Trends of Otters in Scotland

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1 Scottish Natural Heritage TREND NOTE Trends of Otters in Scotland Number 023, November 2015 Prepared using evidence from the Otter Survey of Scotland by Leonie Alexander and Melanie Findlay. Introduction Scotland is a European stronghold for the otter and they are now widespread over the whole of the country. Historically, pesticide pollution of waterways eliminated otters from most of England and Wales and led to substantial declines in Scotland leaving them largely present in the north and west only. The population has since recovered and otters can easily be seen in many areas, but particularly on the west coast and the islands. The most recent population estimate in 2003 suggested that the total Scottish population was around 8,000 at that time (Jefferies et al., 2003). People associate otters with large rivers and lochs, but small burns, canals, marshes, small ponds and even farm ditches can offer excellent foraging habitat. You can find otters in a range of habitats including urban rivers. The centres of Inverness, Edinburgh and other Scottish towns support breeding otters. Old industrial estates and inaccessible corners in towns can offer very secure areas for otters with old culverts and underground mill lades providing secure and dry structures hidden beneath the fabric of towns. Although once restricted as a result of persecution and pollution, today their presence is largely down to the availability of food, primarily fish. Key Points Otter populations have largely recovered from historical lows In , around 80% of sites showed signs of otters There are more sites with evidence of otters in Special Areas of Conservation than in the wider countryside Otters can be found in a range of places including in towns and cities. Otter 2020VISION Otters are largely solitary, semi-aquatic mammals that obtain most of their food from lochs, rivers, or the sea. The Scottish population unusually comprises a particularly high proportion (perhaps 50% or more) of coastal-dwelling individuals that feed almost exclusively in the sea. The coast and islands of western Scotland are particularly important for this species and coastal otters are occasionally referred to as 'sea otters' despite the fact that they are exactly the same species as the animals which inhabit freshwaters further inland. In freshwaters, otters feed mainly on fish such as trout, salmon and eels. In the spring spawning frogs and toads become important prey. Mammals and birds are also taken occasionally. In these habitats, otters are largely (but not exclusively) nocturnal and occupy very large home ranges (around 32 km for males and 20 km for females). In contrast, their 1

2 coastal counterparts are mainly active during the day and, because these productive inshore waters provide so much fish and crustacean prey, they need much smaller home ranges. These can be as little as 4-5 km of coastline. Coastal-dwelling otters require a ready supply of fresh water to wash the salt out of their fur, which would otherwise rapidly lose its insulative properties. Scottish Otter Surveys The rapid contraction of the otter s range in the twentieth century precipitated the first otter survey of Scotland in There have been four subsequent surveys in Scotland to monitor otter distribution. 1. The otter survey of Scotland (Green & Green, 1980) 2. The otter survey of Scotland (Green & Green, 1987) 3. The otter survey of Scotland (Green & Green, 1997) 4. National survey of otter (Lutra lutra) Scotland (Strachan, 2007). 5. Site condition monitoring for otters (Lutra lutra) in (Findlay et al., 2015.) The five surveys varied in sample size and coverage but used the same general field method. Otter field evidence was recorded in sample sections of suitable habitat to a maximum of 600m of one river bank. Spraints (otter faeces) were the most common evidence of otter, but footprints, runs, resting sites (holts and couches), feeding remains and rolling areas were all recorded. Although the presence of spraint clearly indicates the recent or relatively recent presence of an otter, it does not give information on the numbers of individuals present, their age or their sex. In addition, the absence of spraint does not necessarily mean that no otters are present. Holt Tracks Spraint Food remains 2

3 Otter trends from 1977 to 2012 In Scotland there are 44 Special Areas of Conservation (SACs) where the otter is a qualifying feature and these are monitored by SNH. The most recent monitoring was undertaken in which included all 44 SACs and also sampled random 10 km squares in the wider countryside to build up a picture of the current status of the otter in the whole of Scotland. When investigating otter trends over time it was important to ensure that the spatial coverage between surveys was directly comparable. Data from the older, more extensive, surveys were filtered to identify those sites that fell within the SACs or within the randomly selected 10km grid cells from the wider countryside. All other previously surveyed sites that did not fall within these areas were excluded. Otter 2020VISION Percentage of postivie sites SAC 95% confidence interval Survey Period Figure 1. The proportion of sites with signs of otters in each survey period for Scotland as a whole. This analysis suggested that there had been an overall increase in the number of otters from to , followed by a decline in otter presence between the survey and the survey. The apparent dip in the survey was not statistically significant. 3

