Eurasian Otter – Lutra lutra
Otters are part of the Mustelidae family which includes animals such as badgers, weasels and martens. The Mustelids have anal scent glands that produce a strong-smelling secretion the animals use for sexual signalling and for marking territories.
Considered a freshwater species, the otter is typically found in clean water bodies including lakes, reservoirs, rivers, streams and marshes, using tiny waterways, ditches and dry watercourses as regular routes. While some otters are coastal, they are thought to require fresh water to maintain the condition of their coat.
Otters are protected under the Wildlife and Countryside Act (1981) as amended. This means they cannot lawfully be killed, kept or sold (alive or dead), nor can their holts be damaged or obstructed, or otters themselves be disturbed in its resting place (holts, couches etc.) except with a special licence.
Otters are also listed in Appendix 1 of the Convention on International Trade in Endangered Species (CITES). Appendix 1 includes species threatened with extinction and so trade is permitted only in exceptional circumstances.
Otters are listed in Appendix 2 of the Bern Convention as strictly protected fauna species which requires all members of the European Union to “take all appropriate measures to ensure the conservation of the habitats of the wild flora and faunal species”.
European Habitats Directive (92/43/EEC) is the means by which the European Union meets its obligations under the aforementioned Bern Convention, the aims of which are to maintain and/or restore natural habitats and wild species to a favourable status, and protection of habitats and species of importance. Otters are listed under Annex 2 of animal and plant species of community interest, whose conservation requires the designation of special areas of conservation, and Annex 4 of animal and plant species of community interest in need of strict protection.
Eurasian Otters are a semi-aquatic mammal with predominantly light brown fur and a pale ventral area. they can be up to 1.2m in length, with about a 3rd of that length being the tapered tail. They have a long body with short legs, five toes which are webbed. Their ears (small and round), eyes and nose are positioned along the top of their head, so most of their body can be submerged when they are swimming. Their muzzles are thick and broad, more so in males and support long whiskers (vibrissae) that help them when foraging along river beds and in murky waters.
They only ever really confused with American Mink (Mustela vison), an alien species that have significant impacts on native wildlife and so is listed under Schedule 9 of the Wildlife and Countryside Act (1981) which makes it illegal to release or allow its escape into the wild. American Mink is about half the size of Eurasian Otter, with darker fur and a proportionately shorter tail.
Otters don’t display discrete seasonal behaviour aside from changes in diet due to prey abundance. Breeding is a-seasonal and occurs throughout the year, but with peaks during the colder months such as autumn and spring. It is estimated that on average, wild otters live to be 3-4 years of age, with a record of 16 years.
Otter resting places can be roughly divided into three categories:
- Holts are underground or covered refuges. This can include holes underexposed tree roots (ash and sycamore are often used due to their shallow sprawling root system, but also rhododendron, oak and elm), old badger setts, rabbit holes, or log/stick heaps and rock piles. They can readily use artificial structures such as artificial holts, or drains.
- Couches are above ground resting sites which can be relatively exposed. Typically thought to be in reed or willow beds, islands or areas of bracken, which may simply be an area of flattened vegetation in the centre. These sites are usually within 10m of water but may be as far as 50m or more.
- Natal dens are usually in different locations to regular holts. They can be over 150m from water, and don’t display the normal otter signs such as spraints or twig piles found outside regular holts, presumably to reduce their conspicuousness to potential predators.
Eurasian Otters are solitary, spending most of their time alone living within their territories, and while they can defend their territory ferociously there is also evidence of tolerant interactions. Male (dog) otters have larger home ranges, which may overlap that of female (bitch) home ranges. Males seem generally to be found in main river bodies, whereas females and young spend more time in small tributaries or standing water, possibly to avoid attacks on young by adult males. These home ranges provide everything needed by otters in terms of habitat and food, but within otters tend to have a smaller core range where they spend they remain for the majority of the time.
Otters become sexually mature at the age of 2-3 years. Mating occurs in water and has been little studied. Male otters play no part in the upbringing of cubs, leaving him free to potentially mate with other females. Gestation lasts between 60-70 days, leading to the birth of litters of between 1-5 cubs, but typically 2-3. Inland litters are generally larger than coastal litters. Because of the high level of maternal input given to raising offspring, female otters are thought to only reproduce every two years.
- Cubs are not natural swimmers and aren’t always inclined to get into water resulting in the mothers dragging them in when they are around 16 weeks old.
- Cubs will usually stay with their mother for over a year until they become self-sufficient and disperse to find their own territory.
In most habitats across its range, the main part of an otters diet is fish. Which fish species depends on what is available. Otters will feed on a wide range of different sizes of fish, apart from particularly small ones. Eels seem to be particular favourites. In the winter months, cold-blooded fish become slow in the cooler waters, and eels bury themselves in the mud where they can be dug up, making easy prey for otters. When things start to warm up in spring, fish become quicker, and slower moving prey such as amphibians, small mammals, reptiles and young birds become more abundant, and consequently, these start to make up a larger portion of otter diets. In summer, crayfish consumption peaks. Inland otters are primarily nocturnal, whilst coastal otters are less so, thought to be due to the species they generally prey on being less active at night.
Due to their nocturnal habits, its generally quite difficult to see otters, although early mornings are considered the best time, but with such large ranges, they can be difficult to locate. The presence of otters is more often identified through the presence of spraint – otter poo! Its location, smell and content are often enough to give confirmation of the presence of otters.
- Spraints are used by otters to mark territories of individuals, and so it’s deposited at strategic locations such as the base of large trees, prominent boulders, under bridges and at stream confluences. This allows it to be easily located by any passing otter.
- Spraints consist of the indigestible remains of otters prey items, including fish and animal bones, insect exoskeletons, scales and feathers, stuck together by a black, sticky mucus.
- Fresh spraint is dark, wet (affected by environmental conditions), with a sweet musky smell. These can last for 2-8 weeks in the right conditions but need to be regularly replaced as older spraints become dry, grey, crumbly and less odorous.
Footprints can also be used to identify otters, as there are few things to confuse adult otter prints with. Their size (5cm+ width) generally separates them from other mustelids. The presence of five toes separates them from dogs and foxes which have four toes, and their spread around the pad differs from badgers which are clustered in front. In addition, web marks are sometimes visible between the toes. Younger otter prints can be more easily confused with other mustelids such as mink.
Other signs include slides which are smooth areas or areas of flattened vegetation on banksides where otters regularly venture in and out of the water, castles (piles of sandy soil used for sprinting), and holts.
Waterfalls can be good locations to try and view otters as the oxygenated waters attract fish, which in turn attract otters!
A summary of the otter year is shown in the diagram below: