Abstract

Signal crayfish (Pacifastacus leniusculus (Dana)) from Lake Tahoe USA were stocked in 1968-69 in the small Lake Karisjarvi in central Finland. The density, growth, moulting and maturity of the population have been followed almost annually since the introduction. The mean annual catch/trapnight increased 0-2.1 during the 25-year period. The trappable population size increased despite heavy exploitation from 61 individuals in 1974 to 462 in 1988. The population remains sparse; the mean densities for suitable crayfish habitat (6000 m²) were 0.01, 0.02 and 0.08 >80 mm TL signal crayfish) m^-2 respectively. The males grew more rapidly than the females, while growth was similar to that of individuals in southern Finland. The mean size of newly hatched (stage 2) juveniles was 9.7 mm TL and of one-summer­old individuals 30.3 mm. Two-summer-old signal males averaged 84 mm and females 84 mm, and three-summer-old individuals 106 mm and 96 mm TL, respectively. The largest signal crayfish in Lake Karisjarvi was a 159-mm-long male (CL=83.5 mm) and the heaviest was 175 g. Signal crayfish males 80-100 mm TL moulted twice yearly, whereas females (>80 mm) and males (>100 mm) moulted only once yearly. Male signal crayfish (>70 mm) grew a mean of 12.4 mm/moult and females 8.5 mm. The average length incremenVmoult decreased with increasing size. The fraction of sexually mature females varied 41-100% (mean 79%) of the total female population. The smallest mature female was 64 mm TL.

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