Population structure in greater amberjack, Seriola dumerili, from the Gulf of Mexico and the western Atlantic Ocean

Variation in restriction sites of mitochondrial (mt)DNA was examined from 444 greater amberjack (Seriola dumerili) sampled from 11 offshore localities in the northern Gulf of Mexico (Gulf) and along the U.S. southeast Atlantic coast (Atlantic). A total of 49 mtDNA haplotypes (genotypes) were detected. Percent nucleotide sequence divergence among the haplotypes ranged from 0.156 to 2.623 (mean plus or minus SE=0.980 plus or minus 0.015). Nucleon diversity within samples ranged from 0.845 to 0.906, and intrapopulational mtDNA diversities ranged (mean plus or minus SD) from 0.483 plus or minus 0.370 to 0.619 plus or minus 0.419. The latter did not differ significantly from one another. Homogeneity tests of mtDNA haplotype frequencies, F sub(ST) values, analysis of molecular variance (AMOVA), and comparisons of pairwise Phi sub(ST) distances were consistent with the hypothesis that (at least) two subpopulations (stocks) of greater amberjack exist in U.S. waters: one in the northern Gulf and one along the U.S. Atlantic coast. The latter subpopulation includes individuals from the Florida Keys. There was no evidence of phylogeographic structure among mtDNA haplotypes or among sample localities, suggesting either that restricted gene flow between the subpopulations is fairly recent or that gene flow between the two is relatively infrequent. No significant spatial autocorrelations of mtDNA haplotypes was found among samples of greater amberjack from the Gulf, indicating continuous gene flow across the northern Gulf. Long-term effective (female) population sizes of both subpopulations were estimated to be in the range of 90,000-95,000 individuals. The estimates were commensurate with estimates in other, economically important marine fish. Based on suggested differences in mtDNA evolutionary rates between homeothermic and poikilothermic vertebrates, the effective (female) population sizes of both stocks of greater amberjack could be in the range of 500,000 to 1,000,000 individuals.