversity Studies of mitochondrial DNA in 30 wild otters from Denmark Mucci et al 1999 and 76 from Germany Cassens et al 2000 showed low levels of variability Smaller samples from outside their own countries were analysed by these authors and they had similarly low levels of variability with little or no basis for discriminating between populations The authors consider that this may reflect the influence of a genetic bottleneck but in the last Ice Age rather than in recent times and possibly combined either with stabilising selection or an unusually low level of mutation By contrast Dallas Piertney 1998 investigating microsatellite polymorphism in the chromosomal DNA of 32 otters from Britain and Germany found that variability at 13 loci was within the normal range for mammalian species Using 10 of these loci Dallas et al 1999 investigated genetic diversity in otters from Scotland comparing otters on the mainland with those from islands at different distances from the mainland The results revealed a predictable decline in diversity from mainland populations to the more isolated island populations with particularly low levels in the Shetland Islands Mainland otters were categorised as from northern or southern parts of the country and analysed separately since the southern population had undergone a significant decline and was believed to have been separated from the northern one for a period corresponding to several otter generations Despite this the loss of genetic diversity was small and the authors concluded that recovery would soon occur now that the two populations are again in contact with one another Coxon et al 1999 used the technique pioneered by Dallas and his colleagues in an attempt to carry out genetic fingerprinting of DNA extracted from otter spraint from three rivers in south west England and one in southern England They also analysed the tissues of 153 carcasses found in the region This revealed a substantially lower level of genetic diversity in southern otters compared to those in Scotland consistent with a significant bottleneck corresponding to the decline in the 20th Century Among the dead otters sampled almost entirely from the Southwest the number of alleles per locus for nine microsatellites ranged from four to six mean 5 0 compared to 6 10 mean 6 7 for Dallas Piertney s original sample The smaller number of individuals sampled by spraint analysis from the rivers yielded even lower diversity Thus for six loci on three rivers in Devon and Somerset the number of alleles per locus ranged between two and five mean 3 7 n 44 On the river Itchen in Hampshire there was a maximum of three alleles per locus mean 2 3 n 13 and one was monomorphic The isolated Hampshire otters are thought to be derived from a founder population of only three animals released on to the river in 1993 and illustrate one consequence of small scale reintroductions However the recolonisation of neighbouring catchments by natural spread is likely in due course to lead to this population being linked to a larger gene pool 16
Ecology of the European Otter Paul Glendell English Nature Otters can be found in a variety of wetlands including estuaries although fewer individuals have been recorded in estuary sites in Britain than in other habitats Habitat requirements Nature of waterway Otters have been recorded as exploiting virtually all types of water and waterway in the UK Although populations in England and Wales are confined mainly to fresh water they readily exploit suitable coastal habitats in Scotland as well as elsewhere such as Portugal Beja 1995 The importance of estuaries to otters is more difficult to ascertain In the third English survey 25 estuarine sites were surveyed in Devon and Cornwall where otters are widespread and 40 of these had signs of otters compared to 68 of non estuarine sites in the area Otters have been recorded on still waters canals lakes ponds and reservoirs as well as rivers and streams of all sizes Evidence from radiotracking Jefferies et al 1986 and from studies of the distribution of road casualties Chanin 2001 shows that otters will use tiny streams and ditches including dry watercourses as regular routes Jefferies 1988 reports one otter crossing a watershed by travelling 1 8 km overland in Norfolk Riparian habitat Habitat destruction was first identified as a potential threat to otter populations by the Joint Otter Group O Connor et al 1977 largely on the basis that there had been considerable changes to riparian habitats in the previous decade or so which roughly coincided with the period in which otter populations had declined Chanin Jefferies 1978 while concluding that toxic chemicals had been responsible for the initial decline of the otter suggested that one reason for the apparent slow recovery of the otter might be degradation of its habitat in subsequent years Early attempts to assess otter habitat requirements involved seeking correlations between the distribution of spraints and a range of habitat features such as the presence or absence of trees and woodland cover potential den sites human activities and various physical attributes of the waterways Jenkins Burrows 1980 Macdonald Mason 1983 Bas Jenkins Rothery 1984 Macdonald Mason s study was particularly influential being based on surveys at over 50 sites in Wales and the Welsh borders and using multivariate analysis of the distribution of otter signs plus 15 habitat variables They concluded that the presence of ash and sycamore trees was particularly important together with potential den sites the majority of which were found under the roots of these two species as well as rhododendron bushes oak and elm trees 17
Conserving Natura 2000 Rivers Both photos by Geoff Liles Left A typical mid river boulder sprainting site with easy access for otters and spraint deposited on top Right Fresh spraint oily and dark with small fish bones on a boulder Other studies based on spraints have pointed to the importance of the presence of trees and woodland or other cover as well as the impact of human activities while some have also demonstrated a relationship with food supply e g O Sullivan 1993 Prenda Granado Lorencio 1996 Thom 1997 White et al in press Partly as a result of these studies and the Joint Otter Group Report and possibly because it seemed natural there has been a strong tendency for people concerned with otter conservation to put great emphasis on the importance to otters of bankside vegetation particularly trees and scrub that might provide good cover or potential den sites However there is now considerable doubt as to whether this has any direct benefit to otters not least because otters thrive in situations where there are no trees and little cover such as Shetland A major problem with all such studies is in the use of spraint as an indicator of the preferences of otters Most recent authors are careful to describe their studies in terms of predictions of or correlations with otter sprainting activity but this begs the question of what sprainting activity means Many authors have used the numbers or density of spraints in a way that suggests it should correlate with the activity of otters or perhaps the amount of time otters spend in particular places Geoff Liles A well used path of otters through grass with a surveyer checking a sprainting site 18 On inland waters otter spraints are not deposited at random on the waterside but at particular features of the riparian Geoff Liles Otter prints in mud on a lakeshore
Ecology of the European Otter landscape such as the bases of large trees boulders bridges confluences of streams and so on Spraint numbers are known to vary seasonally e g Conroy French 1985 and Thom 1997 found differences in the spatial distribution of signs between seasons as well as differences in numbers In an unpublished study of a river in Devon with a well established otter population Hilary Marshall pers comm found considerable variation in spraint numbers between years During the summer months when rainfall is low June July August the mean number of spraints found per month over a five year period was 32 6 with a maximum of 60 0 and minimum of 4 7 Averaged over whole years there was still a five fold difference between the year with the least spraints and the one with the most Thom 1997 used GIS techniques to investigate the spatial distribution of otter signs in the upper Tyne catchment Attempts to predict the presence or absence of spraint using logistic regression models at various spatial scales did not lead to consistent results but Thom concluded that models including altitude the abundance of minnows and the presence of heavy metals in eel tissues were the most important determinants of spraint distribution Durbin 1998 obtained a much more precise measurement of habitat use from a study based on tracking five otters for periods of between three months and a year on the rivers Dee and Don in Scotland He recorded more than 800 hours of otter activity within a three year period Durbin used the total amount of active time spent in a particular habitat as an indicator of use and compared this with habitat availability as an index of preference Separate indices were used for habitat length and habitat area Although he found that all otters spent more of their time in relatively wide sections of river with large numbers of boulders and many trees when he analysed his data on the basis of area there was no preference for wide streams or trees and most otters spent a disproportionate amount of their time in narrow gravelly streams Durbin found no evidence to suggest that otters actively avoided agricultural areas or areas of human activity either when foraging or for resting In a review of otter habitat use and conservation Kruuk et al 1998 pointed out the difficulty of inferring habitat requirements from studies of habitat preference Individual or species preferences for particular habitats do not necessarily have any significance in terms of survival Using data collected by Durbin 1998 together with information from other radio tracking studies they had undertaken they concluded that the most significant determinant of otter usage of freshwater habitats was abundance of prey Reedbeds and islands were also found to be important as they were frequently used as rest sites while marshy areas were valuable as foraging areas for frogs They found no evidence that the presence of trees woodland or other forms of bankside cover influenced otter activity and suggested that earlier emphasis on the importance of these may have been biased by patterns of otter sprainting activity They also pointed out that otters thrive in areas where there is very little cover such as Shetland Potential benefits of trees and woodland Although it is now clear that otters do not need trees or actively seek out wooded areas there are a number of indirect benefits in allowing or encouraging tree and shrub growth beside waterways Mason Macdonald 1982 demonstrated that such riparian vegetation can significantly add to the availability of invertebrate prey for fish populations which in turn will be of benefit to otters Potential harm Encouraging the growth of trees and shrubs in riparian habitats undoubtedly benefits many organisms other than otters including many species of invertebrates fish and birds but there may also be negative effects in some places For example water voles require dense growth of herbaceous bankside and emergent vegetation Strachan 1998 and the promotion of scrub or planting of trees is detrimental to them Some species of ground and water beetles of national importance depend on the presence of eroding riverbanks Bielinski 1993 and it is important that these conditions should not be disturbed in the mistaken belief that it will benefit otters 19
Conserving Natura 2000 Rivers Dens resting and breeding sites Numerous words have been used to describe places where otters sleep including den holt couch and resting site Holt and couch are words coined by hunters and refer respectively to covered and uncovered resting sites However the word holt is associated by many with the idea of a hole in the ground usually under the roots of a bankside tree and many people regard a couch as an uncovered nest like structure Kruuk and colleagues describe above ground resting sites as couches The terms rock holt and stick pile holt are self explanatory but some other structures do not fit easily into these two categories bundles of reeds with a den in the centre for example A further problem with holts is that of finding them without the aid of a radio tracked otter Many authors have recorded the presence of potential holts which consist of tunnels cavities or other covered structures that may be used by otters It is not usually possible to determine how many of these are actually used although spraints may be found beside some of them Radio tracking studies enable actual resting sites to be identified and it is notable that a high proportion of these consist of places that would not be identified as potential holt sites in a survey e g Green Green Jefferies 1984 So far no one has tried to compare the number of potential sites identified by a survey with the number of actual sites revealed by radio tracking In coastal areas such as Shetland and the Outer Hebrides otter dens holts frequently consist of burrows in peat Kruuk 1995 These may be substantial conspicuous structures some almost as large as a main badger sett though without the large spoil heaps outside Where females with cubs have been in residence impressive piles of spraint may accumulate outside the entrance sometimes up to 0 5 m across and 0 1 m high Coastal holts are mostly within 100 m of the shore and may also be very frequent with densities up to three or four per km in some areas Conroy Kruuk 1995 Both photos by Geoff Liles A typical root system holt in a large sycamore on the River Clwyd Wales 20 In freshwater habitats holts in the form of tunnels are less conspicuous and probably less numerous though resting sites of other
Ecology of the European Otter Otters will use a variety of places as resting sites Top A cavity behind an old collapsing wall in Pembroke town Centre The root plate of a fallen tree with hole leading into a cavity used by a radio tracked otter Bottom Dense bramble scrub on the River Teifi used by radio tracked otters for a daytime resting site All photos by Geoff Liles 21
Conserving Natura 2000 Rivers kinds are much more common Based on his experiences while otter hunting Coghill 1980 summarised the types of resting site recorded in the upper reaches of the Severn catchment He identified 256 sites of which 42 were under the roots of trees 90 ash or sycamore About 20 of sites were described as lying rough in reed or osier beds young forestry plantations islands rhododendron bushes bracken hedges scrub etc Most of these were within 10 m of water but some were up to 50 m away Stick heaps and rock holts formed 13 and 10 of sites and various types of enclosed drain a further 11 The remaining miscellaneous category included badger setts rabbit burrows hollow islands and a car body being used for bank protection Thom 1997 identified 212 potential holt sites in 200 km of the Tyne catchment of which 58 were under trees with half of these under ash or sycamore A further third were in rocky banks stonefilled gabions or caves and the remainder consisted of piles of debris or holes in the bank Thom pointed out that although the overall density was approximately one holt per km of stream the distribution was very clumped with 30 of 5 km stretches of river having none and seven 5 km sections having more than 10 potential holt sites There is no information on how many of these sites were used by otters or of the abundance of other potential resting sites In a radio tracking study of otters in Perthshire Green et al 1984 found that individual otters used considerable numbers of resting sites and frequently lay up in relatively open areas One male used 27 sites over a period of three months and two thirds of these were above ground ranging from dense vegetation or under piles of sticks and branches flood debris to depressions in the bank where the vegetation was no more than 0 3 m high They commented that fewer than 10 of the sites used for resting by otters would have been recognised as such without radiotracking Durbin 1993 found similar numbers of resting sites in his study of five otters with the number recorded for individual otters depending on the length of time they were followed Otters tracked for the longest periods had 27 and 30 holts corresponding to densities of 1 1 sites per km and 0 36 sites per km for a female and a male otter respectively Durbin noted that many sites were only used on one or a few occasions and that otters had smaller numbers of sites that were frequently used Paul Glendell English Nature Woodland areas provide cover and forage for invertebrates that support the otter s fish prey Otters sometimes use wooded areas as resting sites and for natal dens 22
