Review of Contaminant Data for Otters in Scotland & Northern Ireland
The Scottish Environment Protection Agency (SEPA) and the Environment & Heritage Service (E&HS) have a statutory obligation to monitor pollution and the effects of pollutants in Scotland and Northern Ireland, respectively. SEPA is also seeking to develop a suite of environmental indicators that integrate the effects of environmental quality on several environmental components. Otters (Lutra lutra) are top predators of significant conservation importance, being protected by both international and domestic legislation. They are therefore worthy of investigation as possible indicators of the presence of persistent contaminants in aquatic environments.
This project investigated the potential of this species to provide an integrated view of the bioaccumulation of persistent contaminants (organochlorines and mercury) in aquatic food chains. As there appear to be no available published data on the bioaccumulation of contaminants by otters in Northern Ireland the project is restricted to Scotland where there have been three studies of levels of pollutants in otter carcasses during the period 1984-1992: the first confined to Shetland, the remaining two including carcasses from other parts of Scotland.
A brief literature review of published otter contaminant data from outside Scotland and Northern Ireland is provided, with a discussion of results in relation to otter contamination burdens elsewhere.
Existing informal networks for the collection of otter carcasses in Northern Ireland and Scotland are discussed. Suggestions for formalising such networks are also made.
Scottish data suggested that pollutants such as PCBs were present in otters in widely dispersed areas within Scotland, in concentrations that may be relatively high (maximum = 14.40 mg/kg wet wt.). PCBs were a mixture of congeners in which higher-chlorinated ones predominated, especially 138, 153, 170 and 180, a pattern comparable to that observed in otters from continental Europe. The concentration of other pollutants (dieldrin and DDE) appeared to be low and unimportant in a Scottish context.
Although mean levels of DDE and HEOD (dieldrin) contamination were low in the Shetland otters, as they were across other Scottish areas, the mean PCB concentration from Shetland otters (geom. Mean = 2.05 mg/kg wet wt.) was much higher than elsewhere in the country. This is also much higher than the level causing reproductive failure in laboratory mink: 50 mg/kg lipid, or about 1.24 mg/kg wet wt. in liver tissue. Although some very high individual values were also recorded from otters in Grampian, these appeared to have had little, if any, effect on otter densities in these areas.
The distribution of PCB levels in Scottish otters showed high mean levels in the far north (Shetland), with moderately low mean levels throughout mainland Scotland but again higher concentrations in the south-west, near the English border. There were no strikingly high levels of contamination in intensive agricultural regions such as north-east Grampian. The presence of high levels of PCBs in Shetland otters, which feed in the surrounding seas, was consistent with that from other aquatic species from the waters of the north-east Atlantic Ocean, a major environmental reservoir of PCBs. The distribution of PCBs in otters from elsewhere in Scotland was not related in any obvious way to known point sources.
The organochlorine compounds recorded in Scottish studies had not apparently accumulated in otters with age, even when samples were analysed separately by geographic area and sex. Otters are probably able either to metabolise or excrete various organochlorines, including at least some PCB congeners, and relatively high concentrations are found in their faeces 'spraints'. However, liver PCB residues can be relatively dynamic and may not necessarily be good indicators of total PCB burdens or accumulation with age.
Although not a universal pattern, there is a highly significant negative correlation between PCB level and an index of otter body condition throughout Scotland. The mechanism for this correlation is unclear: low body condition could be either a cause or a consequence of high PCB concentrations in the liver.
Scottish otters may have mercury concentrations of up to 45 mg/kg (dry wt), although these levels are well below that (110 mg/kg) recorded in otters which died after being fed mercury-contaminated fish for up to six months. Only 15% of the otters in the Scottish sample had mercury levels above 20 mg/kg and it was concluded that only a few individuals might have accumulated enough mercury to affect their survival. However, it appeared unlikely that otter populations would be suppressed by such contamination, even in the worst affected area (Argyll, n= 18 otter carcasses) where 28% of animals had mercury concentrations exceeding 20 mg/kg and 11% had concentrations exceeding 40 mg/kg.
Although mercury appears to accumulate with age in Scottish otters, age accounted for only 6% of the variation in mercury concentration within Scottish samples and, in some areas, the relationship was absent, or even negative. There is currently no satisfactory explanation for these differences: otters can presumably excrete some of the absorbed mercury with age, but the mechanism is unclear. There was a high negative correlation between otter body condition and mercury concentrations but low body condition could be either a cause of bioaccumulation or a consequence of it. However, as mercury levels are generally rather low, it seems likely that otters with low body condition accumulated mercury in the liver through remobilisation of fat and associated methylmercury and subsequent transfer to the liver.
In Scottish otters, it appears that regional differences in mercury concentration were partly caused by differences in precipitation. Variation in rainfall explained 13% of the variation in mercury residues for mainland otters. Rainfall together with age and body condition, accounted for 23% of the variation. Clearly, other factors (presently unknown) are, if anything, more important.
Data from published otter contaminant studies in Scotland were compared with available contaminant information for "relevant aquatic habitats" from published information from the National Monitoring Programme (NMP: marine habitats) and three SEPA data sets (freshwaters). Although both of the aquatic habitat data sets included analyses of both trace organic contaminants and heavy metals, including all those detailed as otter contaminants, comparisons with the otter data set were not possible. This was due to the mismatch in the geographic distributions of the data sets; there was little, if any, overlap between the distribution of aquatic habitat sites monitored and areas for which otter contaminant data were available.
