Electronic tags that archive or transmit stored data to satellites have advanced the mapping of habitats used by highly migratory fish in pelagic ecosystems. 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 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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Electronic tags have enhanced our understanding of the movements and behavior of pelagic animals by providing position information from the Argos system satellites or by geolocation estimates using light levels and/or sea surface temperatures (SSTs). The ability to geolocate animals that remain submerged is of great value to fisheries management, but the accuracy of these geolocation estimates has to be validated on free-swimming animals. In this paper, we report double-tagging experiments on free-swimming salmon sharks Lamna ditropis and blue sharks Prionace glauca, tagged with satellite telemetry and pop-up satellite tags, which provide a direct comparison between Argos positions and geolocation estimates derived from light levels and SSTs. In addition, the Argosbased pop-up satellite tag endpoints and GPS-based recapture locations of Atlantic bluefin tunas Thunnus thynnus were compared with the last geolocation estimates from pop-up satellite and archival tags. In the double-tagging experiments, the root mean square errors of the light level longitude estimates were 0.89 and 0.55°; while for SST latitude estimates, the root mean square errors were 1.47 and 1.16°for salmon sharks and blue sharks respectively. Geolocation estimates of Atlantic bluefin tuna, using archival data from surgically implanted archival tags or recovered pop-up satellite tags, had root mean square errors of 0.78 and 0.90°for light level longitude and SST latitude estimates, respectively. Using data transmitted by pop-up satellite tags deployed on Atlantic bluefin tunas, the light level longitude and SST latitude estimates had root mean square errors of 1.30 and 1.89°, respectively. In addition, a series of computer simulations were performed to examine which variables were most likely to influence the accuracy of SST latitude estimates. The simulations indicated that the difference between the SST measured by the electronic tag and the remotely sensed SST at a given location was the predominant influence on the accuracy of SST latitude estimates. These results demonstrate that tag-measured SSTs can be used in conjunction with light level data to significantly improve the geolocation estimates from electronic tags.

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