A POPULATION STUDY OF THE WHITE-CLAWED CRAYFISH AUSTROPOTAMOBIUS-PALLIPES (LEREBOULLET) IN AN IRISH RESERVOIR

A population of white-clawed crayfish, Austropotamobius pallipes (Lere boullet), was studied by trapping and underwater observation in Blessi ngton Lake, Co. Wicklow, between 1989 and 1993. Results suggest a spar se, patchily distributed population of some 500 adult crayfish (>30cm carapace length) per 100m(2) of rocky shoreline. Catch per unit effort (CPUE) averaged 0.44 adult crayfish per trap-night over summer, dropp ing to 0.16 during winter, for traps lifted and rebaited every 2-5 day s. Approximately 6% of the catch exceeded 9cm in total length. Males d ominated the larger size classes. The sex ratio of the entire catch of 1356 crayfish was 1.44 males to 1 female, rising to 1.99 males to 1 f emale during egg incubation. Berried females formed about 24% of the c atch during this period. Mating activity occurred from early to mid-Oc tober. Mean pleopodal fecundity in the wild ranged from 55 to 65 eggs per female. Moulting commenced in early June. Growth increments averag ed 9% of carapace length for the early summer moult, falling to about 6.5% for a second moult in the laboratory. This population represents one of the few sizeable lentic populations of A. pallipes remaining in Ireland, perhaps the last European stronghold for this species.