Migratory behaviour of the thornback ray Raja clavata, in the southern North Sea

Although depleted throughout the European continental shelf, the relatively high density of thornback rays Raja clavata, in the Thames Estuary (UK) makes it an important stock centre and potential focus for species management. To describe spatial and temporal distribution, 197 thornback rays were tagged with electronic data storage tags (DSTs) and released in the Thames Estuary in October 1999 and 2000, and 100 rays tagged with conventional tags in 2000. Fifty-one per cent of DSTs and 48% of conventional tags were returned. Fishery-independent estimates of position ('geolocations') between the time of release and recapture using the tidal location method were possible using 75 individual data records of between 31 adn 423 days. Ninety-six per cent of rays were recaptured within the Thames Estuary. The rays were located in water of 20-35 m depth during the autumn and winter, then migrated into shallower water (< 20 m depth) during the spring. Fishery-independent analysis of distribution demonstrated that the rays were more widely distributed in the southern North Sea during the autumn and winter. The range contracted in spring, when the fish moved into the inner Thames Estuary. No gross behaviour differences were observed between males and females. Displacements and dispersion coefficients calculated from geolocation data demonstrated clear annual cycles, indicative of migration. These movements were not apparent from the mark-recapture data, a fishery-related effect. The extent of migration as determined from experiments with DSTs was approximately three times greater than that suggested by conventional tagging data alone.