Abstract

The crayfish plague, caused by the fungus Aphanomyces astaci, has since 1893 caused great losses among native populations of Astacus astacus in Finland. Signal crayfish, Pacifastacus leniusculus, were introduced in 1969 to Lake lso-Majajarvi (61°52'N, 23°51'E) and the development of the population has been studied regularly ever since. Crayfish catches and population sizes have been quite low during the entire study period. The peak occurred in August 1983 when the crayfish catch per trapnight was 0.64 and the population size was estimated at about 670. Population size then declined and has since fluctuated between 200 and 400. It is assumed that the slow development of the population has been caused by environmental factors, especially the small amount of good crayfish habitat, poor water quality and predators. The growth of signal crayfish in Lake lso-Majajarvi is equal to that in other signal crayfish lakes in Finland. Annually about 47-98% of the catchable adult females have been in reproductive condition. The crayfish, introduced into the lake were infected with plague but no mortality has been observed.

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