Seasonal migration of thornback rays and implications for closure management

Abstract

1. 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. © 2006 The Authors.