Publication Abstract

Seasonal migration of thornback rays and implications for closure management

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

Previous studies of the thornback ray, Raja clavata L., have suggested that these form local sub-populations, between which little exchange of individuals is thought to occur. 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.

Here we describe the results of a study in which 197 thornback rays were tagged with electronic data storage tags and released in the Thames Estuary (United Kingdom). We used a method of geolocation based on tidal data recorded when rays remained on the sea-bed over a full tidal-cycle to reconstruct the movements of the fish throughout their liberty period.

Contrary to predictions based on conventional tagging experiments, our results have shown that rays were not restricted to the Thames Estuary, but moved more widely in the southern North Sea, with a seasonal pattern of migration.

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. The impact of closed areas in the Thames Estuary is considered in terms of the likely conserving effect on ray stocks and the potential benefits to the commercial fishery in terms of increased ray stock biomass and yield. We will also consider the impacts of predicted differences in distribution observed from conventional tagging and from electronic data storage tagging experiments.

Reference:

E. Hunter, F. Berry, A.A. Buckley, C. Stewart  and J.D. Metcalfe (2006) Seasonal migration of thornback rays and implications for closure management. Journal of Applied Ecology, 43(4): 710-720