Ecology of the European Otter Kruuk et al 1998 reported that four otters tracked for a total of 669 days spent 58 of their daytime resting periods in couches 29 and the remainder in holts 9 Most couches were in thick cover in shrubs 9 reeds 3 or rushes 3 Three were on islands and the remainder under the bank fallen trees boulders etc Four holts were in artificial embankments formed by boulders three in field drains and two in soil None of these otters used a holt under tree roots while being radio tracked Beja 1996b located a total of 21 resting sites used by three coastal otters in Portugal and one living on an estuarine stream None was underground and all were in thick vegetation such as brambles or among piles of boulders Unpublished observations by Kruuk and colleagues indicate that otters will rest under roads in industrial buildings close to quarries and at other sites close to high levels of human activity These observations clearly indicate that otters are very flexible in their use of resting sites and do not necessarily avoid disturbance in terms of noise or proximity to human activity A common feature is that many of these resting sites are in places where the risk of direct physical disturbance is low islands reeds dense scrub culverts piles of rocks etc Although there is no evidence to suggest that the availability of such sites limits otter populations in the UK some concern has been expressed that this may be the case in areas where riverbanks are completely devoid of natural cover Where this occurs over very long lengths of river kilometres rather than hundreds of metres the provision of even small areas of dense scrub at intervals may be beneficial The construction of artificial dens may serve a similar purpose and there is evidence that these are readily used by otters Cowell et al 2001 found otter fur in 13 of 19 such log pile holts when they rebuilt them On the whole the benefits of dense scrub to wildlife other than otters are such that this should be a preferred option although there is no reason why both should not be provided Natal dens Studies on Shetland show that otters are born in natal holts that may not be the same as those used for rearing young or by adult otters Moorhouse 1988 The piles of spraint which are conspicuous at other holts are not visible and natal holts are difficult to find with simple inconspicuous entrances and little evidence of the presence of otters They are often some distance from the coast On Lunna Ness where Moorhouse carried out most of his observations all were more than 150 m from the shore and one was 500 m from the coast Kruuk et al 1998 have pointed out that three natal holts described by Harper 1981 were occupied by cubs that were several months old which had probably not been born there Thus there are only two descriptions of dens in which otters are known to have given birth Taylor Kruuk 1990 describe one they found in a reedbed on a Scottish loch The den was a few centimetres above water level in a small clearing in the reeds close to a ditch 0 8 m wide and 0 7 m deep It consisted of a mass of reeds 0 9 m by 0 75 m and about 0 4 m high with a tunnel passing under it and a small side chamber The authors believed that the reeds may have been bitten off Durbin 1996 located a natal holt in a narrow strip of deciduous woodland about 150 m from the main river Don and 3 5 m from a small tributary 0 7 m wide The holt was under a pile of boulders 60 m2 and 2 5 m high Individual boulders ranged from 0 5 m to 1 5 m across Five entrances were located but it was not known whether they were connected within the pile The pile had probably been formed during clearance of a nearby arable field and had a covering of vegetation suggesting that this had happened a while ago Location of natal holts during field surveys would appear to be extremely difficult and the above observations suggest that they may be some distance from major rivers and areas frequently used by other otters This might be a defence against cannibalism which has been reliably recorded in one instance Simpson Coxon 2000 There is no evidence as yet to suggest that the availability of natal holts limits the distribution of otters Female otters seem to use sites well away from main watercourses and even appear willing to site them away from water altogether 23
Conserving Natura 2000 Rivers Food availability There are probably more than a hundred reports on the food and feeding habits of the European otter in the literature This is at least in part because this is one of the easiest aspects of otter biology to study since otter spraints are relatively easy to find and collect and the prey remains within them are relatively easy to identify The technique of spraint analysis is not without problems however and the difficulties of extrapolating from the contents of spraints to the actual diet of otters have been widely discussed see particularly Carss Parkinson 1996 Carss Elston 1996 Carss Nelson 1998 Jacobsen Hansen 1996 The results of dietary studies may be presented in a variety of ways and while early studies tended to report the frequency of occurrence or volume of prey remains in spraints more recently many workers have attempted to calculate the biomass or energy value of each prey type ingested A consequence of this is that comparisons between studies are difficult to make other than in the broadest terms In addition many studies only cover part of the year or involve rather small sample sizes Freshwater prey Appendix A lists the results of 15 studies from a range of freshwater sites in Europe where the sample size exceeded 300 spraints and the study covered all seasons Table 3 summarises these data which are all based on studies where frequency of occurrence was reported or could be calculated full details of the calculations and sources are in the appendix Although the results cannot be directly compared a similar pattern emerges in studies based on percentage volume in the diet e g Jacobsen Hansen 1996 Taastr m Jacobsen 1999 estimated biomass of prey ingested e g Beja 1996c 1997 and energy value of prey ingested e g Beja 1996c Table 3 Summary of fifteen dietary studies carried out in Europe between the 1960s and 1990s Fish Amphibia Crayfish Mammals Birds Reptiles Minimum 49 8 0 5 0 0 0 0 0 0 0 0 Maximum 94 0 36 7 34 8 11 4 9 3 9 1 Median 67 7 13 4 3 0 0 6 1 5 0 0 Not surprisingly fish dominate the diets of otters in most places and the only two other groups of prey that normally form a substantial proportion of the diet 33 are crayfish and amphibia mostly frogs Occasionally at certain times of the year these two prey groups may occur more frequently in the spraints than fish but this appears to be unusual e g Erlinge 1967b for crayfish Brzezinski Jedrzejewski Jedrzejewska 1993 Grigorev Egorov 1969 and Weber 1990 for amphibia Crayfish have rarely been recorded as prey of otters in the UK but this is probably because most studies of the otter s diet in Britain have been undertaken in Scotland and southwest England where crayfish are scarce or absent Holdich 2003 Beja 1996c has demonstrated that otters will readily take to preying on an introduced species American red swamp crayfish Procambarus clarkii in Portugal and it seems likely that the same will happen in Britain as otters recolonise areas where crayfish are abundant Beja found that the crayfish were taken more frequently than would have been predicted by their calorific value and considered that this might be because they were frequently encountered when otters were foraging near the bottom where an energetically more valuable prey the eel was also to be found In Scotland Weber 1990 found that amphibia mainly frogs formed 20 of the items identified in the diet through the year with a peak in predation during late winter and spring In one study area he 24
Ecology of the European Otter reported that 55 of prey remains in the spring were amphibian It is probable therefore that in much of the UK only fish and frogs make a significant contribution to the diet of otters in freshwater habitats at present the latter on a seasonal basis In some areas crayfish may become seasonally important as the otter population recovers and there may be other locally important food sources in certain areas such as mountain hares in upland rivers of Scotland Conroy Calder 2000 Fish species A number of authors have attempted to assess the preferences of otters between fish species but these studies are confounded by the difficulties of accurately assessing both the proportion of each fish species in the diet and the availability of each fish species It is generally the case that otters take most of the species that are present and take a much higher proportion of abundant species than of scarce species There is also a widely held belief that otters prefer slower moving species or at least find them easier to catch and that they therefore tend to prey disproportionately on cyprinids and other coarse fish compared to salmonids and on eels in preference to all other fish However there is little unequivocal evidence from the wild to support this Geoff Liles Above and below Otters eat a wide range of fish including perch ruffe and minnows They are also very fond of eels In two studies that involved very careful Geoff Liles estimation of the numbers of each species of prey taken and the numbers present in the study area Thom 1997 found no evidence for selection by species in his study area on the Tyne in Northumberland Conversely Taastr m Jacobsen 1999 found some evidence in their Danish study areas that otters preferred percids perch and ruffe and avoided cyprinids This is the reverse of the situation reported by Wise 1978 who found that the ratio of perch to cyprinids in the diet was much lower than in the lake she was studying The fact that in one study area Taastr m Jacobsen recorded cyprinids in the spraints of otters in January when none was caught by electro fishing clearly illustrates the difficulties of such studies Otters can travel considerable distances many kilometres in the time it takes for food to travel through the gut Although the minimum passage time for one meal is around three hours the remains of a meal can still be detected 24 hours later Jurisch Geidezis 1997 25
Conserving Natura 2000 Rivers In addition most studies assume that the proportion of prey available to the otter is the same as that recorded by electro fishing This is confounded by three factors z z z Electro fishing does not sample all species and sizes of fish equally effectively Eels are particularly difficult to sample Knights et al 2001 Differences in fish behaviour both between species and through the year Different foraging strategies by otters hunting along the bottom versus hunting for mid water species or at the sides of streams rather than in the middle Size of fish taken Otters have been recorded feeding on a very wide range of fish from
Ecology of the European Otter Marine prey The diet of otters living and feeding on the coast is also dominated by fish but generally to a much greater extent than for those foraging in fresh water see review in Appendix A In these studies fish formed more than 90 of the diet in all but one area Mull where Watt 1995 found that crabs formed approximately 10 of the items recorded in spraints He noted however that crabs were taken much more frequently by young otters than by adults and that as the young otters developed and became more efficient at catching fish the proportion of crabs in their diet declined In Shetland Kruuk 1995 found that crabs were taken rarely and mainly by inexperienced otters except in years when fish were scarce On Mull Watt found that the size of fish taken by otters was similar to the size recorded in his fish traps On Shetland however otters tended to take fewer of the very small fish such as butterfish in relation to their abundance and relatively more of the larger fish such as eelpout Food availability and limits to otter populations Studies by Hans Kruuk and colleagues summarised in Kruuk 1995 have demonstrated that in some areas Scottish rivers and islands parts of Africa and Asia otter populations are limited by their food supply Ultimately when otters have fully recolonised the areas in which they declined this is likely to be the case throughout the range of the European otter in Britain At the moment however the absence of otters from some areas is determined principally by historical events the decline from the 1950s to 1970s and the fact that they have not yet had time to recolonise the whole of their former range Strachan Jefferies 1996 concluded that biomass was unlikely to limit otter populations throughout much of England since reports from regional offices of the National Rivers Authority indicated that most rivers reached targets of a sampling biomass of 15 g m 2 for salmonid fisheries and 20 g m 2 for cyprinid fisheries with some exceptions in Wessex and the Thames catchment Nevertheless the availability of food should not be ignored either because a lack of food in some areas may prevent recolonisation by otters or because if there is more food available otter populations will be higher This has conservation benefits since larger otter populations would be more resilient and less vulnerable to other deleterious factors such as road casualties short term pollution events and might also lead to more rapid recolonisation through enhanced recruitment Productivity In an attempt to demonstrate that the otter populations in central England were not limited by food availability Brazier Mathias 2001 tried to estimate the minimum productivity required for an otter population to survive Using data from a wide range of sources they calculated averages for overnight distance travelled by otters daily food consumption by captive otters and the proportion of fish in the diet From these they calculated that each year on average each otter needed to harvest 5 g of fish per m2 of its home range in order to survive In other words rivers with an annual productivity above 5 g m 2 year 1 should support otters but those with a lower level would not There are a number of problems with this approach of which the greatest is in determining the area over which an otter forages Brazier Mathias used the average distance travelled per night multiplied by an average stream width of 6 3 m This overlooks the fact that a considerable part of an otter s travelling time is not spent hunting but moving from one foraging site to another and also makes rather broad assumptions about the sizes of rivers involved in the various studies quoted from A calculation based on home range sizes could have been more accurately determined for at least some of the studies but would not have been usable without accurate information on the numbers of otters foraging within each range More problematic was the fact that information on fish productivity could not be calculated from the contemporary records available to the authors and they had to rely on estimates of productivity based on studies carried out 15 years previously From these data they concluded that fish productivity was 27
