Development of the introduced North American signal crayfish, Pacifastacus leniusculus (Dana), population in a small Finnish forest lake in 1970–1997

Signal crayfish (Pacifastacus leniusculus Dana) originating from Lake Tahoe, California, were introduced into a small (11 ha) lake, Iso-Majajärvi, in Central Finland (61°52´N, 23°51´E) in 1969. Since then, the population has been monitored regularly by trap catching. The stocked signal crayfish (3+ to 5+ years old) were caught until 1975, by when they reached ages of 8+ to 10+ years. First juveniles were born in 1970. Catches and population sizes were low during the study period. The catch peaked in 1983 at 0.6 ind./trap/night. The population size was estimated at 670 trappable specimens. Population size then declined, and has since fluctuated between 200 and 400 individuals. Despite annual variations, mean CPUE indicate a small, but statistically significant, increase in later years. The slow development of the P. leniusculus population has been attributed to environmental factors, especially the combination of limited area of suitable crayfish habitat and the low Ca concentration with periodic acidity. Fifty percent of the female P. leniusculus matured at ca. 84 mm. Consequently, half of the females entered the breeding population in the autumn of their third year (3+). The signal crayfish imported into Finland and stocked were infected with crayfish plague (Aphanomyces astaci), but no mortality has been recorded.