4 Changes in otter distribution from 1977 to 2012 The survey identified regional differences in otter distribution. The north, west and south-west of Scotland, and the Scottish Islands, had strong populations but elsewhere otter distribution was patchy. There was no evidence of otters over large areas of southern Scotland. The second Scottish survey in showed that, in general, otter range was increasing although there was some concern over the slower rate of expansion in the Borders. (This survey excluded the Highland Council area, the Hebrides and the Northern Isles). The third and fourth surveys ( & ) found continued increases in range over Scotland and by the 1990s otters were again considered widespread throughout all of Scotland. The survey recorded evidence of otter at 92% of survey sites. It is generally accepted that pollution was the principal cause of otter declines in the mid twentieth century. In particular pollution from organochlorine pesticides (DDT and Dieldrin) was the main cause, either in isolation or in combination with Polychlorinated Biphenyls (PCBs) and mercury (Carss & Shore, 2004). Use of Dieldrin and PCBs stopped voluntarily in the UK in the 1970s followed by legislation to prohibit their use. Dieldrin levels are now generally low and while individual otters may be carrying high levels of PCBs and/or mercury, these are not thought to have an effect at the population level (Carss & Shore, 2004). Weather during the most recent survey in was generally poor. It is not clear if the conditions (high rainfall, flow rate and spates) affected the overall results. Many factors can influence otter detectability and these can differ between national surveys, SACs and individual survey sites. For example, some survey may have sites with sheltered sprainting areas (e.g. under bridges) whilst others don t; high flow conditions may create flooding of river banks and wash away signs in some surveys/locations; there may be differences in the number of suitable sprainting areas between sites, and so on. Additionally, in coastal areas dry conditions may reduce detectability if fresh-water pools dry out as this means that otters may not always mark the site with spraints. Typical otter habitat in Lochaber SNH Images 4

5 Do otters fare the same in the wider countryside and SACs? SACs were designated because they had good suitable habitat and strong otter populations. Using the results of the survey we looked at whether levels of occupancy differed between SACs and the wider countryside. The survey supported this and sites in the wider countryside generally had a lower level of occupancy than the SAC sites. This relationship was maintained for all survey periods for all of Scotland, and the trend of the wider countryside mirrored that of SACs. 100 Percentage of positive sites All Scotland 30 SAC 20 Non SAC Figure 2. The proportion of sites with signs of otters in each survey period for Scotland as a whole, the SAC sites and the non-sac sites Typical coastal otter habitat in Mull SNH Images Otter 2020VISION 5

6 Environmental change in the future Environmental pressures such as habitat change and the presence of chemical toxins have changed over the last four decades. Levels of the persistent organic pollutants (POPs) thought to have caused the population crash in the mid twentieth century are decreasing, however there are new POPs which are now being recorded in otter tissue samples. Polybrominated Diphenyl Ethers (PBDEs) and closely related compounds which are used as fire retardants may be of concern, but as yet there are no data available on toxicity to otters (Walker et al., 2012). Research by Kean et al. (2013) suggests that the otter population in England and Wales is largely healthy. However, they did note that there was an increase in reproductive abnormalities in males, attributed to exposure to endocrine disrupting chemicals. These findings highlight the importance of ongoing research into pollutants and health indicators in top predators such as otters. Caution otters crossing, Benbecula, Western Isles SNH The implications of aspects of climate change such as change in rainfall on otters and their food supply are difficult to predict. For example, eelpout (Zoarces viviparous), is an important component of the coastal otter s diet where it is present (Kruuk, 2006). There are concerns that the range of this species, and others, may shift northwards as sea temperatures rise (Fisher, 2007). Eels are another important component of the otter s diet. The European eel population collapsed in the 1970s and they are now less frequent in the otter s diet (Kruuk, 2014; Beaton, 2013). Crab remains were noted in coastal spraint which may be indicative of juveniles or lack of fish availability as crabs are considered to be a secondary food item as they provide less energy (Watt, 1993). Road traffic accidents have been shown to account for a number of otter deaths (Grogan et al., 2013). Transport Scotland and the trunk road operating companies have for many years installed mitigation where trunk roads cross rivers that are known to be hotspots for accidents and where a practical solution is possible. Otter fencing and ledges through culverts are options to encourage otters to pass through culverts when the river is in spate as opposed to crossing the road above. These require monitoring and routine maintenance to ensure effectiveness. 6

