Description: Streamlined animal, large flat head, rounded face (mink have pointed face and muzzle), long tapering tail. Small eyes and ears, broad obviously whiskered muzzle, short legs, webbed toes. Brown fur all over except for a paler throat (mink have a white chin patch), can look almost black when wet. It has thick underfur which stays dry even while swimming. In the water, the otter swims very smoothly (unlike rodents and mink which move jerkily) and leaves a clean V shaped wake behind it. It is larger than a mink, and much larger than its close relatives the polecat, pine marten, stoat, and weasel, and is much slimmer than its other mustelid relative the badger. Although otters can be seen at coastal sites around the UK, they are still the same species we see on rivers and lakes here – they are not Sea Otters, which is a separate species not found in Europe.
Size: Otters are much longer than mink or badger. The sizes of male and female otters differ, the total length of male otters range from 96cm-136cm, mean is 119cm. Females range from 94cm – 112cm, mean is 104cm. The tale is also longer in the male than in the female, ranging from 36cm – 47cm (mean 43cm), while females range from 35cm – 42cm (mean 39cm). Sexing at a distance by size alone is not reliable.
Weight: Weight also varies by sex and by season. Males are larger ranging from 5.5kg – 16.8kg, (mean 10.3kg), while female weights range from 6.4kg – 12.3kg (mean 7.4kg). Animals are typically lighter following harsh conditions.
Lifespan: Infant mortality is very high, only 50% of cubs reaching three years. Once adult, wild otters might live 6 years or rarely up to 10 years. In captivity, otters typically live longer lives.
Breeding: Down here in Shropshire, otters follow the mild climate pattern of being able to breed at any time of year. Female otters become sexually mature at 18 months – 2 years, but rarely breed before 2 years old. The female selects and prepares a well protected holt, typically under the roots of a mature tree near to water. Otters can have a litter every year but not all of them do – on average 60% of adult females breed each year and availability of food is an important factor. Some breed every two years, others may breed within less than two full years of their last litter. Again, food is an important factor. Gestation is typically 62-63 days. Litters are mostly recorded as 2 or 3 cubs, but can be 4 or 5 with exceptionally good breeding conditions. The cubs stay with mum for 9 months to a year (which may account for litters being produced every other year in many cases). The cubs are tiny at birth (100-150g) and comparatively slow to develop; eyes open at 31-34 days of age, eat solid food at 49 days (though can still be suckling), and go into the water for the first time at 72 days. Vulnerable as very young cubs, they stay hidden in the birthing den for their first few weeks.
Habitat: Otters don’t live in water – they hunt in it and will travel in it when it suits, but they spend most of their time out of the water (70% of their time is spent in dens or couches [day dens often in long grass]) and a great deal of their time is spent grooming to keep their fur waterproof. But they do need easy access to water (streams, rivers, lakes, ponds, or the sea) and plenty of cover usually provided by thick long grasses, bushes, reeds, and trees. They do not always do well on canalised rivers as they need banks and overhanging trees for holts, despite this they can do well in such places.
Diet: True carnivore. Eels, fish, crayfish, crabs, water beetles, other invertebrates, frogs (not usually toads – but some otters have learned to turn toads inside out to avoid the foul tasting skin), small mammals (including rabbits), occasional birds, chicks and eggs. Carrion will be taken but not frequently, when it has been observed, it has been in the form of large fish such as salmon or pike. Some observations have also been made of otters feeding on dead or dying birds in reed beds.
