Population structure of swordfish across the ICCAT/IOTC management boundary

South Africa is a member of the Indian Ocean Tuna Commission (IOTC) and International Commission for the Conservation of Atlantic Tuna (ICCAT), the two regional fisheries management organisations that are responsible for the management of large pelagic fishes in the Indian and the Atlantic oceans, respectively. The 20°E longitudinal line represents the artificial reporting and management boundary between these two organisations, but it remains uncertain if the artificial indeed reflects a biological meaningful separation of populations of large pelagic fishes. The broadbill swordfish Xiphias gladius is a circumglobally distributed apex predator in temperate pelagic waters and an important target of longline fisheries in all major oceans. Previous studies confirmed genetic differentiation between the Atlantic and Indian Ocean stocks but there is no agreement on the direction of gene flow and where, or indeed if, a population boundary exists. Eleven microsatellite loci were included in this study of the fine scale population structure of swordfish caught in South African waters. Despite the poor quality of old DNA samples, muscle material of 267 swordfish around the entire range of South Africa's coastline was utilised. A map of admixture proportions indicated a potential admixture zone between 14°E and 27°E. Gene flow and migration seem to occur in both directions, but weak differentiation suggests that the Indian Ocean and Atlantic Ocean contain separate stocks which return to their ocean of origin to reproduce. Due to passive drift of larvae and active dispersal of adults, swordfish would be prone to admixture and genetic homogenisation. The swordfish represents one of several species that occur in stocks not only straddling the 20°E reporting boundary, but with significant annual fluctuations in catch reporting resulting from slight changes in distribution across this boundary. Further studies need to be undertaken to ensure this phenomenon does not affect stock assessments of these species on either side of this boundary.