Conserving Natura 2000 Rivers between 10 and 20 times greater than the requirements of otters and that the absence of otters from areas of central and southern England was not due to lack of prey Biomass Since biomass is strongly correlated with productivity and is more easily calculated and data are more readily available it is probably more realistic to use this as an indicator of prey available for otters One approach is to look at figures for biomass in places where otter populations are known to exist and try to determine a lower limit Kruuk et al 1993 demonstrated that otters could successfully exploit oligotrophic streams populated mainly by salmonids with densities between 9 and 14 g per m2 Calculations of the main prey of coastal otters in Shetland showed that the biomass varied mainly between 2 5 and 11 g per m2 throughout most of the year Kruuk Nolet French 1988 but with a dramatic increase during August to 62 g per m2 Otters in this population were shown to be in poor condition at times of year when fish biomass was low They also tended to produce their young in summer so that when metabolic demands on breeding females were greatest shortly before weaning fish were most abundant Ruiz Olmo 1998 carried out an investigation on the influence of altitude on otters in Spain supplementing surveys of otter signs with systematic direct observation as a means of estimating the abundance of otters and detecting evidence of breeding Fish populations were assessed by electrofishing Ruiz Olmo demonstrated a decline in otter abundance with altitude as well as a decline in fish biomass and number of fish species From 300 to 800 m above sea level otters were recorded at between 20 and 30 of sites surveyed fish biomass was between 20 and 280 g per m2 and two to 10 species of fish were present Thirty out of 36 breeding events were recorded in this zone Above this height all values declined progressively There was no evidence of breeding above 1000 m only one species of fish was present at sites above 1200 m and although signs of otters were recorded up to 1800 m they were few in number Biomass at these sites was much lower Ruiz Olmo noted that otters were present at sites with biomass values of 10 20 g per m2 and quoted a study by Nores et al 1990 in which signs of otters were not found at sites with 1 8 3 3 g fish per m2 but were found where the biomass was between 18 and 60 g per m2 From these rather sparse data one may suggest a rule of thumb to the effect that otter populations can survive and breed where fish biomass exceeds 10 g per m2 Where the fish biomass is below this figure throughout the year there is a possibility that they may not be able to do so Thus the UK Environment Agency targets for fish biomass 15 g per m2 for salmonid fisheries and 20 g per m2 for coarse fisheries are well above the level considered to be required by otters and where these targets are achieved food supply is unlikely to prevent otters becoming established although it will still determine carrying capacity Since the number of otters that can be supported by a river system will depend on the food available to them any action that increases the productivity of fish will benefit the otter population The logical extension of this is that the target should be as high as possible while remaining appropriate to the nutrient status of the river At the other end of the scale it is possible that biomass levels lower than 5 g m 2 may be so low as to prevent colonisation by otters Fish availability in seven rivers in England In order to place these values in context an attempt was made to summarise information held by the Environment Agency on the fish populations of seven rivers These data were based on standard electro fishing techniques which tend to underestimate fish populations particularly eels and should thus be regarded as minima Details of the findings are reported in Appendix B and these may be summarised as follows z 28 There have been changes in fish biomass over the past 30 years with strong evidence of an increase in the southwest region
Ecology of the European Otter z z On some rivers in the past 30 years there may have been insufficient fish for otters to survive There is no evidence from these rivers to suggest that otters will be prevented by lack of fish from re colonising UK rivers Declines in fish populations Over the final third of the 20th Century there was evidence that in some places fish populations increased see Appendix B Recently however concern has been expressed about declines in some species notably salmon and eels Declines in adult salmon have been taking place over a long period but although these fish are taken by otters and for short periods may form the staple diet of some individuals Carss et al 1990 they do not form a substantial part of the diet throughout the year There is no evidence that declines in the number of salmon returning to spawn have any impact on otter numbers Declines in fry and parr could be much more significant but only if they result in an overall decline in biomass If declines in salmon fry and parr are compensated by an increase in trout as on the River Otter Appendix B otters will be unaffected Concerned has also been expressed about declines in eel populations Farr Cox 1996 The recruitment of glass eels in Europe has declined since the 1970s and there is also anecdotal evidence of declines in yellow and silver eel stocks Knights et al 2001 Knights et al reviewed the historical evidence for this and carried out surveys on the rivers Severn Dee Piddle and Frome at sites where there was verifiable information on the historic abundance of eels The authors concluded that although there had been a decline in recruitment of glass eels the apparent decline in yellow and silver eels was more likely to be due to changes in fishing effort This was a result of competition from farmed eels leading to lower prices and resulting in lower yields from the wild fisheries They found that most rivers flowing west had higher populations of eels extending further up river than those discharging into the North Sea and concluded that naturally lower recruitment in the eastern rivers further from Atlantic currents had led to lower populations in these rivers The density of eels declined from sea to source in all rivers but upstream lower densities led to faster growth so biomass did not decline to the same degree particularly in western rivers One significant conclusion of this study is that conventional multi species electro fishing techniques result in eel densities being under recorded by a factor of two to five times Thus where eels form a significant part of the fish population biomass estimates from normal fish sampling may be well below the actual level The effect will be correspondingly small where eels are relatively scarce Physical and chemical attributes Although it is possible to demonstrate that some physical attributes of rivers such as width correlate with occupation of sites by otters Chanin unpubl it is most likely that the reasons for this are consequences of otter behaviour and the availability of food rather than direct effects The fact that stream width is negatively correlated with biomass of fish at least in some areas Kruuk et al 1993 Appendix B suggests that any apparent preference for stream size is likely to be influenced by food availability Since stream width is correlated with a number of other variables depth altitude current speed any apparent relationships between these and otter spraint distribution may be due to the same factor Thom 1997 found that altitude was a significant predictor of the distribution of otter signs and suggested that this might be a consequence of thermo regulatory effects However otters successfully forage in Spanish streams at heights exceeding 1000 m Ruiz Olmo 1998 and live in areas of northern Europe where snow cover lasts for many months of the year which suggests that some other factor may be involved Accidental captures of otters in lobster pots show that they are capable of diving to depths of 15 m 29
Conserving Natura 2000 Rivers Twelves 1983 but observations in Shetland showed that 98 of dives were in water less than 7 m deep and more than half of these in water less than 2 m deep In most rivers throughout Britain therefore otters are capable of foraging throughout the water column In very deep rivers lakes and possibly estuaries they may be unable to hunt bottom dwelling fish apart from close to the shore Fish swimming in mid water would still be available to them however In general it seems likely that most physical attributes of waterways only affect otters if they have an impact on food availability or if obstructions such as weirs or dams prevent otters moving along a stream A significant threat may be posed where otters are forced to leave the waterside and cross a busy road Generally otters travelling upstream are more likely to be impeded than those moving down There are three circumstances that might prevent upstream travel z z z Where there is a permanent physical barrier with no opportunity for otters to leave the water to bypass it for example a weir under a road where the waterway has no banks or it is not possible for an otter to clamber on to the bank Where the waterway is constricted so that the water velocity prevents otters swimming upstream and there is no bypass route When there is a temporarily high rate of flow for example during floods and no bypass route Upstream and downstream travel might be prevented z Where a waterway is piped and there is no headroom z When there are temporarily high water levels and no headroom Recommendations for minimising the risks to otters when designing new roads are included in the Design Manual for Roads and Bridges Highways Agency Vol 10 Section 1 Part 9 Paul Glendell English Nature Otters need natural riverbanks with places where they can easily clamber in and out of the water 30
Ecology of the European Otter Substrate Substate characteristics are only likely to affect otters if they affect food supply and it is possible that siltation in particular may have a significant impact Changing farming practices have led to concern about the impact of soil erosion In particular increases in winter cereal cropping reseeding of pasture the use of tramlines removal of hedges and the planting of maize have all added to the amount of topsoil being lost from fields and carried into ditches and streams MAFF 1999a Excessive poaching by cattle along unfenced river banks is a further cause of erosion Extreme levels of input to streams can cause the blanketing and loss of a wide range of plants and animals leading ultimately to decline in fish stocks Lower levels of very fine sediment may have a less obvious but equally serious effect by infiltrating gravel spawning grounds of salmonid fish and preventing the inflow of oxygenated water to the eggs Run off is greatest during the winter at the time when the eggs are developing thereby exacerbating the effect Environment Agency undated a The impact of this on the otter population is difficult to predict but there are extensive areas in England at risk from this form of pollution MAFF 1999b and since the food supply of the otter may be affected its potential significance should not be ignored Channel structure and management In the past the simplification of the channel often involved in flood prevention and drainage works inevitably led to a loss of structural diversity and a reduction in biological diversity and biomass Brooker 1985 reported declines in fish biomass of more than 80 in some schemes as well as declines in fish diversity This has been recognised as an issue in otter conservation management and Otters and River Habitat Management Environment Agency 1999 addresses the point clearly The Water Vole Conservation Handbook Strachan 1998 also provides advice on management of channel structure and since much of this has the potential to lead to an increase in food availability it is therefore of benefit to otters The Environment Agency has published a series of case studies of river restoration which clearly illustrate what can be done to rehabilitate rivers that have been degraded by over engineering Environment Agency undated b Significant benefits could accrue from such schemes where they increase the food supply for otters Man made structures associated with highly regulated rivers such as weirs bridges and bank reinforcements are not avoided by otters and may be used as sprainting sites There are also many instances of otters finding suitable resting sites among boulders and stone filled gabions used in bank reinforcement Water quality requirements Within the range of natural values water chemistry has little impact on otters other than by affecting food supply For example moderate eutrophication may benefit otters by leading to an increase in the abundance of certain fish although excessive eutrophication is detrimental when it leads to the reverse effect Otters are not directly affected by pH values within normal ranges but where acid rain leads to excessive acidity in watercourses it can have an adverse effect on food supply The impacts of toxic pollutants on otters are potentially extremely serious and are considered in the following section Toxic chemicals Overview The first evidence that toxic chemicals might be responsible for the decline of otter populations in Britain came from an analysis of hunting records Chanin Jefferies 1978 The authors concluded from 31
Conserving Natura 2000 Rivers the timing of the decline that the introduction of dieldrin and related pesticides was the most likely cause During the 1980s and 1990s there were a substantial number of reports on pollutants in otters their spraints and their prey in the UK and Europe Mason 1989 Kruuk Conroy 1996 Kruuk Conroy Webb 1997 Simpson 1998 Bradshaw 1999 and Jefferies Hanson 2001 Smit et al 1994 1996 undertook a major investigation into otters and polychlorinated biphenyl compounds PCBs in The Netherlands Development of Otter based Quality Objectives for PCBs DOQOP Mason 1995 provided a detailed overview of otters and pollutants and this is updated in Mason Wren 2001 a review the ecotoxicology in carnivores that includes a substantial section on otters Publications by Smit et al 1994 1996 summarise their work on the DOQOP project and the Proceedings of the International Conference on Otter Toxicology held on Skye in 2000 Conroy Yoxon and Gutleb 2002 includes a number of relevant review papers Types of pollutant Mason 1989 categorised contaminants that might have an effect on otters as Those having an indirect effects mainly on food supply z Organic pollution from sewage treatment works farms and the brewing food and dairying industries z Eutrophication as a result of run off from farms and sewage treatment works z Acidification mainly in the form of acid rain z Acid mine waste Those with mainly direct effects z Oil spillage mainly in coastal areas z Radioactivity Those with effects as a result of bioaccumulation z Metals particularly mercury but also cadmium and lead z Pesticides and PCBs Mason concluded that PCBs have been the most important factor in limiting otter populations in Europe while heavy metals may have had local effects but were not responsible for declines on a wide scale This view is widely held in Europe but there are some who consider that the situation in Britain is not so simple Kruuk Conroy 1996 Kruuk 1997 Jefferies Hanson 2001 In the following review attention is focused on bioaccumulating compounds particularly organochlorine Significance of PCBs PCBs are normally used and found in the environment as mixtures of several components congeners with differing levels of toxicity Mason has argued repeatedly and persuasively for the importance of these compounds both as a cause of the decline of otters in Britain and elsewhere and in hindering its recovery Mason 1997 In particular he has pointed out that z z z z 32 Concentrations of PCBs tend to be lower in countries or regions where otter populations are thriving or widespread and vice versa Otter populations have declined in countries where PCB production has occurred and in areas downwind of production sources PCB levels in fish correlate with those in spraints In Britain the highest occupancy of sites in the national surveys occurs where PCBs in spraints are lowest
