Publication Abstract

Seasonal migration of the thornback ray, Raja clavata L., and implications for closure management.

E. Hunter, A.A. Buckley, F. Berry, C. Stewart and J.D. Metcalfe

Thornback rays, Raja clavata L., are thought to form local sub-populations, between which few individuals are thought to transfer. This feature of the biology of thornback rays coupled with late maturation and low fecundity make these fish highly vulnerable to localised extinction due to fishery exploitation. A significant decline in the abundance of a range of commercially exploited elasmobranchs has further highlighted a growing need for sustainable management strategies. Although subject to a "skate and ray" total allowable catch, there are no additional conservation measures currently in operation in the North Sea aimed at protecting them. To determine seasonal movements and distribution patterns and investigate the potential impacts of closure management on this fishery, we used data from 75 mature thornback rays, tagged with electronic data storage tags (DSTs) and released in the Thames Estuary (United Kingdom). The fish were at liberty between 1999-2001, during which period we also collected commercial fishing catch data. Geo-spatial analytical techniques were applied to integrate data on the seasonal distribution of the fish with a spatial and temporal analysis of catch and fishing effort data. Data from the DST study were compared with a conventional mark-recapture analysis. The impact of closed areas in the Thames Estuary was considered firstly in terms of the likely conserving effect on existing ray stocks, and secondly, the potential benefits to the commercial fishery in terms of increased ray stock biomass and yield. Our results demonstrate that the extent of migration was approximately three times greater than that suggested by conventional tagging data alone. The distribution of catches and catch rates was such that no small area or time closure would return large reductions in fishing pressure. Furthermore, most significant closures would also have a large negative impact on co-occurring, commercially valuable fisheries, notably sole.

Reference:

E. Hunter, A.A. Buckley, F. Berry, C. Stewart and J.D. Metcalfe (2005) Seasonal migration of the thornback ray, Raja clavata L., and implications for closure management. 6th Fish Telemetry Conference held in Europe, 5-11 June 2005, Sesimbra, Portugal