Estimation of the spatial distribution of spawning crabs (Cancer pagurus L.) using larval surveys in the English Channel

A plankton survey in June 1989 showed that crab larvae were widely distributed in the English Channel, Western Approaches, and northern Bay of Biscay, with two main centres of concentration between 0°-l°30'E and 2°—4°W. Hatching began 6—8 weeks earlier in the western area, where water temperatures were several degrees higher than in the eastern Channel. The main concentrations of larvae persisted throughout development and there was only limited dispersal even in the eastern Channel, where predictions of the NORSWAP water parcel following model indicated in an average year there should have been advection into the North Sea. Predictions of limited larval dispersal in northern Biscay were confirmed by the movements of a drogue tracked by satellite. The combined results of the survey and the NORSWAP model simulations showed that the distribution of stage 1 zoeae could be used to predict the distribution of ovigerous female crabs immediately prior to the release of the larvae. This distribution, which was more widespread than previously thought, indicated that fishing effort in the eastern English Channel was not directed at the main centre of spawning, although ovigerous crabs might be vulnerable to other activities such as beam trawling, scallop dredging, and aggregate extraction.