General Ecology: Normally described as ‘crepuscular’ otters will be active in daylight hours when they feel secure enough. Territory size depends on quality in terms of opportunities for denning and feeding. Male otters patrol much larger territories than females and a male’s territory includes the territories of several females. In otters, territories are usually described in length along a water course, but marshes, reed beds, ponds and lakes are important elements of territory. Females with cubs typically have a home range which is 6km or 7km, more if the river or stream is poorly stocked and there is no lake or marsh in the area in which case their territory may need to extend to 10km-12km. Males typically hold territories of between 12km and 20km in length, with an average of 15km. Otters which live at the coast usually have much shorter length territories but then they can swim offshore and in rock pools to fish. Food density isn’t the deciding factor, it’s food availability – not all prey is easy to catch. Solitary and often anti social animals, otters patrol and scent mark territory regularly. Scent marking is by spraints (droppings) which are black, tarry, filled with fish bones and scales, and often left on prominent rocks, piles of earth of tussocks of grass, they have a distinctive smell – like violets and damp hay. Interaction between adult otters can often be aggressive, and bodies of dead otters usually feature old or new wounds (40% in one study). Female otters usually drive youngsters from the natal territory when they are 9-12 months old and resume a solitary lifestyle.
Distribution and main sites: The recovery of the otter is one of the true good news stories in conservation. They are now present in every county, and have re-established strongholds in their traditional range in Shropshire, with particularly buoyant populations found in the south and west of the county where fewer river banks have been cleared of vegetation and rivers haven’t been canalised. Water quality (and its effect on their prey) remains problematic including in its county strongholds.
Local Survey history: Vincent Wildlife Trust, Shropshire Wildlife Trust, Environment Agency, Natural England (on NNRs and SSSIs), Mammal Society, Shropshire Mammal Group, county recorder.
Conservation Status: Legally protected since 1978 following the otter campaign by Friends of the Earth started in 1976. Further protection followed in 1981 (Wildlife & Countryside Act), and further in 1992 (Habitats Directive). The population is recovering but may take a further 30 years before they are as numerous as they were in the 1950s before their almost total disappearance.
Threats: People, directly through (now illegal) persecution, accidental trapping (mink traps, eel and crayfish traps), recreational boat traffic, fishing, road traffic, and indirectly through land management changes (e.g. removal of riverside trees), lowered water levels following over-abstraction, our past use of chemicals especially dieldrin, aldrin, heptachlor, used in seed dressings and in sheep dip, and for treating wool to repel moths, plus pollution from PCBs, oil, etc. Many of the pesticides responsible for the deaths of otters are described as having ‘sub-lethal’ effects – i.e. they do not kill instantly, but they still kill. The decline of aquatic invertebrates and the fish which depend on them has lessened as rivers have been improved.
Action: Shropshire has been the focus of several otter conservation programmes led by the Wildlife Trust and Vincent Wildlife Trust, the Environment Agency and Natural England. There is continuing work with landowners to reduce ongoing threats from pollution, poor water quality, loss of habitat, and road traffic collisions.
Main recording methods: The first national survey was conducted in 1977 which sought to correct the errors in previous small-scale volunteer led surveys. Grid squares of 50km each were surveyed, taking 600m lengths of river and systematically surveyed for evidence of otter occupation (typically fresh spraints, but footprints and today camera trap footage is also used). While an improvement on earlier methods, signs of otter presence don’t automatically equate to numbers of otters present. Over time, the trend has been positive, showing otters are increasing.
Did you know? Recent research shows that otters can ‘smell’ underwater – they do this by blowing a bubble of air and breathing it back in, sensing prey by smell.
Text: Rosie Wood with additions by Andrew Crawford
Toilet or latrine sites: These are often found on high points along the banks of streams, bays or along crossover trails between water bodies. The vegetation is usually flattened out, and the area may contain numerous piles of otter spraint, often comprised of fish scales or crayfish parts.
Irregular, sometimes short, rounded segments, sometimes flattened masses, containing fish bones, scales, or crayfish parts; when fresh, often greenish and slimy. Scat most often found under bridges and on banks of stream or pond, on logs, or on rocks in water.
Slides: The most obvious and best-known evidence of otters — mud or ice/snow. Riverbank slides 8″ (200 mm) wide, much wider with heavy use. Snow slides 12″ (300 mm) wide or wider, up to 25′ (7.5m) long; often on flat ground, sometimes pitted with blurred prints where otter has given itself a push for momentum.