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Abstract Understanding relatedness in sharks is difficult because of uncertain distributions, low population densities and the challenge of sampling across life stages. In Fiji, bull sharks (Carcharhinus leucas) aggregate at the Shark Reef Marine Reserve (SRMR), while gravid females disperse to adjacent rivers to give birth. To investigate reproductive connectivity, female return patterns and kinship structure, we used population genomics to analyse 296 bull sharks from multiple age classes sampled over a decade at the SRMR and in three adjacent rivers. We identified 375 direct genetic links, including first- and second-degree relationships, connecting SRMR adults and subadults with young sharks in the Navua and Rewa rivers, providing evidence of reproductive connectivity and site fidelity. Within rivers, genetic similarities across cohorts indicated reproductive philopatry. In addition, 64 sharks sampled across multiple years were assigned to the same sire–dam pairs, suggesting repeated partner fidelity across breeding seasons and cohorts. Effective population size (Ne) was estimated at 281.9 and increased to 1784.8 when related individuals were excluded. Overall, our findings indicate that bull shark reproduction in Fiji is shaped by reproductive philopatry and repeated partner fidelity, revealing a more structured reproductive system than previously recognized.
Glaus et al. (Wed,) studied this question.