Britain’s only native freshwater crayfish, the white-clawed or freshwater crayfish, Austropotamobius pallipes is in decline and is protected by the Wildlife and Countryside Act and the Berne Convention. The main threats to white-clawed crayfish in the past were loss of habitat and declining water quality. A more recent, and now more significant, threat is the introduction of non-native species, particularly the North American signal crayfish (Pacifastacus leniusculus) which have established themselves in the wild. Signal crayfish out-compete native crayfish for food resources and habitat but also carry a fungus which causes them no harm but leads to crayfish plague in native crayfish. This plague can wipe out whole populations of native crayfish.
The White-clawed Crayfish is best identified by its dirty white underside with two slim claws and eight walking legs, from which it derives its common name. It is pale to dark brown or olive in colour and seldom reaches 10cm in length, making it the smallest crayfish in the county.
Native crayfish inhabit base rich streams, lakes, ponds and gravel pits. They prefer fast flowing, well-oxygenated water systems, with a gravel and boulder bottom. The size of crayfish populations is determined principally by the steepness of the channel banks, the presence of riparian shrubs and trees, and the extension of their roots into the water, particularly alder, willows, and hazel. In conjunction with near vertical banks, tree roots help to produce undercuts which are favoured as resting sites since these shaded areas provide protection against predators. Marginal regions also provide important nursery sites.
The major cause of the native species population decline is through disease, namely Porcelain disease and crayfish plague. Crayfish plague is the more severe of the two. Since the first recorded outbreak in Britain was detected in 1981. It has spread rapidly throughout the country, and has resulted in a widespread disappearance of this species both from whroughout the UK. Other factors affecting native crayfish include river channelisation, pollution, drought, trapping and competition from Signal Crayfish. Although it has undergone dramatic decline throughout most of its geographical range, it is still widespread in Europe, especially France and Britain, making it a priority species for conservation.
The White-clawed Crayfish is classed as a globally threatened species receiving full legal protection under the International Union for Conservation of Nature and Natural Resources. It is included in Schedule 5 of the Wildlife and Countryside Act 1981, making it an offence to kill, injure or take a wild White-clawed Crayfish without a licence and to intentionally damage, destroy or obstruct a resting place used by them. The species has European protection under the Habitats Directive 43/93; being listed in Annex II and V, and is listed in Appendix III of the Bern Convention.