Abstract
Shark species have undergone drastic population declines in recent decades due to overfishing and habitat destruction; thus, establishing connectivity among the populations of various shark species is important to determine the appropriate units and spatial scale for conservation and management, particularly as this group is long-lived with late age of maturation. Consequently, we used DNA variation at 1,317 putatively neutral and 25 potentially adaptive single nucleotide polymorphisms (SNPs) to analyze population genetic structure among 174 unrelated individuals of scalloped hammerhead sharks (Sphyrna lewini) from the Rewa Delta and the Ba Estuary, where documented aggregations of neonates and young-of-the-year occur in the island of Viti Levu, Republic of Fiji. Results of the pairwise FST analysis for the neutral loci revealed a small but significant genetic differentiation (FST: 0.004; P-value = 0.0009). Furthermore, the 25 potentially adaptive loci (i.e., under putative selection) revealed a magnitude of differentiation four times bigger than the estimate obtained using neutral genetic diversity (FST: 0.017; P-value = 0.0009). Interestingly, population assignment tests, using the neutral SNP data set and two different software packages, Admixture and assignPOP, provided evidence for the existence of up to four genetically differentiated populations among our samples. Assignment probabilities ranged from 0.98 ± 0.01 to 0.81 ± 0.03. Admixture and assignPOP assigned the same individuals to the same putative populations for all sampled neonates. Thus, our results provide unequivocal evidence that adult females from multiple genetically differentiated breeding populations contribute to these juvenile aggregation sites.
Highlights
The conservation of elasmobranchs is essential as many shark species have undergone drastic population declines and are some of the most threatened fishes worldwide (Dulvy et al, 2017)
Using 69,164 single nucleotide polymorphisms (SNPs), we identified 1,871 unlinked loci following quality-filtering controls
After determining the cut-off values for full siblings (FS), 83 dyads including 88 individuals were detected for Rewa (Marie et al, 2019), while only 13 dyads and 18 individuals were detected for Ba, and no dyads involving individuals from each of the rivers was detected (Table 1)
Summary
The conservation of elasmobranchs is essential as many shark species have undergone drastic population declines and are some of the most threatened fishes worldwide (Dulvy et al, 2017). The scalloped hammerhead shark, Sphyrna lewini (Griffith, 1834), is a viviparous high trophic level predator, circumglobally distributed in tropical and warm temperate waters over both, neritic and adjacent pelagic waters, with fecundities ranging from 13 to 31 neonates of 42–55 cm total length per litter after a gestation period of 9–10 months (Compagno, 1984). In the Eastern Pacific Ocean, males, juvenile and neonate scalloped hammerhead sharks are uncommon at oceanic aggregations. Several scalloped hammerhead shark populations have been heavily exploited worldwide by both inshore and offshore fisheries and the species is considered endangered by the International Union for Conservation of Nature (IUCN) Red List (Nalesso et al, 2019 and references therein)
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