A spatially structured tagging model to estimate movement and fishing mortality rates for the blue shark (Prionace glauca) in the North Atlantic Ocean

Large numbers of blue sharks are caught as bycatch, and have even become the target species in pelagic longline fisheries in the North Atlantic Ocean. The status of the stock is ambiguous due to the limitations of the fishery-dependent data. This study presents a spatially structured tagging model to estimate blue shark movement and fishing mortality rates in the North Atlantic Ocean. The model uses the blue shark tag-recovery data collected by the United States National Marine Fisheries Service Cooperative Shark Tagging Program (1965-2004). Four major geographical regions (two on each side of the ocean) are assumed. The blue shark fishing mortality rates (F) were found to be heterogeneous across the four regions. While the estimates of F obtained for the western North Atlantic Ocean were historically lower than 0.1 year(-1), the F estimates over the most recent decade (1990s) in the eastern side of the ocean are rapidly approaching 0.2 year(-1). Because of the particular life-history of the blue shark, these results suggest careful monitoring of the fishery as the juvenile and pregnant female segments of the stock are highly vulnerable to exploitation in the eastern North Atlantic Ocean.