Origin of Atlantic bluefin tuna (Thunnus thynnus) in the Bay of Biscay

We used carbon and oxygen isotope ratios (delta C-13 and delta O-18) in otoliths as a tool for identifying the nursery origin of Atlantic bluefin tuna (Thunnus thynnus) caught in the eastern North Atlantic Ocean (Bay of Biscay). Juvenile and adult bluefin tuna were collected over three consecutive years (2009-2011) using the regional bait boat fleet. Otolith delta C-13 and delta O-18 values of bluefin tuna were measured by mass spectrometry, and values were compared with a reference sample of yearling bluefin tuna from eastern (Mediterranean Sea) and western (Gulf of Mexico) nurseries to determine nursery origin. Maximum likelihood estimates based on otolith delta C-13 and delta O-18 values indicated that the overall contribution of western migrants to the Bay of Biscay fishery was <1% and varied over the years assessed. A small number of potential western migrants (2.7%) was detected in 2009, and most of these fish appear members of the abundant 2002 and 2003 year classes. In contrast, the Bay of Biscay fishery was composed exclusively (100%) of eastern origin bluefin tuna in 2010 and 2011, suggesting that this fishery is supported almost exclusively by the eastern spawning area but transatlantic western population may contribute to this fishery in a few years.