Ecology of the European Otter z At a local level both in Wales and around Glasgow sprainting intensity was negatively correlated with PCBs The toxic effects of PCBs are mediated particularly by their effect on vitamin A metabolism Smit et al 1996 Murk et al 1998 Simpson et al 2000 has shown a strong negative correlation between the levels of Vitamin A in otters and the levels of PCBs in road casualties from southwest Britain An alternative view was presented by Strachan Jefferies 1996 who pointed to evidence implicating dieldrin and related compounds at least in the initial phases of the otter s decline This has subsequently been amplified by Jefferies Hanson 2001 who presented data to support their contention that the introduction of dieldrin was responsible for the decline of the otter in Britain from the mid 1950s and that PCBs were not implicated either in the initial decline or in slowing the otter s recovery Jefferies analysed otter tissues from as early as 1962 and pointed out that concentrations of dieldrin were much higher in the 1960s than subsequently with many otters having concentrations exceeding the critical level for foxes 1 mg per kg wet weight By the mid 1970s concentrations in all otters analysed had declined below this In contrast although PCBs were first used in the 1930s they were only found at trace levels in otters during the 1960s and early 1970s increasing in concentration after that time Jefferies Hanson pointed out that there was a close link in time between the introduction of dieldrin and related compounds in 1955 and the recorded onset of the otter s decline in 1957 In addition the very marked coincidence in time with declines in predatory birds such as peregrine falcon and sparrowhawk is very convincing Ornithologists using much more substantial databases were unable to detect any effect of PCBs on populations whereas the evidence against dieldrin is almost overwhelming The most difficult observation to reconcile with the PCB hypothesis is that levels of PCBs in otters in Shetland are considerably higher than elsewhere Kruuk Conroy 1996 despite the fact that this population of otters is known to be thriving Kruuk Conroy also pointed out that individual otters in Shetland and elsewhere in Scotland with high levels of PCBs were in good condition and breeding successfully The evidence that PCBs can have an adverse effect on the physiology of mammals is clear Leonards et al 1994 but Kruuk 1997 has pointed out that it is important not to confuse effects on individuals with effects on populations He also observed that by using inappropriate environmental standards we might condemn as unsuitable areas that are in fact good otter habitat Thus otters in Shetland have a relatively high concentration of PCBs but the population is thriving and increased from the 1980s to the 1990s Kruuk Conroy 1996 The geometric mean for a sample of 14 animals from Shetland was 64 higher than the value at which reproductive failure occurs in mink a frequently quoted level of concern in otters Kruuk Conroy reported levels between 10 and 20 times higher than this in apparently healthy animals one of which was lactating Mason reported in 1998 that following a decline of 8 per year in PCB concentrations in otters over the period 1983 1992 he no longer considered that PCBs were a threat to otters in Britain It is clear that there have been disagreements over the significance of PCBs in the decline and recovery of the otter population in Britain Mason believes PCBs may have been responsible for the initial decline and limited the population during the 1980s and early 1990s but no longer do so Jefferies believes that organochlorine pesticides were responsible for the initial decline of otters in Britain Kruuk questions whether the levels of contamination that Mason has put forward as having an adverse impact on individual otters have had significant impacts on populations The only consensus would appear to be that at the beginning of the 21st Century PCBs are not limiting otter populations in the UK Elsewhere in Europe there is a more general consensus that PCBs have had a significant impact on otter populations but despite this the evidence is not clear cut In Scandinavia Christensen 1995 found no evidence of PCB stress or reproductive failure in the coastal Norwegian otter population and concluded that the lower densities of otters on the western coast could not be explained by PCBs However she noted that high levels of PCBs in fish from southern and central coasts might be a cause 33
Conserving Natura 2000 Rivers for concern and that the disappearance of otters in southern Norway could be caused by PCBs Roos et al 2001 reported that changes in PCB levels correlated closely with changes in Swedish otter populations and suggested that the recovery of the otter population in southern Sweden may be prevented by the high levels of PCBs in that area Pesticides Mason Ford Last 1986 recorded the presence of one or more of lindane HCH dieldrin and DDT and its metabolites in the tissues of 23 otters collected over a wide area of Britain Levels of these compounds were low in otters from Orkney and Shetland but elsewhere the pattern was variable Lindane was generally found in low concentrations below 20 mg per kg of lipid in muscle tissue but dieldrin and DDT reached higher levels up to 66 and 116 mg per kg respectively Similar values to these though a little lower were recorded 10 years later by Kruuk Conroy 1996 for otters in Scotland Meanwhile in southwest England Simpson 1998 found Dieldrin and DDT in all otters analysed 56 but in most cases at low levels and with a significant decline from 1989 96 Lindane was not detectable in 75 of Simpson s samples and was at very low levels in the remainder The sample of 122 otters analysed by Jefferies Hanson 2001 included animals that died over the period 1965 89 They were able to demonstrate a decline in residues of dieldrin over this period but noted that high values were recorded in the period 1965 69 long after the start of the decline of otters and following bans on its use for seed dressings in 1962 and sheep dip in 1966 Roos et al 2001 reported a decline in concentrations of DDT of more than 70 between the 1970s and early 1990s in Sweden but concluded that these changes did not explain changes in otter populations as well as spatial and temporal changes in PCBs They also detected a decline in concentrations of DDT in fish of more than 80 in most areas The Environment Agency s review of its pesticide monitoring programme Environment Agency undated c shows that Environmental Quality Standards EQS for both Aldrin and HCH were breached in 1998 one and eight times respectively All these failures were close to the Thames Estuary rivers Medway and Blackwater with four of the HCH failures in estuarine sites the remainder in fresh water All were at concentrations below 0 1 g l 1 Otters are currently scarce or absent in this area but this is probably because they have not yet had time to recolonise it No other pesticides have been directly linked to otter declines but there were over 200 EQS failures for organophosphate and synthetic pyrethroid compounds used in sheep dips in 1998 Environment Agency undated c Many of these were scattered throughout the sheep farming areas of Wales Northumbria and Cumbria but there was a substantial concentration of sites in the Yorkshire area associated with the textile industry Biological surveys in Wales revealed a loss of invertebrates at 9 of sites visited Sheep dips were the origin of 24 of pollution incidents recorded in 1998 and that number had increased markedly since 1996 This reflects a shift from the use of organophosphates to synthetic pyrethroids which are safer for humans but more toxic to aquatic organisms The impact on otters is likely to be indirect resulting from a reduction in fish that have lost their invertebrate food supply Heavy metals Mercury may be a more serious contaminant of otters than other heavy metals along with cadmium and lead Mason Last Macdonald 1986 found relatively low levels of mercury cadmium and lead in tissue from 36 otters that died between 1982 and 1985 mainly in Wales and Scotland They concluded that these contaminants did not generally reach levels that might cause mortality but were close to levels that led to sublethal effects in other mammals Kruuk Conroy Webb 1997 found mean concentrations of mercury in 112 otters that died between 1986 and 1992 in Scotland to be similar to those recorded by Mason et al 1986 They noted that a small proportion of animals had concentrations of mercury sufficiently high to pose a risk to their 34
Ecology of the European Otter survival for example at times of food stress Nevertheless they concluded that this was not likely to have an impact on the population An indirect effect of heavy metal pollution was suggested by Thom 1997 He considered that otter populations on the South Tyne River might be limited by the availability of minnows whose distribution was negatively correlated with the concentrations of lead in eels Local incidents While nationally there is a downward trend in concentrations of most pollutants in water a number of incidents over the years demonstrate that local effects can have at least temporary effects on otter populations In southwest England there have been a few examples of accidental releases of insecticides from the textile industry and damage to pesticide dumps These have had short term and dramatic effects Similarly sudden influxes of highly enriched water from sewage slurry or silage effluent leading to excessive eutrophication may have a serious if short term impact on the food supply Over longer periods the possibility of leaching from unregulated pesticide dumps cannot be ignored In Sweden Olsson Jensen Reuterg rdh 1978 demonstrated seasonal fluctuations in PCB levels in fish in lakes These increased by two to five times in April when high flows brought in sediments from rivers Similar effects might be expected where high levels of toxic chemicals trapped in sediments are released during dredging or other river management work Standards PCBs Given the unresolved controversy as to whether PCBs have been a significant threat to otter populations in Britain it is difficult to make firm recommendations on water quality standards for these and other toxic chemicals Two principal approaches have been used to set such standards Mason used information on the toxicity of PCBs to mink to set standards for otter tissues and from these calculated appropriate levels for spraints using a mathematical model This has been criticised on two grounds Firstly that the use of mink was inappropriate since the mink are much more sensitive to PCBs than otters Kruuk 1997 Secondly that experimental tests to corroborate the relationship between body burden of PCBs and concentrations in spraint did not yield predicted results Smit et al 1994 Gutleb Kranz 1998 Conroy Carss 2001 Standards for levels in spraints from Mason Macdonald 1994 Critical levels z Concentrations in spraints 16 mg per kg of PCB and dieldrin singly or combined or z Concentrations in spraints of total organochlorines 20 mg per kg Levels of concern z Concentrations in spraints 9 16 mg per kg of PCB and dieldrin singly or combined or z Concentrations in spraints of total organochlorines 16 20 mg per kg Maximum allowable concentrations z Concentrations less than the level of concern but greater than the no effects level No effects levels z Less than 4 mg per kg for all individual contaminants as described above 35
Conserving Natura 2000 Rivers The second approach adopted by the DOQOP project relied on an assay for the physiological effects of PCBs on vitamin A metabolism Smit et al 1996 Rather than calculate the effects of individual components of the toxic burden these authors used a method to directly detect toxic effects at the cellular level This CALUX assay Chemically activated luciferase gene expression measures the effects of the compounds on stages in the production of vitamin A as changes in luciferase activity Assay results are expressed as TEQs TCDD dioxin Equivalents This method has the benefit of measuring one toxic effect directly and does not suffer the disadvantage of requiring separate assays and calculations for each compound in a sample In addition very small samples can be used The questions still arise as to how to extrapolate from effects at the cellular level to impacts on the whole organism or on a population Smit et al used these data to provide quality objectives for PCBs both as TEQs and as concentrations of the sum of seven standard PCB congeners in otters fish and sediments They did not attempt to relate these to concentrations in spraints They used two levels a safe level corresponding to a 1 reduction in hepatic retinoid levels EC1 and a critical level corresponding to a 90 reduction EC90 Standards from Smit et al 1996 a Expressed as TEQs nanograms per g or kg Sample Measurement endpoint vitamin A deficiency Level Safe Critical EC1 EC90 Otter Fish lipid wt ng g 1 fresh wt ng kg 1 lipid wt ng kg 1 organic carbon ng kg 1 dry wt ng kg 1 2 0 7 11 3 0 2 Sediment 5 1 8 29 7 0 4 b Expressed as sum of seven standard congeners nanograms per g or kg Sample Otter Fish Sediment Measurement endpoint vitamin A deficiency lipid wt ng g 1 fresh wt ng kg 1 lipid wt ng kg 1 organic carbon ng kg 1 dry wt ng kg 1 Safe EC1 4 6 88 17 1 0 Level Critical EC90 11 14 233 39 2 2 Organochlorine pesticides No specific independent targets have been set for other organochlorines although they are incorporated in the above standards The US National Academy of Science states that concentrations greater than 0 1mg per kg wet weight ww should not occur in fish Mason 1989 However mean concentrations of dieldrin exceeded this value in 53 of 62 rivers in Britain surveyed by MAFF the Ministry of Agriculture Fisheries and Food and high concentrations of dieldrin were reported from a river in southwest England with a thriving otter population Mason 1989 Heavy metals There is no experimental evidence on which to base safe levels of contamination for otters although some data on toxic effects are presented in Mason 1989 Mason Macdonald Aspden 1982 suggest that a safe level for mercury in fish might be 0 3 ppm ww while Hovens 1992 in Kruuk et al 1997 calculated that a mean level of 0 1ppm would be tolerable Otters in Shetland feed on prey with mean levels up to 0 14 ppm mercury without evidence of significant effects 36
