With recent advances in sequencing technology, genomic data are changing how important conservation management decisions are made. Applications such as Close-Kin Mark-Recapture demand large amounts of data to estimate population size and structure, and their full potential can only be realised through ongoing improvements in genotyping strategies. Here we introduce DArTcap, a cost-efficient method that combines DArTseq and sequence capture, and illustrate its use in a high resolution population analysis of Glyphis garricki, a rare, poorly known and threatened euryhaline shark. Clustering analyses and spatial distribution of kin pairs from four different regions across northern Australia and one in Papua New Guinea, representing its entire known range, revealed that each region hosts at least one distinct population. Further structuring is likely within Van Diemen Gulf, the region that included the most rivers sampled, suggesting additional population structuring would be found if other rivers were sampled. Coalescent analyses and spatially explicit modelling suggest that G. garricki experienced a recent range expansion during the opening of the Gulf of Carpentaria following the conclusion of the Last Glacial Maximum. The low migration rates between neighbouring populations of a species that is found only in restricted coastal and riverine habitats show the importance of managing each population separately, including careful monitoring of local and remote anthropogenic activities that may affect their environments. Overall we demonstrated how a carefully chosen SNP panel combined with DArTcap can provide highly accurate kinship inference and also support population structure and historical demography analyses, therefore maximising cost-effectiveness.
Many shark species are at risk of overexploitation due to their high economic value, slow maturation, and low recruitment compared to most teleosts. However, there is insufficient knowledge about population structure at different spatial scales necessary to optimise fisheries models. We used single-nucleotide polymorphisms (SNPs) obtained through complexity-reduction genome sequencing to quantify the population structure of two highly mobile and commercially fished shark species: bronze whalers (Carcharhinus brachyurus) and dusky sharks (C. obscurus). We applied a comprehensive approach to test several population-structure hypotheses and signal consistency across methods and marker type. We found that C. obscurus was panmictic across Australia and Indonesia and across the Indian Ocean to South Africa based on neutral loci, whereas for C. brachyurus, the westernmost Australian samples appeared to be separate from the rest. The southernmost east Australian samples indicated some difference from the rest of Australia and New Zealand based on candidate loci for C. brachyurus, and potentially also C. obscurus; however, the lack of a reference genome makes the interpretation difficult. Despite similar patterns in both species, subtle and potentially important structure differences emphasise the importance of studying each target species independently rather than assuming similar patterns from closely related species with similar dispersal abilities, as well as considering different marker types in future studies. We found evidence of connectivity across the regions sampled, suggesting that the cumulative effects of regional fisheries and the potential for cross-jurisdictional fishery assessments and management should be considered for Australian, Indonesian, and New Zealand populations.