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 and 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. displacement 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.

Summary1. Sharks and rays are vulnerable to fisheries exploitation because of late maturation and low fecundity, highlighting the need for effective conservation strategies. Area closures have been proposed as an appropriate management option for thornback rays in the southern North Sea, where they appear to form local subpopulations between which there is limited mixing. 2. To gain a fishery-independent estimation of stock distribution, 197 thornback rays Raja clavata tagged with electronic data storage tags (DST) were released in the Thames Estuary in 1999 and 2000. 3. The tidal location method was used to estimate the positions of individual fish between time of release and recapture. The fishery-independent seasonal stock distributions were integrated with landings data and a simple model was developed to estimate monthly fishing effort per International Council for the Exploration of the Sea (ICES) rectangle by different fleets and gear types. 4. The potential impacts of closed area management, in terms of reducing thornback ray mortality in the southern North Sea, were calculated. Spatial closures were applied either (i) as permanent closures of individual ICES rectangles or (ii) seasonally, at the level of the Thames Estuary. Catch reductions were calculated allowing for the redistribution of fishing effort. 5.The results confirmed the importance of the Thames Estuary for thornback rays. However, 77% of rays moved outside the estuary over winter, with seasonal migration into the Thames to spawn between March and August. 6. The effects of closures varied between areas and gear types. Permanent closures of individual ICES rectangles were less effective at reducing fishing mortality on rays than a spring or summer closure of the Thames Estuary as a whole, which would have a major impact on the commercially more valuable sole fishery. 7. Synthesis and applications . The results presented illustrate the potential impacts of a range of closure scenarios, prior to their implementation, as a basis for advice on sustainable exploitation of thornback rays. These models could be further refined by additional studies of juvenile behaviour and of other ray subpopulations in the southern North Sea.

Water currents are a dominant feature of the open sea and often play an important part in the life cycle of the fish that live there. For over three decades, a focus of research at Lowestoft has been the role of tidal water currents as a part of wider understanding of the population dynamics of plaice (Pleuronectes platessa) in the North Sea. Increasingly sophisticated telemetry methods have been developed and used, initially to describe the migratory behaviour of individual fish, and latterly to describe the spatial dynamics of whole populations. Here, the authors review cues and clues for environmental transport at different life stages, with a critical evaluation of the facultative use of selective tidal stream transport in relation to water current speed. The clues that trigger movement into mid-water by adult fish are reconsidered in light of new findings. Finally, the orientation capabilities of plaice, together with evidence of repeat migrations and spawning site fidelity were examined.

Tagged with an electronic data storage tag on 6 October 1999 in the Thames Estuary (UK), a thornback ray, Raja clavata, was recaptured after 504 days at liberty, 276 km north-west of release. The tag recorded 423 days of data. The migration was reconstructed using the tidal location method, and illustrates seasonal migration out of the Thames Estuary to the central southern North Sea during autumn and winter, then a return to the Thames to spawn during spring. This is the first direct evidence of repeated seasonal migration outside the area commonly thought of as the species home range, an observation with significant implications regarding the applicability of closure management as a conservation measure for this species