Mitochondrial DNA control region sequences indicate extensive mixing of swordfish (Xiphias gladius) populations in the Atlantic Ocean

Pacific, Atlantic, and Mediterranean populations of the swordfish (Xiphias gladius) are currently considered as separate fisheries management units, and populations in different regions of the Atlantic are thought to constitute different stocks on the basis of recapture data. To test these hypotheses we sequenced hypervariable segments of the control region of mitochondrial DNA in 35 swordfish from three regions of the Atlantic, as well as in 8 and 7 individuals from the Pacific and Mediterranean regions, respectively. Sixty of the 81 variable sites were confined to a 280 base pair stretch in the left domain of the control region, indicating that this segment is a rich source of genetic markers. Thirty-three haplotypes were found that could be assigned to two clades differing by 3.8% on average, and that diverged approximately 550 000 years ago. Clade I haplotypes were ubiquitous, but haplotypes from clade II predominated in the Mediterranean, and thus likely originated there during Pleistocene marine regressions. Overall, we conclude that there is extensive mixing of swordfish within the Atlantic Ocean. Sister-group relationships of haplotypes from different oceans indicate historical gene flow between these populations, but the co-occurrence of the same haplotypes in different oceans must reflect recent or ongoing dispersal.