Genetic Assessment of Breeding Patterns and Population Size of the Sicklefin Lemon Shark Negaprion acutidens in a Tropical Marine Protected Area: Implications for Conservation and Management

The sicklefin lemon shark (Negaprion acutidens) is found in coastal waters of the Indo-West Pacific where it has been assessed by the IUCN as threatened with extinction. Information on the species’ reproductive ecology and local abundance, which are important considerations for effective management, remain limited. I used genetic analyses of tissue samples collected from juvenile N. acutidens, at the Curieuse Marine National Park (CMNP), Seychelles, between 2014-2017, to (1) estimate the number of adults reproducing at CMNP annually and (2) identify their breeding patterns through pedigree reconstruction. I report strong evidence of philopatry; primarily in females. Over the study period 25 reconstructed females produced multiple litters; the majority (88%) displayed biennial parturition. The remaining 12% displayed annual parturition. Multiple paternity was common (66% of 58 litters; mean number of sires per litter = 1.92). Convenience polyandry provides a likely explanation for this and may be driven by biased operational sex ratios during mating. Male philopatry to CMNP was low (17% of 114 reconstructed males) and may be influenced by habitat availability. Males likely breed over broader geographic scales than females. The breeding patterns I report are similar to those identified in other populations of lemon sharks and are likely applicable across the genus. In Seychelles, shark stocks are in decline due to overfishing. The high female philopatry in N. acutidens suggests protection of parturition sites, such as CMNP, is likely important to the conservation of local populations. However, adult life-stages, particularly males due to wider-ranging behaviour, are still subject to fishing pressure outside the park. Additional management measures are required to prevent further population declines. Species-specific management appears to be the best approach. The introduction of science-based fisheries control measures, for N. acutidens and other shark species, should be an urgent priority in the Seychelles.