Understanding the population structure of a species is important to accurately assess its conservation status and manage the risk of local extinction. The Bull Shark (Carcharhinus leucas) faces varying levels of exploitation around the world due to its coastal distribution. Information regarding population connectivity is crucial to evaluate its conservation status and local fishing impacts. In this study, we sampled 922 putative Bull Sharks from 19 locations in the first global assessment of population structure of this cosmopolitan species. Using a recently developed DNA-capture approach (DArTcap), samples were genotyped for 3,400 nuclear markers. Additionally, full mitochondrial genomes of 384 Indo-Pacific samples were sequenced. Reproductive isolation was found between and across ocean basins (eastern Pacific, western Atlantic, eastern Atlantic, Indo-West Pacific) with distinct island populations in Japan and Fiji. Bull Sharks appear to maintain reproductive connectivity using shallow coastal waters as dispersal corridors, whereas large oceanic distances and historical land-bridges act as barriers. Females tend to return to the same area for reproduction, making them more susceptible to local threats and an important focus for management actions. Given these behaviours, the exploitation of Bull Sharks from insular populations, such as Japan and Fiji, may instigate local decline that cannot readily be replenished by immigration, which can in turn affect ecosystem dynamics and functions. These data also supported the development of a genetic panel to ascertain the population of origin, which will be useful in monitoring the trade of fisheries products and assessing population-level impacts of this harvest.

The blue shark Prionace glauca is a top predator with one of the widest geographic distributions of any shark species, yet classified as critically endangered in the Mediterranean Sea, and Near Threatened globally. Previous genetic studies did not reject the null hypothesis of a single global population across the worldwide species range. Blue shark situation was proposed as a possible archetype of the ‘grey zone of population differentiation’, coined to designate cases where population structure may be too recent or too faint to be detected using a limited set of markers. Here, blue shark samples collected throughout its global range were sequenced using a specific ddRAD method (DArTseq; Georges et al. 2018), which recovered 37,655 genome-wide single nucleotide polymorphisms (SNPs). Two main groups emerged, with Mediterranean Sea and Northern Atlantic samples significantly differentiated from the Indo-west Pacific samples. Significant pairwise FST values indicated further genetic differentiation within the Atlantic Ocean, and between the Atlantic Ocean and the Mediterranean Sea. Reconstruction of recent demographic history suggested the divergence between northern and southern oceanic populations emerged about 500 generations ago and revealed a drastic reduction in effective population size from a large ancestral population. Our results illustrate the power of high-density genome scans to detect population structure and reconstruct demographic history in highly migratory marine species. As the management of the blue shark fishery, either as target or as bycatch, does not account for this delineation, we strongly recommend that the results presented here be considered in future stock assessment and management plans.