Abstract

The exploitation, yield and stock structure of the heavily exploited noble crayfish Astacus astacus in Lake Steinsfjorden, S.E. Norway have been studied tor 14 years, to evaluate the effects of changes in the management scheme. The study is based on catch statistics, data on trap effort, and data from pre- and post- season test fishing for the noble crayfish stock in the period 1979-1992. Legislative changes included reduced legal season and increased trap mesh size. The total yearly accumulated trap effort varied from 72-130 trap nights ha^-1. Mean annual catch per unit effort (CPUE) in the traps of local fishermen ranged from 0.5-1.4 legal sized crayfish trap night, and the yield from 1.1-4.7 kg ha^-1 and year. Both mean CPUE and yield decreased significantly during the study period. We did not register any effect of increasing the trap mesh size. Reduction in the catching season to two weeks resulted in a reduction of the total trap effort by 45% and a substantial decrease in exploitation rate. The reduction in legal season increased the fraction of large crayfish captured, especially males. This is because males now moulted twice between legal seasons instead of once as previously. Yield and population size have been reduced by some 50% during the study period, and we attribute the reduction to a combination of high exploitation and removal of non-legal sized crayfish. It is argued that noble crayfish in lakes may sustain high exploitation rates, when females are allowed to spawn at least once. However, as the mean size of the catch was less than mean size at maturity for females and as the market accept small crayfish, regulations are imperative to avoid overfishing. Regulations should involve a legal season and a minimum size adapted to the local conditions, i.e. size at maturity for females.

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