Abstract

A study of the post-stocking movements of noble crayfish (Astacus astacus) was made by SCUBA-diving in the River Fosselva in southeastern Norway in September and October 1993 and in July 1994; furthermore the area was test fished with baited traps in September 1993 before the stocking and in August 1994. The crayfish stock orginated from both river and lake populations and were stocked at low and high densities, at one point or spread, and with and without 24 h acclimatization. In total 938 crayfish were used in the experiments. The crayfish spread gradually with decreasing rate after stocking. After 106 h, recaptures were registered as much as 380 m downstream and 240 m upstream from the release site. These data correspond to a mean migration speed of 3.6 mh^-1. Crayfish from a river showed a slightly stronger tendency to migrate downstream as compared to those originating from a lake. Crayfish stocked at low density showed a higher downstream migration compared to crayfish stocked at higher density. Acclimatization and spreading slightly reduced short term post-stocking movements. In September 1993, recapture rates did not vary according to origin (lake vs river); however, more females than males were recaptured. Recapture was low when crayfish were acclimatized and spread, and high when point released without acclimatization at high density. In July and August 1994, only one crayfish was registered indicating that virtually all the crayfish had moved downstream and out of the experimental area.

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