Abstract

The occurrence of dark brown or black melanized spots, indicative of the presence of crayfish plague fungus (Aphanomyces astaci Schikora), was investigated in 1979, 1983 and 1985-88 in three lakes inhabited by self-propagating populations of the American signal crayfish (Pacifastacus leniusculus Dana). The animals examined were caught with crayfish traps in Aug-Sep, 3-8 weeks after the molt. In one lake, from which the crayfish population had developed from juveniles imported from Sweden in 1971, crayfish examined had no brown spots. The lake also contained a population of the native noble crayfish (Astacus astacus L.) that also indicated the absence of crayfish plague. In two of the lakes the crayfish populations originated from adults imported from Lake Tahoe, USA, in 1969. The frequency of infected crayfish in the lakes (1,175 examined) was high, 47% and 52% in 1979, but over time the incidence of infection decreased and in 1988 only 11.8% and 10.5% were infected. The decrease occurred even though there was no appreciable change in the environment, trapping effort, or handling of the crayfish. The brown spots most commonly occurred on the walking legs (38% and 40% of spots) and the chelae (30% and 28%), and the average number of spots were 2.3 and 2.0 in each of two Finnish lakes. Many of the spots, 36.2% and 44.4%, were > 3 mm in dia.

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