The only aquatic mammals native to Britain are the otter, water vole and water shrew. The American mink is now well established through much of Britain, despite efforts to eradicate it. Foxes, badgers, stoats and weasels and various rodents make use of bankside and wetland habitat and are important as predators or prey within the food chain, but are not characteristic of the aquatic ecosystem as such.
The otter (Lutra lutra) was once widely distributed throughout Britain, but suffered serious decline through the 1950s due to loss of habitat, persecution and pollution. With the ending of otter hunting, strict control of pesticides, improved river water quality and habitat improvements, otter populations are gradually recovering.
Otters live along rivers and smaller watercourses, marshes and coastal areas, particularly in Scotland, Ireland, Wales and north west and south west England. Otters maintain a home range which varies in size according to the type of habitat, food supply and social interaction with neighbouring otters. On coasts with a rich food supply a home range may be as small as 1km (0.6 mile), but along rivers may extend up to 40km (24 miles). Within the home range, the otter may have up to 30 dens or holts, in the roots of bankside trees, piles of logs, drains or caves. The most secure sites are used for breeding.
Otters, particularly females, are generally intolerant of disturbance. Dog otters are bolder, and may even frequent urban areas provided there is plenty of cover. Except in some coastal areas, otters are nocturnal, and the only way to find out if otters are present is by checking for spraints and tracks.
The main habitat requirements for otters are mature bankside trees with adjacent woodland and scrub. The horizontal roots of ash, oak and sycamore create cavities in the bank which are favoured resting sites. Overhanging trees are also vital in providing the stable, shady, oxygenated conditions for fish, the otter’s main diet. Alder and willow provide useful cover, but their roots are dense and fibrous and do not form the cavities particularly favoured by otters. Deciduous woodland, even if some distance from the river, will be frequented as long as it is accessible by small streams and ditches.
Provided there is an adjacent breeding otter population, good water quality and sufficient food supply, otters may move into new areas. Protection and maintenance of existing tree and scrub cover, tree planting, and other habitat management which favours many types of wildlife is also the best way to encourage otters. Fencing of banks against grazing to allow regeneration of scrub and trees is a useful technique. If possible, fence sections of bank with existing isolated mature trees, which may be used in the future for holts. Piles of logs, brushwood, rubble
and other material may be used as resting sites. Where overhanging trees have to be cut back for navigation or flood prevention, the trees should be pollarded or coppiced so that the root systems remain intact. Management should be done in autumn or winter to minimise disturbance, and work should be concentrated in one area at a time, over several years as necessary. Access may need to be limited to avoid further disturbance.
River catchment management plans for otters have been drawn up and implemented by partnerships of various organisations, including English Nature, the Environment Agency (or their equivalents), Wildlife Trusts and angling organisations. Any habitat work affecting otters should only be done as part of an overall river catchment management plan. Artificial holt construction has been successfully undertaken on several sites. For details of holt construction and other management work see NRA (1993) and RSPB, NRA and RSNC (1994).
The water vole (Arvicola amphibius) was once common throughout England, Scotland and Wales, though absent from Ireland. In recent years it has suffered a decline, in spite of general habitat and water quality improvements which have favoured other aquatic species. The decline may be due to predation by mink. The water shrew (Neomys fodiens), unmistakable with its long snout, lives on various aquatic invertebrates and, occasionally, the spawn and fry of fish including salmon and trout. Both voles and shrews live in burrows in banks. Watercress beds are particularly favoured by shrews. Voles and shrews fall prey to pike, weasels, various hawks and other predators.
The American mink (Mustela vision) was first recorded breeding in the wild in Britain in 1956, on the River Teign in Devon, and is now widespread through much of Britain. Mink catches most of its food around rivers and streams, preying on fish, birds, small mammals, frogs, toads, snails and insects, as well as poultry, game birds and waterfowl. To some extent its great destructiveness may be due to an instinctive attempt to store food for the intense North American winter which never comes. Mink have a more general diet and less exacting habitat requirements than otters, and are more widespread than otters. However, mink do not attack otters and in many areas they co-exist, and it is thought they have little effect on otter numbers. Mink are easily trapped, although they are now so well established this has little effect on numbers. Hunting tends only to disperse the mink and encourages them to increase their range.
Of the fifteen species of bat found in Britain, five are particularly associated with rivers, lakes and ponds, where they forage over open water and bankside vegetation. Mature trees, especially those with holes and cracks are important as summer roosts. The retention of existing trees, plus management to favour a wide range of invertebrates is the best way to ensure that bat populations survive or increase.
