Population Genetic Divergence Of Bonnethead Sharkssphyrna Tiburoin The Western North Atlantic: Implications For Conservation

Sharks are a priority for conservation because numerous species, including small-sized coastal species, are being heavily exploited by commercial and recreational fisheries. Understanding the genetic population structure of sharks is key to effective management, maximizing their evolutionary potential in a rapidly changing environment and preventing population declines. Limited dispersal is an important factor promoting population divergence for several coastal shark species. The genetic variation in 14 microsatellite loci and 21,006 single nucleotide polymorphisms genotyped using restriction-site-associated DNA sequencing was analysed to assess the genetic structure of the bonnethead shark,Sphyrna tiburo, in the western North Atlantic. Genetic differences were identified among three well-defined regions: the western Florida coast, the south-eastern US Atlantic coast, and the southern Gulf of Mexico. Results support previous studies based on mitochondrial DNA sequences in defining differences among these regions and suggest limitations of bonnethead sharks in routinely performing long-distance migrations. The limited connectivity among regions explains the pattern of genetic divergence but also reported divergence at the species/subspecies level. These genetically discrete bonnethead populations have independent evolutionary histories that may include local adaptations to specific areas. Bonnethead sharks are currently managed as two stocks in the USA owing to recent genetic, tagging, and life history studies; however, no stock assessments or management plans exist for Mexico. These results not only serve to reinforce US management strategies and provide critical data about the extent of gene flow and sex dispersal among populations, but also begin the process of effective management in the waters of Mexico to ensure the long-term productivity and resilience of this species. With an absence of gene flow between populations from US waters and the southern Gulf of Mexico, there is a need for management plans based upon independent biological and population dynamics data since limited or no opportunities for populations to interchange individuals may occur to re-establish population viability.