Spatial genetic subdivision among populations of the highly migratory black marlin Istiompax indica within the central Indo-Pacific

The black marlin Istiompax indica is a highly migratory species and as a result is expected to show little genetic population structure throughout its broad geographic range. Tissue samples from 183 I. indica were collected from three geographic regions within the central Indo-Pacific and analysed using mitochondrial and nuclear DNA markers. Nuclear genetic heterogeneity was found among populations in the south-western Pacific Ocean, eastern Indian Ocean and South China Sea (significant F-ST values of 0.013-0.037). Combining information from nuclear markers with published movement and reproductive data suggests that reproductive philopatry plays a role in maintaining contemporary I. indica population structure. Analyses of the mitochondrial control region did not reflect this pattern; however, it identified historical population structure. Differing patterns of genetic population structure revealed by mitochondrial and nuclear markers demonstrated that a transition must have occurred between historical and contemporary population structures. This restructuring presumably reflects a species whose populations have become genetically isolated before experiencing a period of secondary contact. The spatial subdivision evident among populations indicates that I. indica in this central Indo-Pacific region should be managed as three independent stocks, to guide the sustainability of this fisheries resource