Mixed stock origin of Atlantic bluefin tuna in the U.S. rod and reel fishery (Gulf of Maine) and implications for fisheries management

Abstract

The highly migratory Atlantic bluefin tuna (Thunnus thynnus) has a distribution that spans the North Atlantic and two distinct spawning populations, an eastern population originating in the Mediterranean Sea and a western population originating in the Gulf of Mexico. Atlantic bluefin tuna are managed as two separate management units (east and west) in the North Atlantic, despite observed mixing that occurs across the management boundary. Characterizing the effects of stock mixing has been identified as a priority for improving the management of Atlantic bluefin tuna. Identifying the stock composition of landings from the Gulf of Maine is of particular importance, because approximately 70 % of the U.S. western Atlantic total allowable catch is removed from this region annually. The aim of our research was to apply otolith chemistry techniques to characterize the origin of bluefin tuna caught in the U.S. rod and reel fishery in the Gulf of Maine and to demonstrate how this information can be applied in fisheries management. Prior research established otolith stable isotope chemistry (δ13C and δ18O) as an effective and reliable stock identification tool, and we applied this approach to determine the population of origin of bluefin tuna collected from fishery dependent sampling (recreational and commercial) in the Gulf of Maine. Results indicated that the majority of fish caught in the Gulf of Maine from 2010 to 2013 were eastern origin. We found the highest proportion of eastern origin fish were caught in 2012 and the proportion of eastern origin fish was greater in late summer to fall. Although the majority of fish in small and intermediate size classes were eastern origin, fish in the largest size class (>250 cm) were predominantly western origin. Using these data, we demonstrated an approach for integrating mixed stock composition information into fishery-specific harvest data (U.S. rod and reel catch, catch-at-age, and catch-per-unit-effort). This information can be used to monitor mixed stock composition of the fishery, partition catch to population of origin, and to inform management decisions aimed at controlling population of origin harvest.