Cross-cohort kinship in the Critically Endangered Largetooth Sawfish (Pristis pristis) from a northern Australian river

The sawfishes (family Pristidae) are one of the most threatened groups of vertebrates in the world. Global declines are associated with overfishing, habitat loss, and their K-selected life histories. The Largetooth Sawfish (Pristis pristis) is the only euryhaline sawfish, with the last stronghold for this species being found in the remote waters of northern Australia. Using thousands of genome-wide single-nucleotide polymorphisms (SNPs) and long-read mitochondrial data, we examined the patterns of relatedness in 113 individuals collected over 13-years from the Daly River, Northern Territory. We identified 20 full- and 55 half-sibling pairs, with most half-siblings sharing a mitochondrial haplotype. Estimates of both the effective number of breeders per cohort and the generational effective population size were low (85.7–318.0 and <600 individuals, respectively). We reveal that relatedness is positively associated with both pairwise capture year and capture location. These findings provide further evidence for female reproductive philopatry and highlight the vulnerability of P. pristis to local extinction from the Daly River, a critical nursery area, as it falls under increasing anthropogenic pressures.