Abstract

The original population of the native crayfish, Astacus astacus, disappeared in 1960 abruptly from a small (4.2 ha) lake in southern Finland, most probably due to the crayfish plague, Aphanomvces astaci. The lake was stocked in 1971 with 900 newly-hatched juveniles of the plague-resistant American crayfish, Pacifastacus leniusculus. Later in the same year, a few Astacus were caught in the lake, probably originating from specimens which had escaped the plague infection. Later investigations indicated that the Astacus population had been reproducing at least since the mid-1960s. Pacifastacus were recaptured for the first time in 1974, i.e. 3 years after the stocking. The length of the 4-summer old Pacifastacus varied from 11.5 to 13.0 cm, The stocked Pacifastacus seem to have reproduced for the first time in 1975. In test fishings made with traps in August 1977, the number of Astacus caught were 0.7 specimens/trap/night and Pacifastacus 0.1 specimens/trap/night. According to catches, the population of Astacus seems to be about 7 times greater than the population of Pacifastacus. This is in good agreement with the appraisal of the population sizes made with marking-recapture method in 1977. In these studies, the size of the catchable, adult (over 70 mm) population of Astacus was estimated at about 740 specimens and the population of Pacifastacus at 95 specimens. According to test fishings made in the lake, the catches of both Astacus and Pacifastacus have only slightly increased during the last years. The densities of both species vary greatly in different parts of the lake. Astacus have been caught i n all parts of the lake but the catch during the summer of 1977 was about 40 times greater in the best habitats than in less-suitable areas. Pacifastacus have so far been found only at the best biotopes in the lake. In the best areas the catch of Pacifastacus comprised about 30% of the total crayfish catch, i.e. about three times more than expected according to the size of Pacifastacus population in the lake when compared with the size of Astacus population. Both crayfish species seem to prefer biotopes of the same type: hard bottoms of rock, stones, or gravel or bottoms suitable for digging, covered with sunken tree trunks, twigs, litter, and vegetation which provide shelter for the crayfish.

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