Ecology of the European Otter Kruuk et al 1997 illustrate the difficulty of extrapolating safe levels for humans to otters by calculating that otters in Shetland have an annual intake of mercury approximately 40 times higher than that recommended for people Monitoring Even if there is no agreed basis on which to set targets or standards for levels of pollutants there is agreement that high levels of PCBs organochlorine pesticides and heavy metals are all potentially detrimental to otters and it is therefore important to monitor the levels of these in otter populations and their environments Simpson 1998 and Bradshaw 1999 demonstrated the benefits of carrying out large scale long term monitoring of dead otters killed mainly in road accidents although these can only provide useful samples when looked at over a relatively large area Sampling of prey or water via the Environment Agency s monitoring schemes provides much more detailed information at the catchment or even stream level though this may not provide a good indication of impact on otter populations The use of spraints for monitoring pollutant levels in otters is still controversial and at the International Conference on Otter Toxicology held on Skye in 2000 it was agreed that further work is necessary before a full understanding of this will be possible Mason Macdonald s 1994 calculations of safe and critical levels were based on a mathematical model and some attempts to test this experimentally have led to unpredicted results Smit et al 1994 Gutleb Kranz 1998 For example it is not clear whether the concentrations of pollutants in spraints reflect the most recent meal consumed by an otter or are influenced by food consumed over a longer period of time Until these issues are resolved the use of spraints as a monitoring tool should be undertaken with caution High levels of contaminants in spraints should certainly give cause for concern since they indicate that the otters have been exposed to the pollutants However the standards set by Mason may not be appropriate Further work is needed to confirm this Conclusions Further information on levels of toxic chemicals in several river catchments is set out in Appendix C From these data and the reports cited above some conclusions can be drawn z z z z z z In the UK and parts of Europe levels of toxic chemicals in otters their prey and in fresh water are declining This has taken place alongside a recovery in the otter population but there is no agreement as to how the two phenomena are linked It is possible perhaps likely that different pollutants have affected otter populations in different areas and at different times The possibility of new pollutants such as synthetic pyrethroids having an impact on otter populations either directly or via their prey cannot be ignored and vigilance for future contaminants is essential While various standards have been set some on a firm physiological basis there is not universal agreement that these are appropriate Effects on individuals do not necessarily mean that there will be effects on populations and there is evidence that populations can thrive when otters carry pollutant burdens higher than some of the standards that have been suggested Ultimately the most desirable objective is to eliminate toxic chemicals from waterways entirely and the only logical target is zero or at least as low as possible In practice as otter populations recover and contamination of waterways by many pollutants declines the most important conservation objective should be to continue monitoring to ensure that known pollutants continue to decline and to detect the appearance of new ones The possibility of local possibly temporary high levels of pollutants having a significant effect on otters or their prey should not be ignored 37
Conserving Natura 2000 Rivers z z On these grounds monitoring of pollutant levels in otters and their environment is essential and it would be desirable to agree a national strategy for doing this It is clear that the volume and complexity of the literature on otters and toxic chemicals is such that a full review is beyond the scope of this project With the publication of Jefferies data on toxic chemicals in otters dating back to the 1960s Jefferies Hanson 2001 it may be timely to consider a full review of studies of pollutants and British otter populations incorporating the published literature and using data on watercourses held by the Environment Agency Summary of the effects of physical and chemical attributes Although there is no evidence to suggest that either physical or chemical attributes of waterways are inhibiting the continued spread of otters both of these have the potential to limit the ultimate size of the population by their effects on the food supply Currently the effects are most likely to be mediated by the impacts of siltation inappropriate channel structure and the use of certain pesticides There are regional variations in the extent of these but it is probable that nationally significant efforts to remedy or mitigate them could have a substantial impact of the ultimate size of the otter population rather than its distribution Disturbance Anthropogenic disturbance As with habitat destruction the suggestion that human activity might cause significant disturbance to otters dates back to the report of the Joint Otter Group In the UK this appeared to be supported by the fact that the otter population declined most in the central southern and southeastern parts of Britain where the human population is highest while otters survived well in Scotland and to a lesser extent in southwest England where human densities are lower Despite the fact that there is increasing evidence that otters habituate readily to many forms of human disturbance there is still a widespread belief that disturbance is detrimental to otters Jefferies 1987 reviewed much of the anecdotal evidence available at the time and demonstrated that otter activity was not significantly affected by various forms of anthropogenic disturbance including walkers anglers and dogs though he noted that there was less evidence available for female otters with cubs and suggested that these might have more stringent requirements Since then further evidence has accrued A common response to the sounds of anglers or walkers with dogs is for radio tracked otters to move to a position where they can see the origin of the disturbance then dive and swim underwater for 50 m or so before surfacing and resting on the bank for five to 30 minutes then resuming their previous activity Durbin 1993 Perhaps the most dramatic demonstration of the ability of otters to live in highly disturbed areas is the recolonisation of Glasgow During the first survey of Scotland few sites within the conurbation of Glasgow had signs of otter but this had increased to 12 by the time of the second In the third survey 63 of sites within the conurbation had signs of otters and breeding had been recorded at four urban sites Green Green 1997 In England officers of the Otters and Rivers Project OARP have reported the presence of otters in 80 cities and towns and they are described as being frequent or resident in 49 of these There was evidence of otters breeding in nine towns and near several others In Shetland where the otter population is considered to be healthy otters regularly breed under the islands ferry terminals and under the jetties of Europe s largest oil terminal at Sullom Voe From these reports it seems safe to conclude that the recovery of the otter population is not being impeded by human disturbance 38
Ecology of the European Otter Introduced species The fact that feral American mink first began breeding in Britain at the start of the otter s decline led to it being suspected as a causal factor through competition predation of otter cubs or as a carrier of disease The best evidence for a significant interaction between mink and otter comes from the analysis of distribution records collected during the national otter surveys of England Strachan Jefferies 1996 They demonstrated that in southwest England where otters had a high level of site occupancy in 1991 94 mink occupancy had declined In East Anglia where otters were scarce mink occupancy had remained stable or increased The Severn Trent region was intermediate in both respects The authors concluded that the most likely cause of this decline in mink was aggression and possibly predation by otters Substantive threats in different areas of the UK The evidence pointing to a substantial recovery of the otter population is irrefutable Strachan Jefferies 1996 and the most important factor leading to the continued absence of otters from many areas is almost certainly time Strachan Jefferies pointed out that although otters can travel considerable distances and could cross central England in less than two weeks they have a low breeding rate and recolonisation is slow They calculated that it took 14 years for the rolling front of recolonisation to travel a distance of 50 km from the Welsh borders into the East Midlands and estimated that even by 2020 signs of otters will be found at only about 72 of survey sites in England Reviewing a wide range of factors that might threaten the recovery of the otter population Strachan Jefferies 1996 concluded that only road deaths and the possible spread of contagious diseases from captive and feral mustelids gave them cause for concern One further factor that may have a regional effect is the acidification of waters which occurs mainly in Wales and Scotland Disease Post mortem examinations of over 200 otters reported by Bradshaw 1999 and Simpson 1998 reveal remarkably few infectious diseases However there was a single report of a dying otter found in Paul Glendell English Nature Otters are at risk from the acidification of waters which mainly occurs in Wales and Scotland 39
Conserving Natura 2000 Rivers Norfolk with symptoms and lesions very similar to those of Aleutian disease an immune deficiency virus associated with captive mink Wells Keymer Barnett 1989 DNA from this virus was isolated from a single otter in Spain though the pathogenicity of the strain identified was unknown Ma as et al 2001 Road casualties Investigations Concern about the increasing numbers of otters found dead on roads led the Highways Agency to commission research into this phenomenon Philcox Grogan Macdonald 1999 Grogan Philcox Macdonald in prep Philcox et al obtained data on 673 casualties over the whole of Britain between 1971 and 1996 and noted a marked increase in numbers from the mid 1980s Part of this could be attributed to an increasing awareness of the need to report and collect otter casualties but it was not possible to determine the extent to which increases in road length traffic and otter numbers influenced this trend A high proportion of these casualties 22 occurred in southwest England Apart from the Scottish Islands Philcox et al found that this region had the highest number of casualties per unit area In Wales Liles Colley 2000 2001 undertook a study of road deaths for the Environment Agency Wales Welsh Water and the Wildlife Trusts The first report identified sites where mitigation should be carried out and established priorities while the second described procedures for implementing these measures Information on adjacent habitat and the nature of each mortality site was recorded and a preliminary effort was made to identify factors that could be used in a predictive model but none was presented However 58 of casualties occurred away from bridges or culverts at sites described by the authors as watersheds short cuts or enigmas Two further studies on road casualties have been commissioned by the Highways Agency one regional in southwest England carried out in 2000 and one national carried out during 2001 and 2002 An investigation into otter mortality on the trunk roads and motorways in Devon and Cornwall identified over 200 actual or potential casualty sites along 320 km of the A30 A38 and M5 Chanin 2001 Potential sites were places where the road crossed or ran adjacent to a river but seven of the 30 sites where casualties had occurred were away from water Thirty seven casualties were recorded on these roads between 1990 and 2000 a disproportionate number occurring on the A30 in Cornwall where it ran along the watersheds between rivers draining to the north and to the south of the peninsula Apart from crossings where the passage of otters was permanently blocked by impassable weirs for example it was not possible to predict where otters were most likely to be killed A unique feature of this study was that sites where otters had not been killed were examined as well as those where they had It was therefore possible to compare the frequency of casualties at particular types of structure with the abundance of those structures along the roads It was notable that although otters were unlikely to be killed at high viaducts there was no relationship between size or type of structure and the likelihood of casualties occurring Following this a national project to identify sites for mitigation on trunk roads was carried out during 2001 and 2002 The project was managed by Halcrow and guided by an advisory panel including representatives from statutory and non statutory organisations involved in otter conservation as well as knowledgeable individuals Impact on otter populations Since 1992 the Southwest Region of the Environment Agency has arranged for otter carcasses to be collected and subjected to post mortem examination Simpson 1998 and has maintained records of all casualties reported to it From 1990 to 2000 more than 250 otters have been killed on the roads of the Southwest Figure 4 and the increase in numbers over this time has given some cause for anxiety with a total of 50 animals recorded in 1999 However these data include animals killed in areas where otters are still recolonising notably Somerset Dorset and Hampshire and if only otters reported in Devon and Cornwall are included there is some evidence to suggest that the annual kill is levelling off Figure 4 40
Ecology of the European Otter 40 Number killed 35 30 25 20 15 10 5 0 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 Figure 4 Reports of otter road casualties in Devon and Cornwall recorded by the Environment Agency Southwest Region Whether this reflects a serious regional threat to otters is open to question The use of DNA fingerprinting enabled a total of 23 otters to be identified on the River Tone catchment Coxon et al 1999 These were recorded over one year in an area 400 km2
