GENETIC ANALYSES OF ATLANTIC NORTHERN BLUEFIN TUNA CAPTURED IN THE NORTHWEST ATLANTIC OCEAN AND THE MEDITERRANEAN SEA

tuna, Thunnus thynnus thynnus, reproduces as a panmictic unit. To address this question, we examined both mitochondrial DNA control region nucleotide sequences and nuclear gene ldhA allele frequencies in replicate samples of northern bluefin tuna from the Mediterranean Sea and the northwestern Atlantic Ocean. AMOVA analyses of both types of data revealed no significant differences between samples from the two regions. These results demonstrate the importance of analyzing multiple year classes and large sample sizes in stock structure analyses. In addition, larval samples from the Gulf of Mexico and the Mediterranean Sea were not significantly different from each other or from the other samples when control region sequences were compared. However, despite the strong evidence presented here, failure to find genetic evidence for population substructure does not constitute evidence for a single panmictic population. It is possible that multiple subpopulations do exist, and that genetic differentiation at the loci analyzed in this study has not occurred because of large population sizes and/or low levels of reproductively successful migration between the subpopulations.