Abstract

Signal crayfish, Pacifastacus leniusculus, escaped into a tributary of the River Wharfe in Yorkshire, England in 1987 and soon spread to the main river. Although initially co-existing with the native or white-clawed crayfish, Austropotamobius pallipes, this study found P. leniusculus replaces the native species over 4-5 years. The average rate of downstream spread has been 1.2 km per year. There appears to be pulsed expansion rather than uniform spread which may be related to the availability of suitable habitat. The River Wharfe is a large upland river with highly variable flow and stony substrate. Density estimates of crayfish were obtained by systematic manual searching and night-time viewing of active animals. Both species prefer the slower flowing runs and margins overhung by trees. This study found a sharp edge to the distribution of signal crayfish in 1997. It appears that P. leniusculus colonises a new area, breeds and grows rapidly, and ousts A. pallipes. Once refuges are occupied, continued increase in population may encourage individual signal crayfish to move to new areas, beyond the temporary barrier posed by fast-flowing riffles and other unfavourable habitat.

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