Conserving Natura 2000 Rivers Population size Barring further large scale changes in the environment it seems likely that otters will continue to colonise those parts of Britain where they declined to extinction during the third quarter of the 20th Century Long term monitoring by national surveys will enable this recovery to be charted but whether the population densities in those areas recover to former levels will be more difficult to ascertain National long term conservation objectives for otters tend to be set on the basis of their distribution that by a certain date otters should be found over a particular area rather than on the basis of population size or density However judging from recent correspondence there is also a need to obtain information on numbers of otters inhabiting particular areas Special Areas of Conservation SACs and National Parks for example Good estimates of the population of otters in Shetland with defined confidence limits have been determined by Kruuk et al 1989 and Conroy Kruuk 1995 but other estimates of otter numbers and density in Britain are based on inadequate data and should be treated with considerable caution Determining the population in freshwater habitats is only likely to be practical in the near future by the use of DNA fingerprinting of otter spraints a technology that is promising but still in its infancy If there is a need to know numbers of otters for conservation purposes a higher priority should be given to developing this technique and serious consideration should be given to seeking funding and commissioning further research Population ecology in lowland areas To date all studies of otter movements social organisation and habitat use in the UK other than those using spraints have been undertaken in Scotland While there is no reason to believe that otters in lowland areas of England would behave differently comparative data are lacking and there is considerable scope for studies using radiotelemetry and DNA fingerprinting to investigate otter biology in these areas as the population recovers There are substantial difficulties in carrying out such work in terms of resources development of techniques DNA fingerprinting and catching the animals radiotelemetry Nevertheless consideration should be given to promoting such research particularly in areas where preliminary studies have been carried out such as parts of Yorkshire and the Itchen catchment in Hampshire Diet in lowland areas In reviewing the literature on otter diet it has become clear that although numerous studies have been carried out in Europe as a whole there are remarkably few from lowland Britain Apart from two projects on a eutrophic lake in south Devon in the 1970s Chanin 1981 Wise Linn Kennedy 1981 most substantial studies of the diet of otters in Britain have been undertaken on the coast or on upland rivers and streams in Scotland As otters return to central and southern England the opportunities for such a study continue to increase and although it is unlikely to have a major impact on otter conservation there would be value in promoting such work One situation where such information may usefully support conservation measures is where otters come into conflict with still water fisheries and consideration should be given to commissioning research on this topic Review of toxic chemicals in otters Considerable numbers of otters and spraints have been analysed for toxic chemicals Reports are scattered through the literature and it is very difficult to obtain an overview of the data The longestterm study dating back to the 1960s and carried out by DJ Jefferies is now approaching publication Jefferies Hanson 2001 The Centre for Ecology and Hydrology at Banchory Aberdeenshire holds a large number of unanalysed samples dating back to the early 1990s and covering much of Scotland James Conroy pers comm 42
Ecology of the European Otter The data on water quality obtained for this report and described briefly in Appendix D demonstrate that there is information available to provide a background to the pollutant burdens of otters A review of these data covering the whole of Britain over the past 40 years and describing changes in space and time would make it possible to provide a coherent picture of the impact of toxic chemicals on the UK otter population in the second half of the 20th Century This would also provide a valuable foundation for evaluating future changes Monitoring National Surveys The repeated surveys of otters in Britain provide a unique record of changes in the distribution of a threatened species over a period of nearly 25 years The fact that there is not yet a policy for the future of these surveys is a serious cause for concern There is an urgent need for a review of the method and approach to otter surveys and data storage as a basis for determining a policy for their future Such a review should not only take into account the need for continuity but also national arrangements for monitoring mammals should these be agreed and the proposed scheme for monitoring otters throughout Europe Reuther et al 2000 Road casualties and postmortems The current system of collecting and recording otter road casualties has developed on an ad hoc basis in the three main regions where otters are abundant southwest England Wales and Scotland There is no formalised scheme for Northern Ireland Conroy Carss 2001 There is a need to develop a national policy on dealing with road casualties ideally with regional mechanisms for collecting carcasses and forwarding them for postmortem examination There should also be nationally agreed protocols for data recording and for carrying out postmortem examinations Data from postmortems should be regularly reviewed as a means of monitoring changes in the health of the otter population in terms of condition clinical diseases and pollutant burdens Data from road casualty sites should be regularly reviewed in order to identify places where mitigation might usefully be carried out The Highways Agency has put considerable resources into investigating otter road casualties revised its Design Manual for Roads and Bridges to incorporate the needs of otters and undertaken to carry out mitigation where appropriate However its responsibilities only extend to trunk roads and motorways and there is an additional need to involve local authorities with responsibility for roads in monitoring and mitigation schemes Local and regional offices of the Environment Agency would form a useful focus for such work Priorities for action High priority z Population monitoring schemes National Surveys z Monitoring of road casualties postmortems z Monitoring of road casualties for purposes of mitigation z Dietary studies where there are conflicts with still water fisheries Medium priority z z Review of the impact of toxic chemicals Development of DNA fingerprinting Low priority z Studies of otter population biology in lowland areas of England and Wales z Dietary studies in lowland areas of England and Wales 43
Conserving Natura 2000 Rivers Conclusions This review has shown that at present there is no evidence to suggest that the recolonisation of Britain by otters will not continue The impact of some factors that had previously been considered to impose constraints on otters ability to exploit some waterways may have been over estimated anthropogenic disturbance and the availability of suitable resting sites for example While the actual effects of toxic chemicals in the past are only now being elucidated there is a general consensus that all the likely candidates for causing the decline in otter populations are now present at low levels and that they are unlikely to inhibit recolonisation in much of Britain There is scope for further clarification of the biology of otters particularly in lowland areas and it is important to ensure that the population continues to be monitored both in terms of its distribution and various aspects of the health of individuals disease pollutants etc The greatest potential for enhancing the long term conservation status of otters lies in tackling factors that reduce the food supply below its natural level One of these the effects of pollution by sheep dips is being undertaken by the Environment Agency which has promoted a Sheep Dip Strategy involving the production of a code of practice under the Groundwater Regulations 1998 Environment Agency undated c The agency is also aware of and addressing the issues of siltation and channel management but significant improvements in these areas will involve a wide range of interests such as the construction and road building industries agriculture forestry and flood prevention English Nature has produced position papers on managing floodplains to reduce flood risk and enhance biodiversity and on reducing pollution from diffuse agricultural sources the latter focusing particularly on SACs and SSSIs Sites of Special Scientific Interest In the past the otter has been used as a flagship species for conservation activities on a relatively small scale Whether it can be used to promote long term sustainable strategies for management of waterways at the catchment level is unclear but there is no doubt that if it can there would be significant benefits to the otter population as well as the aquatic and riparian environment generally Steve Chadd English Nature 44
Ecology of the European Otter Acknowledgements I would like to thank many colleagues who have provided me with information ideas contacts and other forms of help Ann Skinner s patience and support is particularly appreciated The following have been particularly helpful in making certain sources of information available to me and discussing their views on otters and conservation Jessa Battersby JNCC Johnny Birks Vincent Wildlife Trust Jim Conroy Don Jefferies Hans Kruuk Tony Mitchell Jones English Nature Lisa Schneidau and staff of the Otters and Rivers Project see Appendix D The help and support I have had from Environment Agency staff has been overwhelming Lyn Jenkins Teg Jones and Andrew Crawford have been particularly kind and I would like to thank Nichola Salter for helping me get to the bottom of the silt problem The patience of staff in the water quality and fisheries sections of the Southwest Thames and Midland regions listed in appendices B and D is very much appreciated Piran White Adam Grogan and Beth Simmons have kindly helped me by making unpublished data and papers available I am extremely grateful to Graham Scholey Jim Conroy Liz Halliwell Ruth Warren Tim Sykes and Tony Mitchell Jones for their helpful comments on the first draft of this report References Andrews E Howell P Johnson K 1993 Otter Survey of Wales 1991 Vincent Wildlife Trust London Ansorge H Schipke R Zinke O 1997 Population structure of the otter Lutra lutra Parameters and model for a central European region Z Saugetierk 62 3 143 151 Bas N Jenkins D Rothery P 1984 Ecology of otters in northern Scotland 5 The distribution of otter Lutra lutra faeces in relation to bankside vegetation on the river Dee in summer 1981 Journal of Applied Ecology 21 2 507 513 Beja PR 1995 Structure and seasonal fluctuations of rocky littoral fish assemblages in south western Portugal implications for otter prey availability Journal of Marine Biological Association UK 75 4 833 847 Beja PR 1996a Seasonal breeding and food resources of otters Lutra lutra Carnivora Mustelidae in south west Portugal A comparison between coastal and inland habitats Mammalia 60 1 27 34 Beja PR 1996b Temporal and spatial patterns of rest site use by four female otters Lutra lutra along the southwest coast of Portugal Journal of Zoology 39 4 741 753 Beja PR 1996c An analysis of otter Lutra lutra predation on introduced American crayfish Procambarus clarkii in Iberian streams Journal of Applied Ecology 33 5 1156 1170 Beja PR 1997 Predation by marine feeding otters Lutra lutra in south west Portugal in relation to fluctuating food resources Journal of Zoology 242 3 503 518 Bielinski A 1993 Otters in Northumberland In Morris PA ed Proceedings of the National Otter Conference Mammal Society Bristol 38 40 Bradshaw AV 1999 Aspects of otter Lutra lutra mortality in England and Wales Unpublished PhD thesis University of Cardiff Wales Bradshaw AV 2001 Unpublished paper presented at the Mammal Society Annual Conference April 2001 Brazier M Mathias L 2001 Otters fish prey availability biomass and sustainability a preliminary report R D Technical Report W256 Environment Agency Bristol 43 pp Brooker MP 1985 The ecological effects of channelization Geographical Journal 151 63 69 Brzezinski M Jedrzejewski W Jedrzejewska B 1993 Diet of otters Lutra lutra inhabiting small rivers in the Bialowieza National Park eastern Poland Journal of Zoology 230 3 495 501 45
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Conserving Natura 2000 Rivers Hovens JPM 1992 Microverontreinigingen in water bodems en visbiota in Nederland de visotter Lutra lutra als normsteller Stichting Otterstation Groningen Nederland Jacobsen L Hansen HM 1996 Analysis of otter Lutra lutra spraints 1 Comparison of methods to estimate prey proportions 2 Estimation of the size of prey fish Journal of Zoology 238 1 167 180 Jefferies DJ 1987 The effects of angling interests on otters with particular reference to disturbance In Angling and wildlife in fresh waters ITE symposium No 19 Institute of Terrestrial Ecology Grange over Sands Jefferies DJ 1988 Otters Crossing Watersheds J Otter Trust 2 2 17 19 Jefferies DJ Hanson HM 2001 The role of dieldrin in the decline of the otter in Britain the analytical data In Conroy JWH Gutleb A Yoxon P eds Proceedings of the Otter Toxicology Conference Skye 2000 International Otter Survival Fund Broadford Skye Jefferies DJ Wayre P Jessop RM Mitchell Jones AJ 1986 Reinforcing the native otter Lutra lutra population in East Anglia an analysis of the behaviour and range development of the first release group Mammal Review 16 2 65 79 Jenkins D 1980 Ecology of otters in northern Scotland I otter Lutra lutra breeding and dispersion in midDeeside Aberdeenshire in 1974 79 J Anim Ecol 49 713 735 Jenkins D Burrows GO 1980 Ecology of otters in northern Scotland III the use of faeces as indicators of otter Lutra lutra density and distribution J Anim Ecol 49 755 744 Jurisch C Geidezis L 1997 Minimum passage rate of fishes through the digestive tract of otters Lutra lutra Linne 1758 Mustelidae Mammalia 61 1 123 126 Knights B Bark A Ball M Williams F Winter E Dunn S 2001 Eel and elver stocks in England and Wales status and management options R D Technical report W248 Environment Agency Bristol Kruuk H 1995 Wild otters Predation and populations Oxford University Press Oxford Kruuk H 1997 The significance of PCBs in otters a reply IUCN Otter Spec Group Bull 14 2 54 56 Kruuk H Conroy JWH 1996 Concentrations of some organochlorines in otters Lutra lutra L in Scotland implications for populations Environmental Pollution 92 2 165 171 Kruuk H Carss DN Conroy JWH Durbin L 1993 Otter Lutra lutra L numbers and fish productivity in rivers in north east Scotland Symp Zool Soc Lond 65 171 191 Kruuk H Carss DN Conroy JWH Gaywood MJ 1998 Habitat use and conservation of otters Lutra lutra in Britain a review Symp Zool Soc Lond 71 119 134 Kruuk H Conroy JWH Moorhouse A 1987 Seasonal reproduction mortality and food of otters Lutra lutra L in Shetland Symp Zool Soc Lond 58 263 278 Kruuk H Conroy JWH Webb A 1997 Concentrations of mercury in otters Lutra lutra L in Scotland in relation to rainfall Environmental Pollution 96 1 13 18 Kruuk H Moorhouse A Conroy JWH Durbin L Frears S 1989 An estimate of numbers and habitat preferences of otters Lutra lutra in Shetland UK Biol Conserv 49 4 241 254 Kruuk H Nolet B French D 1988 Fluctuations in numbers and activity of inshore demersal fishes in Shetland Journal of Marine Biological Association UK 68 601 617 Leonards PEG Smit MD de Jongh AWJJ van Hattum B 1994 Evaluation of dose response relationships for the effect of PCBs on the reproduction of mink Mustela vison Institute for Environmental Studies Free University of Amsterdam Amsterdam Liles G Colley R 2000 Otter Lutra lutra road deaths in Wales Unpublished report to the Environment Agency Cardiff Liles G Colley R 2001 Otter Lutra lutra road mortalities a procedure for the implementation of mitigation measures Unpublished report to the Environment Agency Cardiff Macdonald SM Mason CF 1983 Some factors influencing the distribution of otters Lutra lutra Mammal Review 13 1 1 10 48
