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Scottish Natural Heritage
TREND NOTE
Number 023, November 2015
Trends of Otters in Scotland
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 2011-12, 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
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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 1977-79. There have been four subsequent surveys in Scotland to
monitor otter distribution.
1. The otter survey of Scotland 1977-79 (Green & Green, 1980)
2. The otter survey of Scotland 1984-85 (Green & Green, 1987)
3. The otter survey of Scotland 1991-94 (Green & Green, 1997)
4. National survey of otter (Lutra lutra) Scotland 2003-04 (Strachan, 2007).
5. Site condition monitoring for otters (Lutra lutra) in 2011-12 (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
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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 2011-12 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
100
90
80
Percentage of postivie sites
70
60
50
40
30
20
SAC
10
95% confidence interval
0
1977-79 1984-85 1991-94 2003-04 2011-12
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
1977-79 to 2003-04, followed by a decline in otter presence between the 2003-04 survey
and the 2011-12 survey. The apparent dip in the 1984-85 survey was not statistically
significant.
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Changes in otter distribution from 1977 to 2012
The 1977-79 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 1984-85 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 (1991-94 & 2003-04) found continued increases in range
over Scotland and by the 1990s otters were again considered widespread throughout all of
Scotland. The 2003-04 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 2011-12 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
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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
90
80
Percentage of positive sites
70
60
50
40 All
Scotland
30
SAC
20
10 Non SAC
0
1977-79 1984-85 1991-1992 2003-2004 2011-2012
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 Otter © 2020VISION
Images
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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.
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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,
http://www.brc.ac.uk/mammals/recording.php
or your local Biological Records Centre.
http://www.brisc.org.uk/Sources.php
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 http://www.otterproject.cf.ac.uk/found_an_otter.html
Otter © 2020VISION
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References
Beaton, H. 2013. Are eels a declining food source for otters in Scotland? British Wildlife, 25,
103-108.
Carss, D. N. & Shore, R. 2004. 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. 2015. Site condition monitoring for otters (Lutra
lutra) in 2011-12. Scottish Natural Heritage Commissioned Report No. 521.
Fisher, R., 2007. Chilled out Baltic fish feel the heat. [Online] Available at:
http://www.newscientist.com/article/dn10887-chilled-out-baltic-fish-feel-the-heat.html
[Accessed 4 March 2013].
Green, R. & Green, J. 1980. The otter survey of Scotland 1977-79. Vincent Wildlife Trust,
London.
Green, R. & Green, J. 1987. The otter survey of Scotland 1984-85. Vincent Wildlife Trust,
London.
Green, R. & Green, J. 1997. The otter survey of Scotland 1991-94. Vincent Wildlife Trust,
London.
Grogan, A., Green, R. & Rushton, S. 2013. The Impacts of Roads on Eurasian Otters (Lutra
lutra). IUCN Otter Specialist Group Bulletin 30 (1), 44-57.
Jefferies, D.J., Strachan, C. & Strachan, R. 2003. 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 1996–1998 with a history of the long-term changes in the status of both
species and their causes, pp. 188–197. Vincent Wildlife Trust, Ledbury.
Kean, E. F., Lyons, G. & Chadwick, E. A. 2013. Persistent organic pollutants and indicators
of otter health: other factors at play? CHEM Trust report.
Kruuk, H. 2006. Otters ecology, behaviour and conservation. Oxford University Press,
Oxford.
Kruuk, H. 2014. Otters and Eels: Long-term Observations on Declines in Scotland. IUCN
Otter Specialist Group Bulletin 31 (1), 3-11.
Strachan, R. 2007. National survey of otter Lutra lutra distribution in Scotland 2003-04.
Scottish Natural Heritage Commissioned Report No. 211.
Walker, L.A., Moeckel, C., Pereira, M.G. et al. 2012. 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. 1993. Ontogeny of hunting behaviour in otters (Lutra lutra L.) in a marine
environment. Symposia of the Zoologicial Society of London, 65, 87-104.
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