Population structure of istiophorid billfishes

The phylogeny of istiophorid billfishes was recently revised based on analyses of nuclear and mitochondrial DNA loci, and a variety of molecular genetic characters has been used to investigate population structuring within several species of billfish. Despite these efforts, the population structure of most istiophorid billfishes is not well understood. This paper reviews genetic insights into the phylogeny and population structure of the billfishes with an emphasis on recent studies. In general, the results of genetic studies indicate significant heterogeneity between Atlantic and Indo-Pacific samples of circumtropical species. Within ocean basins, levels of population structure range among the species from no statistically significant heterogeneity detected among samples to small, but statistically significant genetic divergence between geographically distant samples. The apparent high level of genetic connectivity within oceans for some species seems to contrast with inferences derived from studies employing conventional and satellite tags that suggest limited trans-equatorial or trans-oceanic movements. This disparity may result, in part, from the different time scales underpinning evolutionary and ecological population structure; however, the power of previous genetic studies to elucidate population structuring has been limited by opportunistic sampling. To effectively assess population structure within the highly migratory billfishes, future genetic research will not only require larger sample sizes, both in terms of individuals and molecular markers, but also the development of biologically meaningful sampling designs that incorporate information on the movement patterns and life histories of these pelagic fishes.