Electronic tagging and population structure of Atlantic bluefin tuna

Bluefin tuna in decline The Atlantic bluefin tuna is at the centre of an international debate in fisheries conservation. Last summer the western Atlantic bluefin tuna fishery collapsed and some conservationists say it will not recover unless the International Commission for Conservation of Atlantic Tunas adopts a tougher regime. Results from an electronic tagging programme on bluefin tuna are presented this week, and they make alarming reading. Western tagged bluefin tuna are shown to migrate freely across the international stock boundary into the eastern Atlantic, where they are vulnerable to European fisheries; and both known spawning grounds, in the Gulf of Mexico and the Mediterranean Sea, are shown to be linked to the endangered western Atlantic fishery. Electronic tags that archive or transmit stored data to satellites have advanced the mapping of habitats used by highly migratory fish in pelagic ecosystems 1 , 2 , 3 , 4 , 5 , 6 . Here we report on the electronic tagging of 772 Atlantic bluefin tuna in the western Atlantic Ocean in an effort to identify population structure. Reporting electronic tags provided accurate location data 7 , 8 , 9 that show the extensive migrations of individual fish ( n = 330). Geoposition data delineate two populations, one using spawning grounds in the Gulf of Mexico and another from the Mediterranean Sea. Transatlantic movements of western-tagged bluefin tuna reveal site fidelity to known spawning areas in the Mediterranean Sea. Bluefin tuna that occupy western spawning grounds move to central and eastern Atlantic foraging grounds. Our results are consistent with two populations of bluefin tuna with distinct spawning areas that overlap on North Atlantic foraging grounds. Electronic tagging locations, when combined with US pelagic longline observer and logbook catch data, identify hot spots for spawning bluefin tuna in the northern slope waters of the Gulf of Mexico. Restrictions on the time and area where longlining occurs would reduce incidental catch mortalities on western spawning grounds.