Edible crab ( Cancer pagurus) larvae surveys off the east coast of England: implications for stock structure

Edible crab ( Cancer pagurus) fishing areas are distributed along the English North Sea coast, with significant fisheries offshore of North Norfolk and the River Humber. Previous tagging studies showed directed northward movements of mature females along the English east coast, suggesting that recruitment was strongly influenced by larval drift southwards. The results from a crab larvae survey undertaken in July 1999, when compared with those from two previous surveys in 1976 and 1993, showed that the epicentre of crab spawning, as perceived by the density of zoea I larvae, has been consistently approximately 70 km offshore south-east of Flamborough Head. Recent hydrographical evidence suggests the existence of a seasonal, jet-like circulation associated with a thermal front, which is formed in April/May and breaks down in October/November. Transport along this front extends southwards along the north-east coast of England, as far as Flamborough Head, before turning offshore and passing along the northern edge of the Dogger Bank, with little interchange across the frontal system. South of the Dogger Bank the circulation is weak, with only a slow eastward residual flow. The areas to the north and south of the front are effectively isolated during the period of crab spawning activity, challenging the accepted wisdom that the main crab fishery in the south is reliant upon recruitment from spawning areas to the north. Indeed, the crab population south of the Dogger Bank may be a separate, self-sustaining stock providing recruitment of mature female crabs to northern areas.