1. Tuna Research and Conservation Center, Stanford University, Hopkins Marine Station, Pacific Grove, California 93950, USA
2. Monterey Bay Aquarium, 886 Cannery Row, Monterey, California 93940, USA
3. Dalhousie University, Biology Department, Halifax, Nova Scotia, B3H 4J1 Canada
4. *These authors contributed equally to this work

Correspondence to: Barbara A. Block¹ Correspondence and requests for materials should be addressed to B.A.B. (Email: bblock@stanford.edu).

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.

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