Abstract

Following an initial survey covering the period of August 1982-May 1983, population parameters (length frequencies, sex ratios, molt status, and female reproductive status) were recorded for crayfish caught in small mesh traps in 12. 0 ha Valkealampi Pond in the summers of 1984, 1985, and 1986. The population was initially characterized as stunted with fewer than 10% being legal harvest size (100 mm total length). The sex ratio was 1.00:1.00; only half of the females had developed ovaries in the autumn. Intense trapping and removal of crayfish were begun in the summer of 1982. There was a decline in catch of legally harvestable crayfish from 1982 to 1984 but an increase from fewer than 10% of all crayfish to more than 30% in males and 15% in females by 1986. The sex ratio was skewed toward females in 1984 and 1986. There was poor recruitment to the adult size classes in all years. About half of the summer caught females had well developed ovaries in each year. However, based on the presence of egg case debris on unmolted females, fewer than 30% was laying eggs. Many mated females resorbed ovaries. Ovarian egg counts were comparable to other Scandinavian populations. Thus, in response to exploitation, the crayfish generally increased in size but there was no change in fecundity.

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