Genomic applications in fish traceability and fishery stock management: phylogeography and population structure of the Mediterranean-Atlantic blue shark, Prionace glauca

The abundance of blue sharks, Prionace glauca, Linnaeus 1758 (BS), has dramatically declined over the last century in the Mediterranean Sea (a reduction of 75% over the last 30 years). In this study, the combination of a near-capillary sampling strategy of BS in several areas of the basin together with the development and use of genomic markers - that can represent both neutral and selective markers - offers the opportunity to assess and visualize for the first time for this poorly studied and vulnerable species population structure and potentially local adaptation. The phylogeography between Mediterranean Sea and North-Eastern Atlantic Ocean, inferred using mtDNA (control region, Cytb), highlighted no obvious haplotypic pattern of geographical differentiation, while Φst analyses indicated significant genetic structure among four geographical groups. Successively, spatial genetic differentiation of 203 Mediterranean and North-eastern Atlantic BS was estimated using 3,451 species specific SNP loci, developed using the ddRAD technology. Neutral SNPs differentiation was null or very low (pairwise Fst ranged from 0.2 to 0.5%) indicating a near or fully-panmictic population. A subpanel of potentially under-selection loci revealed significant higher Fst values (pairwise Fst ranging from 0.01 to 0.09), clustering analyses failed to evidence strong signals of differentiation among areas, probably due to the combination of migrants-mediated gene flow and the big population size. Based on these results, combined with the biology of this species, such the extreme vagility e complex population dynamics, it seems fair to suggest that the Mediterranean BS population could represent a meta-population, with spatially separated populations which interact through migrant specimens. This work has advanced knowledge on Mediterranean Blue Shark population biology and ecology relating ex-novo the Mediterranean BS population and the fishery stock to the Atlantic biological and management units, being the Mediterranean BS often not considered in the review of data and ecological relationships.