Ecology of the European Otter Macdonald SM Mason CF 1994 Status and Conservation need of the otter Lutra lutra in the western Palaearctic Council of Europe Strasbourg 54 pp MAFF 1999a Controlling soil erosion an advisory booklet for the management of agricultural land MAFF Publications London MAFF 1999b Controlling soil erosion a manual for the assessment and management of agricultural land at risk of water erosion in lowland England MAFF Publications London Ma as S Ce a JC Ruiz Olmo J Palaz n S Domingo M Wolfinbarger JB Bloom ME 2001 Aleutian mink disease parvovirus in wild riparian carnivores in Spain J Wildlife Diseases 37 1 138 144 Mason CF 1989 Water pollution and otter distribution a review Lutra 32 2 97 131 Mason CF 1995 Habitat quality water quality and otter distribution Hystrix 7 195 207 Mason CF 1997 The significance of PCBs in otters at national and regional scales IUCN Otter Spec Bull 14 1 3 12 Mason CF 1998 Decline in PCB levels in otters Lutra lutra Chemosphere 36 9 1969 1971 Mason CF Macdonald SM 1982 The input of terrestrial invertebrates from tree canopies to a stream Freshwater Biology 12 305 311 Mason CF Macdonald SM 1986 Otters Ecology and conservation Cambridge University Press Cambridge Mason CF Macdonald SM 1988 Metal contamination in mosses and otter distribution in a rural welsh river receiving mine drainage Chemosphere 17 6 1159 1166 Mason CF Macdonald SM 1989 Acidification and otter Lutra lutra distribution in Scotland Water Air and Soil Pollution 43 3 4 365 374 Mason CF Macdonald SM 1994 PCBs and organochlorine pesticide residues in otters Lutra lutra and in otter spraints from SW England and their likely impact on populations Science of the Total Environment 144 305 312 Mason CF Ford TC Last NI 1986 Organochlorine residues in British otters Bulletin of Environmental Contamination and Toxicology 36 5 656 661 Mason CF Last NI Macdonald SM 1986 Mercury cadmium and lead in British otters Bulletin of Environmental Contamination and Toxicology 37 6 844 849 Mason CF Macdonald SM Aspden VJ 1982 Metals in freshwater fishes in the United Kingdom 1980 81 Vincent Wildlife Trust London Melquist WE Hornocker MG 1983 Ecology of river otters in west central Idaho Wildlife Monographs 83 1 60 Moorhouse A 1988 Distribution of holts and their utilisation by the European otter Lutra lutra L in a marine environment Unpublished MSc thesis University of Aberdeen Mucci N Pertoldi C Madsen AB Loeschcke V Randi E 1999 Extremely low mitochondrial DNA controlregion sequence variation in the otter Lutra lutra population of Denmark Hereditas 130 3 331 336 Murk AJ Leonards PEG van Hattum B Luit R vander Weiden MEJ Smit M 1998 Application of biomarkers for exposure and effect of polyhalogenated aromatic hydrocarbons in naturally exposed European otters Lutra lutra Environmental Toxicology and Pharmacology 6 2 91 102 Nores C Hern ndez Palacios O Garc a Gaona JF Naves J 1990 Distribuci n de se ales de nutria Lutra lutra en el medio ribere o cant brico en relaci n con los factores ambientales Revta Bio l Uni Oviedo 8 107 117 O Connor FB Sands TS Barwick D Chanin P Frazer JFD Jefferies DJ Jenkins D Neal E 1977 Otters 1977 The report of the Joint Otter Group Nature Conservancy Council London O Connor FB Wood MS Chanin P Jefferies DJ Jenkins D Neal E Rudge J Sands TS Weir V 1979 Otters 1979 The second report of the Joint Otter Group Society for the Promotion of Nature Conservation Lincoln Olsson M Jensen S Reuterg rdh L 19780 Seasonal variation of PCB levels in fish an important factor in planning aquatic monitoring programs Ambio 7 66 69 49
Conserving Natura 2000 Rivers O Sullivan WM 1993 Studies of the distribution ecology habitat requirements and mortality of the otter Lutra lutra in Co Cork and effects of drainage and toxic contamination of the species Unpublished PhD thesis University College Cork Philcox CK Grogan AL Macdonald DW 1999 Patterns of otter Lutra lutra road mortality in Britain J Appl Ecol 36 5 748 762 Pikulik MM Sidorovich VE 1996 Seasonal prevalence in otter reproduction and amphibian availability Doklady Akademii Nauk Belarusi 40 6 80 83 Prenda J Granado Lorencio C 1996 The relative influence of riparian habitat structure and fish availability on otter Lutra lutra L sprainting activity in a small Mediterranean catchment Biol Conserv 76 1 9 15 Reuther C Dolch D Green R Jahrl J Jefferies D Krekemeyer A Kucerova M Madsen AB Romanowski J Roche K Ruiz Olmo J Teubner J Trinidae A 2000 Surveying and monitoring distribution and population trends of the European otter Lutra lutra Habitat 12 1 148 Roos A Greyerz E Olsson M Sandegren F 2001 The otter Lutra lutra in Sweden population trends in relation to DDT and total PCB concentrations during 1968 99 Environmental Pollution 111 457 469 Ruiz Olmo J 1998 Influence of altitude on the distribution abundance and ecology of the otter Lutra lutra Symp Zool Soc Lond 71 159 178 Ruiz Olmo J Delibes M Zapata SC 1998 External morphometry demography and mortality of the otter Lutra lutra Linnaeus 1758 in the Iberian peninsula Galemys 10 239 251 Shore RF Rattner BA eds 2001 Ecotoxicology of Wild Mammals John Wiley London 730 pp Sidorovich VE 1991 Structure reproductive status and dynamics of the otter population in Belorussia Acta Theriol 36 1 2 153 161 Simpson VR 1998 A post mortem study of otters Lutra lutra found dead in south west England R D Technical report W148 Environment Agency Swindon Simpson VR Coxon KE 2000 Otter Cannibalism British Wildlife 11 423 426 Simpson VR Bain MS Brown R Brown BF Lacey RF 2000 A long term study of vitamin A deficiency and polychlorinated hydrocarbon levels in otters Lutra lutra in south west England Environmental Pollution 110 267 275 Smit MD Leonards PEG Hattum B van de Jongh AWJJ 1994 PCBs in European otter Lutra lutra populations Institute for Environmental Studies Free University of Amsterdam Amsterdam 77 pp Smit MD Leonards PEG Murk AJ de Jongh AWJJ van Hattum B eds 1996 Development of otter based quality objectives for PCBs Institute for Environmental Studies Free University of Amsterdam Amsterdam 129 pp Stephens MN 1957 The otter report Universities Federation for Animal Welfare London Strachan R 1998 Water vole conservation handbook Wildlife Conservation Research Unit Oxford Strachan R Jefferies DJ 1996 Otter Survey of England 1991 1994 Vincent Wildlife Trust London Taastr m HM Jacobsen L 1999 The diet of otters Lutra lutra L in Danish freshwater habitats comparisons of prey fish populations Journal of Zoology 248 1 1 13 Taylor PS Kruuk H 1990 A record of an otter Lutra lutra natal den Journal of Zoology 222 4 689 692 Thom TJ 1997 Factors affecting the distribution of otter Lutra lutra L signs in the upper Tyne catchment NE England Unpublished PhD Thesis University of Durham Twelves J 1983 Otter Lutra lutra mortalities in lobster creels Journal of Zoology 201 4 585 588 Watt JP 1993 Prey selection by coastal otters Lutra lutra L Unpublished PhD Thesis University of Aberdeen Watt J 1995 Seasonal and area related variations in the diet of otters Lutra lutra on Mull Journal of Zoology 237 2 179 194 Weber JM 1990 Seasonal exploitation of amphibians by otters Lutra lutra in north east Scotland Journal of Zoology 220 4 641 651 50
Ecology of the European Otter Weir V Bannister KE 1973 The food of the otter in the Blakeney area Trans Norf Norw Nats Soc 22 377 382 Wells GAH Keymer IF Barnett KC 1989 Suspected aleutian disease in a wild otter Lutra lutra Vet Rec 125 9 232 235 White PCL McClean CJ Woodroffe GL Baker JE Hutchings CE in press A spatial analysis of habitat use by a reinforced otter population using GIS J Appl Ecol Wise MH 1978 The feeding ecology of otters and mink in Devon Unpublished PhD Thesis University of Exeter Wise MH Linn IJ Kennedy CR 1981 A comparison of the feeding biology of mink Mustela vison and otter Lutra lutra Journal of Zoology 195 181 213 51
Conserving Natura 2000 Rivers Appendix A The diet of the European otter The following tables summarise the diets of otters determined by a wide range of studies throughout Europe Criteria for inclusion in the list were that the study had to cover the whole year include more than 300 spraints or observations and be compatible with other data In each case the values used were recalculated to express the diet in the form of percentage occurrence of items excluding non significant matter plant material insects small crustaceans or molluscs Summaries of the data are provided and included in the main text All studies in fresh water are based on analysis of spraints Some studies in coastal areas are based on spraint analysis others on direct observation Table A1 Diets of otters in freshwater habitats expressed as frequency of occurrence Country N Fish Mammal Bird Reptile Amphibia Ireland Poland Spain Spain Sweden 1709 379 367 598 14615 62 6 80 0 57 0 66 2 67 7 0 2 0 3 5 5 0 6 0 6 1 5 0 0 1 3 0 3 9 3 0 0 0 0 0 9 2 2 0 0 10 1 18 3 0 5 12 3 8 1 25 6 1 3 34 8 18 3 14 3 England England 353 607 94 0 91 4 4 3 7 1 1 1 1 0 0 0 0 0 0 6 0 5 0 0 0 0 England Scotland Spain 858 1180 2145 86 6 74 9 70 4 0 4 3 0 0 4 4 9 1 8 6 7 0 0 0 0 9 1 8 1 20 3 13 4 0 0 0 0 0 0 Ireland Portugal 2349 2883 51 1 53 2 0 0 0 0 2 0 0 5 0 0 1 4 15 9 16 4 20 9 28 6 Hungary Finland Belarus 873 1506 641 76 5 65 2 49 8 1 5 11 4 0 2 7 2 3 2 1 4 0 8 0 0 0 0 14 0 17 1 36 7 0 0 3 0 11 8 N sample size See list at end of appendix for sources of data Table A2 Summaries for freshwater habitats Fish Mammal Bird Reptile Amphibia Crayfish 52 Crayfish Maximum 94 0 11 4 9 3 9 1 36 7 34 8 Minimum 49 8 0 0 0 0 0 0 0 5 0 0 Mean 69 8 2 4 2 8 1 0 12 8 10 6 Median 67 7 0 6 1 5 0 0 13 4 3 0
Ecology of the European Otter Table A3 Diets of otters in coastal areas expressed as frequency of occurrence Country Type Portugal Spr Norway N Fish Mammals Birds Reptile Amphibia Crab 2267 91 7 0 1 0 3 0 5 5 3 2 1 Spr 1074 97 9 0 0 0 9 0 0 0 0 1 3 Ireland Spr 1026 97 3 0 0 1 2 0 0 1 2 0 4 Scotland Obs 2030 96 8 0 1 0 0 0 0 0 0 3 0 Scotland Spr 948 86 8 0 0 1 0 0 0 0 9 11 3 Type Spr based on spraint analysis Obs based on direct observation See list at end of appendix for sources of data Table A4 Summaries for coastal habitats Fish Mammals Birds Reptile Amphibia Crab Maximum 97 9 0 1 1 2 0 5 5 3 11 3 Minimum 86 8 0 0 0 0 0 0 0 0 0 4 Mean 94 1 0 0 0 7 0 1 1 5 3 6 Median 96 8 0 0 0 9 0 0 0 9 2 1 References Otters in fresh water Adrian MI Delibes M 1987 Food habits of the otter Lutra lutra in 2 habitats of the Donana National Park SW Spain Journal of Zoology 212 3 399 406 Beja R 1996 An analysis of otter Lutra lutra predation on introduced American crayfish Procambarus clarkii in Iberian streams Journal of Applied Ecology 33 5 1156 1170 Breathnac S Fairley JS 1993 The diet of otters Lutra lutra L in the Clare River system Proc R Ir Ac B 93 3 151 158 Callejo A Delibes M 1987 Diet of the otter Lutra lutra Linnaeus 1758 in the upper reaches of the River Ebro catchment northern Spain Miscellania Zoologica 11 353 362 Chanin P 1981 The diet of the otter and its relations with the feral mink in two areas of southwest England Acta Theriol 26 83 95 Erlinge S 1967 Food habits of the fish otter Lutra lutra L in south Swedish habitats Viltrevy 4 371 443 Harna G 1993 Diet composition of the otter Lutra lutra in the Bieszczady Mountains south east Poland Acta Theriol 38 2 167 174 Kyne MJ Smal CM Fairley JS 1989 The food of otters Lutra lutra in the Irish Midlands and a comparison with that of mink Mustela vison in the same region Proc R Ir Ac B 89 3 33 46 Lanszki J and Kormendi S 1996 Otter diet in relation to fish availability in a fish pond in Hungary Acta Theriol 41 2 127 136 Lopeznieves P Hernando JA 1984 Food habits of the otter in the central Sierra Morena Cordoba Spain Acta Theriol 29 26 3 383 401 Sulkava R 1996 Diet of otters Lutra lutra in central Finland Acta Theriol 41 4 395 408 Sidorovich VE Kruuk H Macdonald DW Maran T 1998 Diets of semi aquatic carnivores in northern Belarus with implications for population changes Symp Zool Soc Lond 71 177 190 Webb JB 1975 Food of the otter Lutra lutra on the Somerset Levels Journal of Zoology 177 486 491 Weber JM 1990 Seasonal exploitation of amphibians by otters Lutra lutra in north east Scotland Journal of Zoology 220 4 641 651 53
Conserving Natura 2000 Rivers References Coastal otters Beja PR 1991 Diet of otters Lutra lutra in closely associated freshwater brackish and marine habitats in southwest Portugal Journal of Zoology 225 1 141 152 Heggberget TM Moseid KE 1994 Prey selection in coastal Eurasian otters Lutra lutra Ecography 17 4 331 338 Murphy KP Fairley JS 1985 Food of otters Lutra lutra on the south shore of Galway Bay Proc R Ir Ac B 85 4 47 55 Kruuk H Moorhouse A 1990 Seasonal and spatial differences in food selection by otters Lutra lutra in Shetland Journal of Zoology 221 4 621 637 Watt J 1995 Seasonal and area related variations in the diet of otters Lutra lutra on Mull Journal of Zoology 237 2 179 194 54
Ecology of the European Otter Appendix B Fish population changes A small survey was carried out to collate information on fish populations from a number of rivers in England and part of Wales There were three principal objectives z To provide a brief overview of the nature of data available z To investigate changes in fish populations through time z To compare biomass values in these rivers with those obtained from areas where otters were known to be present Requests for information were made to fisheries staff in three Environment Agency regions for records from the 1970s to the present Table B1 lists the regions and rivers with information on otter surveys where appropriate and comments on the status of otters Information on pollutants in these rivers was also sought and the results are presented in Appendix C Information was obtained from all rivers except the Teme In addition while collecting data for the River Kennet some records for the Kennet and Avon Canal were also obtained Records were sent mainly as spreadsheets or copies of reports For one region a visit was made to the office where paper reports were held Table B1 Rivers selected for analysis showing number of sites searched during national otter surveys number positive at each survey and brief notes on otter presence Midland Thames Southwest Teme Avon Windrush 52 64 0 12 18 41 0 0 4 Kennet 22 0 0 0 Camel Teign 14 0 8 9 11 Otter 9 0 1 4 Present in early 1970s Sporadic records no otters resident up to 1999 Sporadic reports from 1990s No resident otters Serious pollution incident in 1988 Present in early 1970s but signs much less frequent than mink Signs much more frequent than mink in 1998 Absent in early 1970s personal records Chanin 1976 Nature of records Most early surveys were carried out with fishing interests in mind There was therefore a tendency to focus on target species such as salmonids in the Southwest and salmonids plus coarse fish in the other areas Fish numbers and in many cases sizes were recorded in most instances but estimates of biomass were much less frequent For three rivers a time series showing changes in numbers over a period of about 30 years can be presented Biomass data are usually limited to one or two sampling sessions for each river and much of this was obtained after 1985 Extraction of data from the written reports is not always easy and it is noticeable that there is considerable variation in the comparability of the data On some rivers there are records over a large period from the same sampling sites Elsewhere sampling was carried out on a more ad hoc basis with different sites used at different times Despite this the records form a valuable resource and the trend for extracting data and recording it in spreadsheets or databases for ease of access should be encouraged 55
Conserving Natura 2000 Rivers Summary of information received A full analysis of these data is beyond the scope of this report and the information is therefore presented in a series of illustrative tables and graphs Biomass There were no cases were biomass could be compared for more than three sampling years and it was not always possible to compare large numbers of identical sites For the rivers Kennet and Windrush and the Kennet and Avon Canal comparable data were obtained from enough sites for statistical tests to be carried out but analyses of variance revealed no significant differences between samples taken at different times Table B2 Mean fish biomass on rivers and the Kennet and Avon Canal 2 Upper Avon Lower Avon River Windrush River Kennet Kennet Avon Canal River Teign River Otter 1985 1992 1996 1993 1986 1993 1986 7 1993 4 1986 9 1993 4 1979 1978 23 20 11 27 10 10 18 18 21 21 15 12 7 8 9 8 14 9 11 6 24 7 29 2 31 8 27 6 34 4 31 7 10 1 23 4 While most rivers mostly had a biomass in excess of 20 g per m2 it is noticeable that the Teign an oligotrophic moorland stream had a mean value of only 10 g m 2 in 1979 and the potentially more productive River Avon was below 8 g m 2 in its upper reaches in the mid 1980s Kruuk et al 1993 demonstrated that fish biomass and productivity was strongly correlated with stream width in a river dominated by salmonids A similar picture is found on the river Teign Figure B1 where the biomass of salmonids was 10 g m 2 or less in sections of stream greater than 7 m wide and 10 g m 2 or higher in all but one section greater than 7 m wide On the River Otter there were similar differences for salmonids but coarse fish were distributed more evenly Table B3 Since the proportion of eels varied between parts of the river and these are known to be under recorded by multi species electro fishing the estimates for non salmonid fish on this river are likely to be low Table B3 Biomass g m 2 of different types of fish in the main river Otter with that in the upper reaches and tributaries Salmonid Non salmonid Total 3 43 10 58 14 01 13 36 10 01 23 37 15 57 17 28 32 85 On tributaries of the River Kennet there was no correlation between width and biomass On the River Windrush there was a tendency for smaller streams to have a higher biomass but this was much less pronounced than on the River Teign Figure B2 illustrates this for 1986 A similar situation prevailed in 1993 though with higher values 56
Ecology of the European Otter Biomass g m 2 40 30 20 10 0 0 2 4 6 8 10 12 14 Stream width m Figure B1 The relationship between fish biomass and stream width on the River Teign in 1979 Biomass g m 2 80 0 60 0 40 0 20 0 0 0 0 5 10 15 Stream width m Figure B2 The relationship between fish biomass and stream width on the River Windrush in 1986 The maximum biomass recorded during these surveys was 195 g per m2 on the River Kennet but values below 10 g per m2 were frequently recorded and there were also significant numbers of sites with less than 5 g per m2 The proportion of these varied but in some places may have been sufficient to have had an impact on otters had they been present at the time For example otters were known to have been present on the River Teign when a third of sites had a biomass of less than 5 g per m2 Table B4 illustrates these data and incorporates a small sample from the Upper Nene from an unpublished Anglian Water Authority report Interpretation of these data requires some caution since conditions in the River Avon a slow moving deep river are very different to those of the mainly shallow fast flowing Teign Nevertheless it is interesting to note that as recently as the early 1990s a high proportion of sites on the Avon had low values for biomass and the Nene in the 1970s was in an even poorer condition 57
Conserving Natura 2000 Rivers Table B4 Proportion of sites where biomass estimates were below 5 and 10 g m 2 Date Upper Avon Upper Avon Lower Avon River Windrush River Windrush River Kennet River Kennet Kennet Avon Canal Kennet Avon Canal River Teign River Otter River Nene 1992 1996 1993 1986 1993 1986 7 1993 4 1986 9 1993 4 1979 1978 1979 Sites 20 11 27 18 21 20 29 23 25 15 12 12 Biomass
Ecology of the European Otter Fish per 100 m2 40 30 20 10 0 1975 1980 1985 1990 1995 1990 1995 2000 Year Fish per 100m2 80 60 40 20 0 1975 1980 1985 2000 Year 140 Fish per 100 m2 120 100 80 60 40 20 0 1970 1975 1980 1985 1990 1995 2000 2005 Year Figure B3 Changes in salmonid density on the rivers Otter top Teign centre and Camel bottom 59
Conserving Natura 2000 Rivers Fish per 100 m2 60 50 40 30 20 10 0 1975 1980 1985 1990 1995 2000 Year Figure B4 Change in species composition on the River Teign Open circles salmon Closed circles trout It is notable that the absolute densities of salmonids on these streams vary widely being lowest on the River Otter and highest on the River Camel These differences are independent of differences in biomass potential food for the otter and are due to the presence of larger fish on the Teign and particularly the River Otter Trout on the River Otter grow much faster than those on the Teign with the result that the mean sizes for 1 trout on the Otter ranged between 16 and 25 cm at different sample sites while the means for 2 trout on the Teign ranged between 16 and 19cm Overview These data show that the biomass of fish in the rivers investigated varied widely There is clear evidence of an increase in fish densities in southwestern rivers and possibly an increase in biomass over time elsewhere It would appear that in the past 30 years there have been places where fish populations have been so sparse that otters may have found it difficult to successfully exploit them although there is no firm evidence that this is the case now Sources of information Information was obtained from unpublished records held by the Environment Agency in its local and regional offices Assistance in obtaining records was provided by the following Southwest Region Simon Steel and Rob Wood Thames Region Darren Bedworth and Paul Lidgett Midland Region Jo Mosley References Chanin PRF 1976 The ecology of the feral mink Mustela vison Scheber in Devon Unpublished PhD thesis University of Exeter Kruuk H Carss DN Conroy JWH Durbin L 1993 Otter Lutra lutra L numbers and fish productivity in rivers in northeast Scotland In Dunstone N Gorman ML eds Mammals as Predators Oxford Scientific Publications Oxford 171 191 60
Ecology of the European Otter Appendix C Changes in water quality in rivers Background Information on water quality in the rivers Camel Teign Otter Kennet Windrush Avon and Teme was sought from Environment Agency offices in the Southwest Thames and Midland regions The amount of information available on this topic is immense and it soon became clear that only the most superficial analysis would be possible The most complete information came from the Midland Region and constituted 114 000 analyses from the River Avon covering 209 sites and 2126 sampling dates between 1974 and 2001 A similar listing from the River Teme was about a quarter of the size These data included analyses of the effects of pesticides on otters but not of PCBs For the River Windrush annual summary sheets were obtained for a number of sampling stations that reported mean minima and maxima for a range of organic and inorganic compounds together with other water quality data turbidity dissolved oxygen etc These included a wide range of organochlorine pesticides and PCBs No data were obtained for the Kennet Summary data were obtained from the Southwest Region for 1974 2001 for various pesticides and PCBs at selected sampling stations on the three rivers These included mean minima and maxima Values The summarised data for the three southwestern rivers show low levels of PCBs when measured and organochlorine pesticides throughout the period Maximum yearly values of PCBs and pesticides are all below 0 01mg l 1 except for lindane HCH for which all values fall within the range 0 01 to 0 15 mg per litre Similar maximum values are found for the Windrush and although there appear to be declines in concentrations over the periods for which data are available many measurements are at the lower limit of detection and this has decreased with time rendering analysis impossible The most comprehensive set of data came from the Avon and the Teme Here too there was a decline in concentrations with time also confounded by changes in sensitivity of the analyses It is also clear that on the Avon there is at least one hot spot a heavily polluted site where samples are taken from a borehole At this site a maximum value for lindane of 3 7 mg per litre was recorded in 1986 and a value of over 1 mg per litre was obtained for DDT in the same year In order to illustrate changes that have taken place and particularly to demonstrate that high levels of toxic chemicals were present in both rivers over an extended period figures E1 and E2 include only samples where concentrations exceeded 1 0 mg per litre Data from boreholes are also excluded and toxic compounds are grouped to clarify the picture Note that the broad patterns of high levels are similar in the two catchments and although it may appear that pollution on the Avon was more severe approximately four times as many records were received for this river A clear trend of declining values is not unexpected but it is perhaps significant that values in excess of 100 mg per l of several pollutants were recorded in these rivers more than 20 years after the start of the decline of the otter 61
Changes in concentration g l 1 Conserving Natura 2000 Rivers Dieldrin HCH DDT E 10000 1000 100 10 1 1970 1975 1980 1985 1990 1995 Dieldrin includes values for Dieldrin Aldrin and Endrin HCH includes all isomers of HCH DDT E includes all isomers of DDT and DDE Figure C1 Changes in concentrations mg l 1 of some organochlorine pollutants in the River Avon between the 1970s and the 1990s Changes in concentration g l 1 Dieldrin HCH DDT E 1000 100 10 1 1970 1975 1980 1985 1990 1995 Dieldrin includes values for Dieldrin Aldrin and Endrin HCH includes all isomers of HCH DDT E includes all isomers of DDT and DDE Figure C2 Changes in concentrations mg l 1 of some organochlorine pollutants in the River Teme between the 1970s and the 1990s Sources of data Thanks are due to the following for permission to use the data and for extracting them Southwest Region Rob Moore and Nigel Morris Thames Region Jason Gash and John Eastwood Midland Region Andrew Sayer and Sue Stocks 62
Ecology of the European Otter Appendix D The Otters and Rivers Project Brief questionnaires were sent to all project officers in the Otters and Rivers Project with a view to assessing z The extent to which records of habitat management and holt construction have been kept z Local knowledge of any studies of otters in progress or completed z z The number of distribution surveys carried out and the extent to which the results are published or recorded in local or other recording centres Knowledge of the use of towns by otters Seventeen of the 24 officers responded Monitoring conservation work on otters All respondents stated that records of current conservation activities including holt construction were being kept In most cases information is stored by the local Wildlife Trust although this is not universal and several respondents stated that information was kept as personal records In three areas records are maintained by the Environment Agency and in one information is kept at the local Record Centre Only five areas have complete records of management though for some of the others data may be available in the Vincent Wildlife Trust archives Nine had full records of holts constructed in their areas Monitoring distribution Fifty seven local distribution surveys were reported all but seven of these taking place since the beginning of 1995 Several were large scale taking in whole counties 13 surveys or other large geographical areas for example the Lake District National Park Most of the remainder were river or catchment based To date records from only 12 surveys had been sent to a recording centre though in several cases there was no suitable centre available Only 15 reports were published either by the local Wildlife Trust as internal documents or as reports to sponsoring organisations Otters in towns Otters were reported from 80 cities and towns and described as resident frequent or regularly occurring at 49 of these The status was unknown at three and occasional or rare at the remaining 28 There was evidence of breeding in nine towns and just outside a further four Breeding was considered probable at two other towns and females with cubs were recorded in three others Comments There is clearly a great deal of work being done by the Otters and Rivers Project With a history of otter conservation work extending back over more than 20 years in some areas much has been achieved It is important to ensure that records are maintained and particularly to make sure that survey data are adequately recorded and used appropriately The OARP is devising an appropriate recording system for surveys and other work carried out under the project and its antecedents Lisa Schneidau pers comm The survey work has the potential to complement and significantly add to the monitoring undertaken by the National Surveys and every effort should be made to support this initiative 63
Conserving Natura 2000 Rivers It would be beneficial to ensure that distribution records are integrated into the National Biodiversity Network Areas from which responses were received Devon Yorkshire Northumberland Cheshire Cornwall Derbyshire Dorset Norfolk and North Suffolk Nottinghamshire South Suffolk Essex Somerset Staffordshire Warwickshire Worcestershire Sussex and Kent 64
Conserving Natura 2000 Rivers Ecology Series 1 2 3 4 5 6 7 8 9 10 11 Ecology Ecology Ecology Ecology Ecology Ecology Ecology Ecology Ecology Ecology Ecology of the White clawed Crayfish Austropotamobius pallipes of the Freshwater Pearl Mussel Margaritifera margaritifera of the Allis and Twaite Shad Alosa alosa and A fallax of the Bullhead Cottus gobio of the River Brook and Sea Lamprey Lampetra fluviatilis L planeri and Petromyzon marinus of Desmoulin s Whorl Snail Vertigo moulinsiana of the Atlantic Salmon Salmo salar of the Southern Damselfly Coenagrion mercuriale of the Floating Water plantain Luronium natans of the European Otter Lutra lutra of Watercourses Characterised by Ranunculion fluitantis and Callitricho Batrachion Vegetation Monitoring Series 1 2 3 4 5 6 7 8 9 10 11 A Monitoring Protocol for the White clawed Crayfish Austropotamobius pallipes A Monitoring Protocol for the Freshwater Pearl Mussel Margaritifera margaritifera A Monitoring Protocol for the Allis and Twaite Shad Alosa alosa and A fallax A Monitoring Protocol for the Bullhead Cottus gobio A Monitoring Protocol for the River Brook and Sea Lamprey Lampetra fluviatilis L planeri and Petromyzon marinus A Monitoring Protocol for Desmoulin s Whorl Snail Vertigo moulinsiana A Monitoring Protocol for the Atlantic Salmon Salmo salar A Monitoring Protocol for the Southern Damselfly Coenagrion mercuriale A Monitoring Protocol for the Floating Water plantain Luronium natans A Monitoring Protocol for the European Otter Lutra lutra A Monitoring Protocol for Watercourses Characterised by Ranunculion fluitantis and Callitricho Batrachion Vegetation These publications can be obtained from The Enquiry Service English Nature Northminster House Peterborough PE1 1UA Email enquiries english nature org uk Tel 44 0 1733 455100 Fax 44 0 1733 455103 They can also be downloaded from the project website www riverlife org uk
Life in UK Rivers was established to develop methods for conserving the wildlife and habitats of rivers within the Natura 2000 network of protected European sites Set up by the UK statutory conservation bodies and the European Commission s LIFE Nature programme the project has sought to identify the ecological requirements of key plants and animals supported by river Special Areas of Conservation In addition monitoring techniques and conservation strategies have been developed as practical tools for assessing and maintaining these internationally important species and habitats The European otter is a top predator and important in maintaining the equilibrium of a freshwater ecosystem It declined dramatically over most of the UK after the 1950s with the introduction of pesticides However it is gradually making a comeback and can now be found over much of its former range The otter is an important indicator of the environmental quality of its habitat This report describes the ecological requirements of the otter in a bid to assist the development of monitoring programmes and conservation strategies that are vital for its future Information on Conserving Natura 2000 Rivers and Life in UK Rivers can be found at www riverlife org uk This document was produced with the support of the European Commission s LIFE Nature programme and published by Life in UK Rivers a joint venture involving English Nature the Countryside Council for Wales the Environment Agency the Scottish Environment Protection Agency Scottish Natural Heritage and the Scotland and Northern Ireland Forum for Environmental Research
