Unraveling the genetic and environmental factors that influence blacktip reef shark (Carcharhinus melanopterus) reproduction success

A global assessment (in 2021) reported that a third (32.6%, 391 on 1199 species) of chondrichthyan species (sharks, rays, chimaeras) were threatened by extinction. This assessment might have been underestimated as 12.9% of these species are currently listed as data deficient. With 90% of teleost fish stocks being exploited, shark species are the most lucrative target with the expansion of shark fin markets. The scarce knowledge in combination with the tremendous declines in shark species densities highlights the urgency to conduct studies on chondrichthyans. Here, we aimed at studying a blacktip reef shark (Carcharhinus melanopterus) population that was monitored and sampled for fin clips over the last decade. The objectives were to explore the over-time density variations in the juvenile population and their origins, then we reconstructed the genealogy (adult and juvenile sharks) shedding light on the (reproductive) biology of sharks. Since 2007 the blacktip reef shark population of Moorea Island (French Polynesia) has been sampled for fin clips (dorsal fin tissue samples) and morphometrics in ten sites around the island which provided a database of 1780 individuals over a 15-year timespan. We modelled the spatial and temporal density changes over the last decade (together with inter-specific competition with Negaprion acutidens and with water temperature as explanatory variables) in the blacktip reef shark population of Moorea Island and then we developed a species-specific microsatellite markers bank to reconstruct the genealogy of the shark population. The genealogy (first highly conservative pedigree reconstructed for a shark species) was used to define life history traits for blacktip reef sharks (maximal longevity, sexual maturity, litter size, dominant individuals) and explore their behavior (female reproductive behavior and philopatry). A slight negative trend was found in juvenile shark body condition over time together with variability among sites, life stages and shark families. Lifetime reproductive success was equal for males and female sand was high in two sites that were found to have a significant negative effect on body condition likely because juveniles in these nurseries frequently reached the independent foraging capacity, allowing them to survive on their own and reach the adult life stage. This research has consequences for the management and conservation of shark populations as we identified priority areas, having higher overtime shark densities and survival, and we documented life history and reproductive traits.