Global stock structure of the Silky shark (Carcharhinus falciformis, Carcharhinidae) resolved with high-throughput DNA sequencing

Silky sharks (Carcharhinus falciformis, Carcharhinidae) are currently the secondmost harvested shark species in the pelagic longline fisheries and populations are declining. Thisspecies is currently managed as a single stock per Regional Fisheries Management Organizationdue to a lack of data on stock delineations suggesting other options for population-levelmanagement. Genetic population structure can resolve dispersal boundaries and provide anappropriate foundation for defining distinct stocks, which is crucial for any successful wildlifemanagement. This study investigated the global stock structure of silky sharks using high-throughput sequencing of genomic data from pooled individuals (pool-seq) based on 628specimens collected by scientists on research charters, observers aboard commercial vessels, andfish markets across 11 circumglobal regions. Single nucleotide polymorphisms (SNPs) indicate significant population structure between all 11 regional samples. FST values calculated from mitochondrial (mtDNA) markers ranged from 0.012 - 0.160 and from 0.014 - 0.035 for nuclear (nDNA) markers within oceans,with much higher values between oceans (mtDNA: 0.383 - 0.844, nDNA: 0.042 - 0.078). Manypelagic sharks show population structure on the spatial scale of different ocean basins as seenhere, but these within-ocean results are unprecedented for a pelagic shark and challenge thesingle stock per RFMO management scheme currently in place. Here we report evidence formultiple stocks within the jurisdiction of each of the regional fisheries commissions. Theseresults argue that multiple stocks per RMFO need to be established to ensure these smaller stocksare not over exploited.