Freshwater mammals

Otters are one of Britain’s largest carnivores. They’re solitary mammals and their territories can cover up to 20kms of riverbank. They mark their territory with faeces (spraint), which can be found in prominent places such as on rocks.

Otters live in both coastal and freshwaters. They create dens (known as holts) in riverbanks, generally in tree root systems, holes or under rocks. They mainly eat salmonid fish, eels and crayfish although sometimes eat birds such as coots and ducks and, in the spring, frogs and toads. They need clean rivers with an abundant, varied supply of food, plenty of bank-side vegetation to provide cover for lying up during the day and secluded sites for holts. Wetlands provide an important habitat for raising their young as well as frogs and toads for food. Otters breed all year.

The otter population in Britain and western Europe declined dramatically between the mid-1950s and the mid-1970s. This was largely due to the introduction of organochlorine pesticides but also due to habitat loss and hunting. In the 1977-81 national survey, signs of otter were only found in 6% of sites in England.  Since then the population has slowly recovered due to the ban of organo-chlorine pesticides, legal protection in 1978 and better water quality.  

Otters are found on all Devon’s rivers and the population is thought to be healthy. However, as they remain vulnerable to impacts such as pollution, and as Devon is a national stronghold, the Environment Agency has said they should be a Focus Species in the LNRS. They are a UK Priority Species, are legally protected and also a qualifying feature of the Dartmoor SACSpecial Area of Conservation More.

Beavers are Britain’s largest rodent. They live in freshwater habitats such as rivers, streams and ponds, preferring areas that are surrounded by woodland. Their river territories are around 3kms.  

Beavers dig burrows (known as lodges), which have their entrance below water level. They live in family groups of an adult pair and their offspring. Between one and six young, called kits, are born between May and June. They leave when they’re around two years old to set up a new territory. 

Like water voles, beavers are herbivores. In winter they eat tree bark and in Devon favour willow, poplar, hazel and birch. In summer they eat a wide range of non-woody vegetation. In autumn they sometimes create a food reserve of branches and twigs, which they store underwater close to the entrance of their lodge. 

Beavers are great swimmers but slow on land so they prefer to be near water that’s at least 70cms deep. They’re known as ecosystem engineers because they build dams of mud, wood, plant material and stones across shallow watercourses and ditches to create deep ponds. They also build bunds across fields to create new ponds and canals to help them move vegetation back to their lodge.

Beavers were once widespread across Britain but by the end of the 16^th century had been hunted to extinction. However, since 2000, there have been official and unofficial beaver releases  across the country. In 2013, beavers were seen on the River Otter in Devon (see Inspiration below) and there are now wild beavers in the Otter, Tamar, Taw and Exe catchments.

Beavers’ engineering works have many benefits. They create habitats for a wide range of other species, including water voles. New wetlands slow the flow of water and can help to prevent flooding downstream, as well as acting as a buffer to watercourses and improving water quality. While this will benefit fish populations there are concerns that dams may create barriers and prevent fish such as shad (see Fish) from reaching critical spawning grounds. There are also potential conflicts with landowners and highways as beavers can flood land and fell trees across roads.

Water voles are our largest voles. They mainly live along well-vegetated banks of slow flowing rivers, ditches and lakes (and sometimes reed beds and marshes) where they have cover from predators and can feed on a wide range of vegetation. They have a territory of around 130m along the banks and are thought to travel up to 3km to establish new territories.

Water voles dig extensive burrow systems in banks with entrances above and below the water. They can also make aerial nests in wetlands when there are no banks to burrow into (has this ever happened in Devon?). They usually have three or four litters a year. 

Unlike many other British mammals, water voles tend to be active during the day. Signs that they’re present include closely grazed ‘lawns’, sometimes with piles of chopped food near the burrow entrance, a characteristic ‘plop’ when they enter the water and deposits of distinctive shiny black faeces. 

During the 1980s and 1990s, the once widespread water vole population declined by 90%. This was partly due to loss of habitat but was mainly because of predation by American mink. Water voles are classed as endangered, are a UK Priority species and are legally protected. In Devon they were declared extinct in 2002 and various reintroductions are taking place across the county.  What’s the mink situation?