7 The future for Scotland s otters The recent survey has shown that Scotland s otter population is relatively widespread. There are limits to what a survey on occupancy can show us and advances in monitoring methods such as using DNA to identify individuals, marking otters to find out movements and improved field surveys in the future, will all help fill in the picture of what s happening with otters. Scotland s otters are internationally important and it is vital that we continue to monitor and research them to understand the changes. The pressures on the otter population will continue to change through habitat change, changes in weather and climate, new roads and increasing traffic, changes in fish populations, changes in the types of pollutants in the aquatic environment. Otters are not only top predators but also enigmatic and engaging animals. Their presence attracts visitors to Scotland, and the sight of an otter swimming in the sea or splashing along the shore is something that can be a lifetime wildlife moment for a lot of people. If you see an otter you can send your sighting to either The Mammal Society these sightings will contribute to the Mammal Atlas, or your local Biological Records Centre. If you find a dead otter Cardiff University are currently running a national scheme collecting otters found dead in England, Scotland and Wales for post mortem examination. Please contact Otter 2020VISION 7

8 References Beaton, H Are eels a declining food source for otters in Scotland? British Wildlife, 25, Carss, D. N. & Shore, R Review of contaminant data for otters in Scotland and Northern Ireland. Report to Scotland and Northern Ireland Forum for Environmental Research (SNIFFER). SEPA, Sterling. Findlay, M., Alexander, L. & Macleod, C Site condition monitoring for otters (Lutra lutra) in Scottish Natural Heritage Commissioned Report No Fisher, R., Chilled out Baltic fish feel the heat. [Online] Available at: [Accessed 4 March 2013]. Green, R. & Green, J The otter survey of Scotland Vincent Wildlife Trust, London. Green, R. & Green, J The otter survey of Scotland Vincent Wildlife Trust, London. Green, R. & Green, J The otter survey of Scotland Vincent Wildlife Trust, London. Grogan, A., Green, R. & Rushton, S The Impacts of Roads on Eurasian Otters (Lutra lutra). IUCN Otter Specialist Group Bulletin 30 (1), Jefferies, D.J., Strachan, C. & Strachan, R Estimated numbers of the three interacting riparian mammals in Britain using survey data. In: Jefferies, D.J. (Ed) The water vole and mink survey of with a history of the long-term changes in the status of both species and their causes, pp Vincent Wildlife Trust, Ledbury. Kean, E. F., Lyons, G. & Chadwick, E. A Persistent organic pollutants and indicators of otter health: other factors at play? CHEM Trust report. Kruuk, H Otters ecology, behaviour and conservation. Oxford University Press, Oxford. Kruuk, H Otters and Eels: Long-term Observations on Declines in Scotland. IUCN Otter Specialist Group Bulletin 31 (1), Strachan, R National survey of otter Lutra lutra distribution in Scotland Scottish Natural Heritage Commissioned Report No Walker, L.A., Moeckel, C., Pereira, M.G. et al Polybrominated Diphenyl Ethers (PBDEs) in Eurasian otters (Lutra lutra) collected from Britain in 2010: Predatory Bird Monitoring Scheme (PBMS) Report. Centre for Ecology & Hydrology, Lancaster, 14pp. Watt, J Ontogeny of hunting behaviour in otters (Lutra lutra L.) in a marine environment. Symposia of the Zoologicial Society of London, 65,