Crab (Cancer pagurus) migrations in the English Channel

Abstract

The development of the persistent suture tag has permitted the study of long-term migrations of large decapods like the European edible crab (Cancer pagurus L.). A tagging study of the growth and migrations of C. pagurus was part of a research project undertaken to provide advice on the effective management of the English Channel crab fishery. During the years 1968–74 a total of 15296 suture-tagged crabs was released; the majority in the main fishery off south Devon and Cornwall, but releases were also made in the eastern half of the Channel. The early releases were mainly inshore, within 22 km (12 nautical miles), but as the fishery expanded offshore releases were made in mid-Channel.Most of the recaptures occurred within two years of release, with the highest recapture rates being in the most intensively fished areas during the main fishing season. From the total of 2714 crabs recaptured, 2255 had reliable recapture information. In the inshore release areas 90 % of the males and 76 % of the females were recaptured within 18 km (10 nautical miles) of their release position. Offshore the corresponding proportions were considerably lower at 76 % and 59%, with 11 % of the males and 21 % of the females having moved > 90 km (49 nautical miles).When males did move > 18 km they were mainly the larger males and their mean displacements were 14–31 km in several directions. The offshore releases of males in the middle and eastern Channel showed more extensive movements, which appeared to be directed, most of them being in a westerly direction, with mean displacements of 41–114 km. With only one exception, the female inshore releases had a mean direction of movement of between south and west, with mean displacements of 12–84 km. Overall, those females making the more extensive movements were larger than those caught in the vicinity of the release positions. The last inshore release at Portland Bill gave the first indication of extensive westerly movements from the eastern part of the English Channel.Movements of females released on the offshore Devon and Cornwall grounds were mainly in a westwards direction, although several inshore movements and longer distance movements of up to 302 km towards the French coast were recorded. The recaptured females from the middle and eastern Channel releases showed greater mean displacements (110–158 km), all in a westerly direction. Only 10% of the mid-Channel recaptures were made within 18 km, with 73% being caught > 90 km from their release position. The highest mean velocity was 23 km/week, with an individual maximum of 183 km/week.Most of the recaptures of male C. pagurus released inshore were made near the release position, and those movements which were observed would appear to be the result of nomadism. Offshore males seemed to be more mobile, but a nomadic existence for larger male C. pagurus seems more likely than migration. Female C. pagurus in the English Channel are more mobile than males. Large, mature, female crabs leave the eastern Channel and congregate in the autumn off south Devon. During the winter and spring there may be some nomadic movements. These migratory movements are reflected in the size composition and sex ratio. The migrations seem to be related to the breeding behaviour, ensuring a suitable sea-bed substrate for berried crabs and/or allowing distribution of the larval phase on the easterly residual currents in the English Channel.