Crustaceans are key components of marine ecosystems which, like other exploited marine taxa, show seasonable patterns of distribution and activity, with consequences for their availability to capture by targeted fisheries. Despite concerns over the sustainability of crab fisheries worldwide, difficulties in observing crabs’ behaviour over their annual cycles, and the timings and durations of reproduction, remain poorly understood. From the release of 128 mature female edible crabs tagged with electronic data storage tags (DSTs), we demonstrate predominantly westward migration in the English Channel. Eastern Channel crabs migrated further than western Channel crabs, while crabs released outside the Channel showed little or no migration. Individual migrations were punctuated by a 7-month hiatus, when crabs remained stationary, coincident with the main period of crab spawning and egg incubation. Incubation commenced earlier in the west, from late October onwards, and brooding locations, determined using tidal geolocation, occurred throughout the species range. With an overall return rate of 34%, our results demonstrate that previous reluctance to tag crabs with relatively high-cost DSTs for fear of loss following moulting is unfounded, and that DSTs can generate precise information with regards life-history metrics that would be unachievable using other conventional means.
Trawl surveys in the estuary of the River Medway in the autumns of 1987-1991 have shown that the distribution of juvenile sea bass Dicentrarchus labrax is strongly associated with the warm-water outflow from Kingsnorth power station. In years of low abundance of first-year bass, very few were caught outside the warm-water discharge channel, whereas proportionately more fish of abundant year classes occupied the main river. About 15% of the available juvenile bass population died on the cooling-water intake screens at Kingsworth in the autumn and winter of 1987 and 1988. However, growth and survival of first-year bass in the Medway Estuary may be enhanced by the power station's warm-water effluent, such that overwinter mortality due to inadequate nutritional reserves and low temperatures may be considerably reduced. 1999 The Fisheries Society of the British Isles F. 1. Chart showing the River Medway and places named in the text, the location of Kingsnorth power station, trawl sampling stations (), limits of marking areas (heavy dashed line), and high (solid estuary boundary) and low (light dashed line) water marks.
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