DNA sequence polymorphisms contained in intron 6 of the lactate dehydrogenase-A (ldh-A) gene were used to examine the genetic population structure of Pacific swordfish (Xiphias gladius Linnaeus, 1758). Seven alleles defined by five polymorphic sites were identified among
305 swordfish. Comparisons of allele frequency were conducted for 11 samples, including Chile (multiple years), Ecuador (multiple years), Mexico, Hawaii (multiple years), eastern Australia, and western Australia. Although there was evidence of genic differentiation, global differentiation
was low (Fst = 0.001). To increase the power of the tests of differentiation, samples within each region were pooled into four regional samples. No deviations from hardy-weinberg equilibrium were observed, and the global fixation index increased more than ten-fold (Fst
= 0.013). Global exact tests of genic and genotypic differentiation were significant, and so were the pair-wise comparisons between the south-eastern Pacific Ocean (SEPO) sample from Chile, and all other regions. In addition, the north-eastern Pacific Ocean (NEPO; Ecuador to Mexico) was different
from the north-central Pacific Ocean (NCPO; Hawaii), which in turn was different from the south-western Pacific Ocean (SWPO; pooled eastern and western Australia). These results may have important implications for the fishery management of Pacific swordfish, particularly because of the heterogeneity
observed between SEPO and NEPO.
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