Historical demography and genetic differentiation inferred from the mitochondrial DNA of the silky shark (Carcharhinus falciformis) in the Pacific Ocean

The population genetic structure of the silky shark Carcharhinus falciformis in the Pacific Ocean wasstudied by analyzing 14 haplotypes identified by sequencing a 732 bp fragment of the mitochondrialDNA control region from 353 silky sharks. The mean haplotype (0.48 ± 0.03) and nucleotide diversity(0.0009 ± 0.00008) of the silky shark population were lower than in other shark species. The AMOVAanalysis comparing Western and Eastern Pacific regions showed a low, but significant, level of varianceassociated with differences between groups (ΦCT= 0.0199, P = 0.008). Here, we present the first populationgenetic study of the silky shark in the Pacific and provide evidence of the existence of distinct Eastern andWestern Pacific populations. Despite the significance found on the population subdivision level, it was notpossible to definitively reject the hypothesis of panmixia due to the small differences registered as a resultof the low levels of mtDNA genetic variation. The population history of C. falciformis was examined usingneutrality tests, mismatch distributions and coalescence analyses based on Bayesian approximations. Theresults were consistent with a population expansion occurring approximately 12,000-40,000 years ago. The results from IMa showed that the time of divergence was approximately 170,161 years ago, and thatin terms of coalescence, the migration that supported the population expansion occurred from west toeast across the Pacific. Similarly, the effective population size estimates were consistent with a reductionin the silky shark populations when compared to the ancestral levels. Based on our results and on thepopulation exploitation levels, we suggest evaluating the adoption of a two-stock management strategyto sustain the long-term use of this resource.