However, superficial comparison of otter and aquatic habitats data sets did highlight the large geographical disparity between much of the contaminant data for otters and that for aquatic habitats. There were large areas for which otter data are available but for which comparable environmental data are lacking (i.e. the Northern Isles, much of coastal Argyll, and Dumfries & Galloway). Similarly, large areas covered by both NMP and SEPA datasets contain no information from otters (i.e. the Moray, Tay, Forth, Clyde and Solway Firths and eastern Scotland from Tayside to the Borders).
The literature review flagged up that there are multiple sources of methodological variability found in the analyses and reporting of otter contaminants. There are substantial differences between studies in the types of contaminants recorded, the sample types analysed (e.g. muscle, liver, spraints) and the analytical methods used. This lack of consistency in the choice of body tissue for analysis means that it could be very difficult or impossible to compare data between studies.
As a consequence of the lack of methodological and reporting consistency, this report takes a pragmatic approach, being restricted to the 23 studies included in a recent review of variation in contaminant analyses methods and reporting in peer-reviewed journals (1981-2000). Furthermore, given these serious constraints, anything more than a qualitative comparison of studies would be invalid.
Eighteen studies involving PCB analyses were collated. Overall, there was wide (i.e. over four orders of magnitude) variation within the range of PCB concentrations given. Data from Scotland suggest that PCBs were present in individual otters in widely dispersed areas, in concentrations which may be relatively high. Mean PCB concentrations from Scotland tended to be lower than those elsewhere but, in terms of order of magnitude at least, were in accordance with many of the minimum values reported. At a pan-European scale, there was little evidence that Scottish otters have abnormally high PCB concentrations: although high levels may occur in some individuals, and relatively high levels in animals from certain areas, there is no evidence for any negative effects at the population level.
Eighteen studies involving dieldrin analyses were collated. Overall, there was wide (i.e. over four orders of magnitude) variation within the range of dieldrin concentrations given. Mean dieldrin concentrations from Scotland tended to be similar to those elsewhere, in terms of order of magnitude at least. Data from Scotland suggested that dieldrin is present in otters in widely dispersed areas, in concentrations that appear to be 'low and unimportant'.
Sixteen studies involving DDE analyses were collated. Overall, there was wide (i.e. over four orders of magnitude) variation within the range of DDE concentrations given. Mean DDE concentrations from Scotland tended to be lower than those elsewhere but, in terms of order of magnitude at least, were in accordance with many of the minimum values reported. Data from Scotland suggested that DDE is present in otters in widely dispersed areas, in concentrations that appear to be 'low and unimportant'.
Six studies involving mercury analyses were collated. Overall, there was less variation within the range of mercury concentrations given than for organochlorines but it was still considerable (i.e. over three orders of magnitude). Mean mercury concentrations from Scotland tended to be among the higher levels reported. Data from Scotland suggested that mercury concentrations were highest in regions to the north and west and were correlated with annual rainfall, a finding consistent with an atmospheric origin of mercury pollution. Nevertheless, it appeared unlikely that otter numbers here are affected by mercury.
There is reasonable consensus that, at the population scale at least, otter numbers do appear to have changed in association with the increase and subsequent decline in the use of bioaccumulating contaminants. Even if the exact processes and consequences of contaminant accumulation in otters remain uncertain, this species is clearly a good bioindicator of contaminants in aquatic systems.
Recommendations for future research
Although the otter does not fully meet the bioindicator criteria identified in the literature, it remains one of the best bioindicators in freshwater systems of the risk posed particularly by persistent organic contaminants which bioaccumulate along food chains. It is therefore recommended that analysis of pollutant burdens in otters should be continued because the results can be compared on a regional/national level and give an indication of the toxicological risk to this species. In addition, a regular monitoring programme of otter tissue would enable new, potentially toxic chemicals to be identified and their threats to the aquatic environment determined.
Tissue samples from 474 otters are currently stored frozen at CEH Banchory and have yet to be analysed. In addition, samples from about 40 otters are currently held at the Scottish Agriculture College's (SAC) veterinary laboratory, Inverness.
To gain a more detailed analysis of changes in environmental contamination (and associated effects on health), complementary analysis of eels (Anguilla anguilla) is recommended. Eels are relatively easy to sample and a monitoring programme that gives complete spatial coverage (including areas from which otters are currently absent or rare) could be developed. Eels would give both an index of the current contaminant status of the aquatic environment, and enable identification of the pathways of contaminants into the upper levels of the food chain.
Analysis of otter faeces ('spraints') using a structured sampling programme over relatively large spatial scales may be of value where the specific objective is to determine wide-scale changes over time in the exposure of otters to particular contaminants. However, various uncertainties with this method of monitoring remain and the validity of using spraints for monitoring purposes requires further research.
It should be relatively easy to establish a network for the collection of otter carcasses in both Scotland and Northern Ireland using the existing agencies and voluntary groups. The network would need to identify the key agencies prepared to store, prepare and analyse otter carcasses and would obviously require a funding commitment for the programme over a number of years.
Keywords: Pollution, Eurasian otter, Lutra lutra, Contaminants, Organochlorines, Polychlorinated Biphenyls, Heavy metals, Mercury , Scotland, Europe, Bioindicator, Aquatic environment.
Copies of this report are available from the Foundation, in electronic format on CDRom at £20.00 + VAT or hard copy at £25.00, less 20% to FWR members
N.B. The report is available for download from the SNIFFER Website