Regional population genetics and global phylogeography of the endangered highly migratory shark Lamna nasus: Implications for fishery management and conservation

1. The porbeagle sharkLamna nasusis a large highly migratory shark distributed in cold and temperate marine waters of the North Atlantic and Southern Hemisphere (SH). According to the International Union for Conservation of Nature, the porbeagle is assessed globally as Vulnerable and regionally as Critically Endangered in the North Atlantic and the Mediterranean Sea. This study explored, for the first time, the population genetics ofL. nasusat a regional (south-east Pacific Ocean) and global scale. 2. In this study, the null hypotheses of no genetic discontinuities among populations (i) within the SH, and (ii) between the SH and Northern Hemisphere (NH) were tested. Also, the demographic history ofL. nasusin different ocean basins was assessed. Two mitochondrial markers (Control Region [CR] andcox1) well suited for population genetics inferences in sharks were used. 3. Spatial-genetic analyses suggested two genetic clusters co-occurring in the south-eastern Pacific Ocean. A two-way ANOVA using thecox1but not the CR mtDNA fragment detected an effect of genetic identity on shark body size. Phylogeographic analyses, haplotype networks, and analyses of molecular variance demonstrated genetic differences between populations from the NH and SH but not among populations in the SH. 4. Migration estimates indicated limited current maternal gene flow between the two hemispheres but high gene flow within hemispheres. Two well-defined haplotype groups with star-like shapes inhabited all ocean basins in the SH. These results could reflect a historical scenario of reproductive isolation and more recent mixture among previously isolated populations in the SH. A Bayesian skyline plot analysis indicated sudden population expansion in the SH occurring similar to 100-125 kya. 5. This study highlights the need for additional studies focusing on the population genomics (using nuclear markers, i.e. single nucleotide polymorphisms) and the general biology ofL. nasusto explore the existence of genetically dissimilar populations in the SH. Such studies will help implementing efficient genetic monitoring programmes.

Automated summary

This study examined the population genetics of the porbeagle shark at regional (south-east Pacific Ocean) and global scales using mitochondrial markers. Genetic analyses revealed two clusters in the south-eastern Pacific Ocean, with an effect of genetic identity on shark body size. Limited gene flow was found between hemispheres, but high gene flow within hemispheres, suggesting historical reproductive isolation and recent mixture among populations. The study emphasizes the importance of further research on population genomics and general biology of the porbeagle shark in order to implement efficient genetic monitoring programs.