The white clawed crayfish (Austropotamobius pallipes) is a freshwater crustacean at imminent risk of extinction, largely due to the introduction of American signal crayfish (Pacifastacus leniusculus) to Britain. With the purpose of determining how white clawed crayfish respond to habitat and spatial variables, this study correlated white clawed crayfish distribution over a 35 km length of the River Wansbeck, Northumberland, to physical variables at three-spatial scales. White clawed crayfish were present throughout the study area at an average density of 5.3 individuals per square metre. The realised niche of white clawed crayfish was very broad; the only available areas crayfish could not make use of were those with microhabitat scale D50 smaller than 8 mm. Within their wide realised niche, crayfish showed significant responses to habitat. The strongest response was to grain size, with crayfish preferentially selecting cobbles as refuges. Distance downstream and lateral distance did not influence distribution or density of white clawed crayfish but crayfish were more abundant in the upstream half of the study area, reflecting the higher availability of favourable habitat in low order streams. Patchiness in distribution was only evident at the sub-metre scale, suggesting crayfish are only directly responding to microhabitat scale heterogeneity. Habitat based conservation actions should be conducted at this scale. However, habitat variables operating at the kilometre section and site scale (100 m) influenced the suitability of microhabitats. The abundant, dense population of white clawed crayfish on the River Wansbeck makes it a site of international importance. It is therefore recommended for designation as a Special Area of Conservation
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