Genetic identity of YOY bluefin tuna from the eastern and western Atlantic spawning areas

We used 320 young-of-the-year (YOY) specimens of the highly migratory and overfished Atlantic bluefin tuna, Thunnus thynnus, Linnaeus 1758, to evaluate the hypothesis that Atlantic bluefin tuna comprises 2 stocks with spawning grounds in the Gulf of Mexico and in the Mediterranean Sea. Significant genetic differentiation at 8 nuclear microsatellite loci (FST 5 0.0059, P 5 0.0005) and at the mitochondrial control region (UST 5 0.0129, P 5 0.0139) was detected among YOY Atlantic bluefin tuna captured on spawning grounds in the Gulf of Mexico (n 5 40) versus the western (n 5 255) and eastern (n 5 25) basins of the Mediterranean Sea. The genetic divergence among spawning populations, combined with the extensive trans-Atlantic movements reported for juvenile and adult Atlantic bluefin tuna, indicates a high degree of spawn-ing site fidelity. Recognition of genetically distinct populations necessitates independent management of Atlantic bluefin tuna on spawning grounds and warrants evaluation of the level of mixing of populations on feeding grounds. The genetic pattern might not have been detected unless juvenile specimens or actively spawning adults had been sampled. The Atlantic bluefin tuna, Thunnus thynnus, Linnaeus 1758, is a highly migratory species that supports a fishery throughout the North Atlantic Ocean and Mediterranean Sea. Increas-ing catches over the last several decades have resulted in severe overfishing and a rapid decline in abundance (ICCAT 2003). The Atlantic bluefin tuna is now considered to be one of the most depleted tuna species (Safina 1993; Magnuson et al. 1994) and was listed as endangered at the 8th Conventio