Abstract

For many years large numbers of A. pallipes have been taken from the River Lea but there has been no previous systematic collection of data concerning numbers present. During 1974-1978 the river was heavily dredged and a new weir and rock-lined stilling basin constructed. Invading crayfish numbers were monitored by trapping and mark and recapture. By 1981 numbers had risen from zero to an estimated 2.5 per m²; the population of juveniles could not be sampled. The population in the newly built works was stable with little evidence of migration back to the surrounding area. The broken rock construction was of obvious importance. In 1980 a tributary was being cleared of the aquatic grass, Glyceria maxima, using a 0.73 m dredge. This removed up to 137 crayfish at each operation stroke. Modification of the operation by rinsing the dredge returned a high proportion of animals to the water. In 1981 mortality of the study populations occurred, no other species was affected. Aphanomyces astaci was suspected but was not isolated until 1984, when it was found on experimentally caged crayfish. The mortality has spread upstream and the River Lea is now virtually devoid of crayfish. This was one of the first known occurrences of Aphanomyces astaci in the UK.

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