Population and catch structure of the edible crab (Cancer pagurus) in the English Channel

Abstract

Cancer pagurus is widely distributed in the English Channel, supporting a fishery worth over £ 1 million along the south and southwest coasts of England. The most important area is off Devon where fishing extends up to 60 km offshore. Landings and catch rates vary seasonally, with the main fishery depending upon large mature female crabs with developing gonads, caught in the autumn on the offshore grounds at catch rates of up to 265 kg/100 trap hauls. Large male crabs, up to 267 mm carapace width and 4200 g in weight are caught, particularly inshore, during the spring. Larger crabs are found in deeper water and at the western end of the Channel. In the autumn the proportion of females in the catches increases, as does their mean size. Mean sizes have varied little in the period 1967 to 1974. Selection of the landed catch by fishermen is dependent upon market preferences, with average minimum selection size of about 150 mm; the legal minimum size is 115 mm carapace width. Carapace width-total weight relationship regression equations indicate allometric growth for males and isometric for females. Unlike stocks in the North Sea, where a well-defined moulting period exists, soft-shelled (recently moulted) crabs occurred throughout most of the year, though generally more were found in the summer. Development of the ovaries takes place from August to December, with up to 90% of the females, in November, having well-developed ovaries. Generally egg-bearing females were absent from the catches; a few were observed, mainly in the spring, with sizes ranging from 133 mm to 205 mm. Smaller females were found with plugs in the oviducts, indicating that mating had taken place, but they were not necessarily mature. The hepatopancreas/gonad meat yield of females varied seasonally, being related to moult condition and gonad development. Comparison between the population and catch structure of the English Channel and North Sea crab stocks shows considerable differences, with much higher catch rates and higher mean sizes in the Channel. Extensive migratory movements of mature female C. pagurus down the English Channel result in a higher proportion of females in the catches, compared with the eastern end of the Channel, and a higher mean size. This concentration in the autumn of female crabs with well developed gonads on the grounds off south Devon, is probably related to the breeding cycle, with the requirement for a suitable bottom substrate for spawning, and the distribution of the larval stages by residual currents.