Population genomic and size distribution data suggest high genetic connectivity in the sandbar shark (Carcharhinus plumbeus) along a 2700 km coastline

Abstract

Understanding patterns of connectivity in exploited elasmobranchs is vital to their successful management and ongoing sustainability. In this study we use 1480 nuclear SNP loci to investigate fine-scale genetic connectivity in sandbar sharks (Carcharhinus plumbeus) along the Western Australian coastline into the Timor Sea with high statistical power. The genetic dataset was combined with latitudinal and depth size distribution data to increase our understanding of population structuring of this species along the continuous Western Australian coastline. Genetic analyses based on traditional and Bayesian approaches found no evidence of restrictions to nuclear gene flow in sandbar sharks between Albany on the south coast and the Timor Sea to the north. The size distributions reveal a size-segregated population, with juveniles predominantly found in deeper water in the south of the state, and adults predominantly in shallower northern waters. The combination of these genomic and size datasets indicates that sandbar sharks form a single population within Western Australia. Levels of genetic diversity, e.g. observed and expected heterozygosity and allelic richness, in this population were high, implying that the size of the population is relatively large. The results have important management implications; despite forming a single biological stock, the species’ size segregation highlights the importance of ongoing management to minimize the likelihood of growth overfishing in the south of the state. Also, the stock extends into the Timor Sea where unmanaged Indonesian fishing may impact sustainability in Western Australian waters.