Stock structure of the world's istiophorid billfishes: a genetic perspective

Istiophorid billfishes are highly migratory species that inhabit the tropical and subtropical, epipelagic waters of the world's oceans, a large, relatively homogeneous environment that lacks significant physical barriers. Based on these observations alone, one would not expect marlins, sailfish and spearfishes to exhibit substantial stock structure. This assumption has been evaluated with a variety of techniques, including analyses of morphological characters, adult distribution, tag and recapture data, the spatial and temporal distribution of spawning and, recently, molecular genetic characters. This paper focuses on inferences of istiophorid billfish stock structure derived from investigation of several different classes of molecular markers, and reviews our current understanding of the genetic basis of stock structure of striped marlin, white marlin, blue marlin, sailfish and black marlin. Significant genetic differences exist between Atlantic and Indo-Pacific populations of blue marlin and sailfish, and the presence of distinct mitochondrial DNA lineages suggests that ocean populations were isolated in the past. However, the occurrence of identical genotypes in both oceans is evidence of recent genetic contact. The genetic data do not support recognition of separate Atlantic and Indo-Pacific species of blue marlin or sailfish. White and striped marlin are separated by about the same level of genetic divergence as Atlantic and Indo-Pacific populations of blue marlin and sailfish, but preliminary analysis of the mitochondrial DNA control region suggests that, unlike Atlantic and Indo-Pacific populations of blue marlin and sailfish, white marlin and striped marlin represent independent evolutionary units. If white and striped marlin are valid species, they are of very recent origin. Significant intraspecific genetic heterogeneity was found among collections of striped marlin and sailfish within the Indo-Pacific; both species exhibited a clear spatial partitioning of genetic variation among geographically distant collection locations. There was no genetic evidence for within-ocean population structuring for other istiophorids examined. Inferences of billfish stock structure derived from studies of molecular markers complement those obtained using other methods of analysis, and together these studies demonstrate substantial differences in the level of population structuring among istiophorid billfishes, information critical for effective management of these highly migratory species.