Growth of the signal crayfish, Pacifastacus leniusculus, in a small forest lake in Finland

The growth, growth per moult and moulting frequency of adult Pacifastacus leniusculus were studied in 1975–1977 and 1987–1988 in Iso-Majajärvi, an 11 ha oligotrophic lake with a very low calcium concentration (mean 1.6 mg l–1) in central Finland (61°52'N, 23°51'E). Growth was determined from the relationship between premoult and postmoult carapace lengths (CL) and measured on marked specimens. The mean absolute per moult increment (MI) in CL was 5.3 mm (i.e. 10.6 mm in total length, TL) in males and 4.1 mm (i.e. 8.2 mm in TL) in females. The percentage moult increase (PCMI) in CL ranged from 4.7% to 20.0% in males and from 1.5% to 13.6% in females. The MI and PCMI of both sexes diminished with the increase in size. Males moulted more often than females. About 27% of adult males apparently moulted twice and 73% once. Among females, 30% moulted twice and about 70% once. Due to their rapid growth, more than half of P. leniusculus (the majority males) reached harvesting size (> 100 mm TL) at the age of 3+, by which time some were up to 115 mm (TL). Data on length/weight and carapace/total length ratios are given. The growth of adult P. leniusculus in Iso-Majajärvi was of the same order of magnitude as in two other Finnish lakes with a higher Ca concentration.