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.

The deployment of electronic data storage tags that are surgically implanted or satellite-linked provides marine researchers with new ways to examine the movements, environmental preferences, and physiology of pelagic vertebrates. We report the results obtained from tagging of Atlantic bluefin tuna with implantable archival and pop-up satellite archival tags. The electronic tagging data provide insights into the seasonal movements and environmental preferences of this species. Bluefin tuna dive to depths of >1000 meters and maintain a warm body temperature. Western-tagged bluefin tuna make trans-Atlantic migrations and they frequent spawning grounds in the Gulf of Mexico and eastern Mediterranean. These data are critical for the future management and conservation of bluefin tuna in the Atlantic.

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Electronic tags were used to examine the seasonal movements, aggregations and diving behaviors of Atlantic bluefin tuna (Thunnus thynnus) to better understand their migration ecology and oceanic habitat utilization. Implantable archival tags (n = 561) were deployed in bluefin tuna from 1996 to 2005 and 106 tags were recovered. Movement paths of the fish were reconstructed using light level and sea-surface-temperature-based geolocation estimates. To quantify habitat utilization we employed a weighted kernel estimation technique that removed the biases of deployment location and track length. Throughout the North Atlantic, high residence times (167±33 days) were identified in four spatially confined regions on a seasonal scale. Within each region, bluefin tuna experienced distinct temperature regimes and displayed different diving behaviors. The mean diving depths within the high-use areas were significantly shallower and the dive frequency and the variance in internal temperature significantly higher than during transit movements between the high-use areas. Residence time in the more northern latitude high-use areas was significantly correlated with levels of primary productivity. The regions of aggregation are associated with areas of abundant prey and potentially represent critical foraging habitats that have seasonally abundant prey. Throughout the North Atlantic mean diving depth was significantly correlated with the depth of the thermocline, and dive behavior changed in relation to the stratification of the water column. In this study, with numerous multi-year tracks, there appear to be repeatable patterns of clear aggregation areas that potentially are changing with environmental conditions. The high concentrations of bluefin tuna in predictable locations indicate that Atlantic bluefin tuna are vulnerable to concentrated fishing efforts in the regions of foraging aggregations.

The movements of Atlantic bluefin tuna (Thunnus thynnus thynnus) have captured the interest of scientists and fishers since the time of Aristotle. This tuna is unique among bony fish for maintaining elevated body temperatures (21°C above ambient) and attaining large size (up to 750 kg). We describe here the use of a pop-off satellite tag, for investigating the Atlantic-wide movements and potential stock overlap of western and eastern Atlantic bluefin tuna. The tag also archives data on water temperatures. The objectives of this study were to evaluate the effectiveness of the technology, study the movements of Atlantic bluefin tuna, examine their thermal niche, and assess survivorship of tagged fish. The pop-off satellite technology provides data independent of commercial fisheries that, when deployed in sufficient quantity, should permit a critical test of the stock structure hypotheses for Atlantic bluefin tuna.Atlantic bluefin tuna are endothermic fish (1), seasonally distributed over the north Atlantic Ocean and Mediterranean Sea, where they are exploited intensively by commercial and recreational fisheries. Despite international management, exploitation has reached a peak in the past decade (2) and the sustainability of the fishery is in question. Bluefin tuna in the Atlantic and Mediterranean are managed as separate eastern and western stocks. Considerable debate centers on the appropriateness of this subdivision given the evidence for transoceanic mixing (3). To understand the life history of bluefin tuna and develop competent management strategies, temporal and spatial movement patterns must be identified (4). Data on bluefin tuna dispersal patterns have been difficult to obtain because of the limited resolution of analytical tools available for studying pelagic fish. To date, tag and recapture programs using conventional tags (fisheries-dependent indicators of movement) have been the method of choice for describing the distribution of Atlantic bluefin tuna, as in most tuna fisheries (5). Conventional tag and release data indicate that transAtlantic movements of all size classes of Atlantic bluefin tuna occur (3, 6-8). Recently, microprocessor-based, data storage tags (archival tags) have been developed for monitoring the geoposition (based on ambient light levels), thermal physiology, and diving behavior of large pelagic vertebrates (9-13). Although the data intensity of archival tags is high (2 megabytes), their major limitation is the need to recapture the animal to access the data. This requires deployment of large numbers of tags in species with high exploitation rates. In addition, the multinational nature of most oceanic fisheries complicates the coordination of archival tag recoveries. Archival tags have been deployed recently on Atlantic bluefin tuna (9), but significant numbers of returns take years to retrieve. Satellite tags (conventionally towed or attached) have been employed to study the large-scale movements and physiology of marine mammals, birds, and sea turtles (14-18). These ta...