Partitioning genetic variation in swordfish (Xiphias gladius L.): Analysis of sample variance and population structure

Swordfish are highly mobile pelagic fishes of significant economic importance. The global distribution of swordfish makes the management of this species particularly challenging due to multinational exploitation. Studies examining the population structure of swordfish (Xiphias gladius) using mitochondrial DNA (mt) have revealed a significant amount of genetic differentiation between ocean basins despite. However, the maternal mode of inheritance of mtDNA reflects solely the history of maternal lineages and offers no insight into males reproductive behavior.; If the observed differences in mtDNA truly reflect limited gene flow between oceans basins then heterogeneity in allele frequencies at nuclear loci should also be detected. Therefore, investigations using polymorphic nuclear intron markers were made to test the null hypothesis of population homogeneity to the alternative hypothesis of heterogeneity in nuclear allele frequencies across ocean basins. Genetic variation was partitioned to examine the extent differences in sample collection (fishing trips) contributed to the overall observed regional genetic variation. Similarly, within region yearly samples were tested for temporal stability and the proportion of the total genetic variation assigned to this component was examined. Finally, partitioning of genetic variation between regions was examined to test for both concordance in mtDNA and nuclear markers and the genetic homogeneity of regional samples.; No significant differences were observed for any of the within region comparisons of trips or yearly samples. However, a small but significant portion of the genetic variation observed was attributable to differences among regions. In addition both nuclear and mtDNA markers revealed similar patterns of genetic differentiation. Therefore, the null hypothesis of homogeneity of nuclear allele frequencies was rejected. These data provide strong evidence for population differentiation in this highly mobile pelagic species.; Finally, population growth rates were assessed under two different management scenarios to examine the effectiveness of a size regulation on the north Atlantic swordfish fishery. Simulated populations of swordfish were exposed to varying fishing pressure using a Leslie matrix. The analyses indicate population growth rate decreases with size regulations in place. Undersize fish have the potential for contributing to a large percentage (25%) of biomass harvested. These results suggest that in a fishery where hook mortality is high (<80%) and gear selectivity is low, size regulations may have an opposite effect than the one intended. Area closures may positively affect population growth rates, however the decrease in mortality of undersized swordfish would have to be substantial (estimated here to be 1/5 the actual fishing pressure). A combined approach of eliminating size regulations and implementing area closures may help to increase swordfish population growth rates.