Nursery Resource Use Dynamics in Great Hammerheads (Sphyrna mokarran) Across Ontogeny

Dependence on nursery areas often requires young sharks to make trade-offs between access to prey and avoiding the risk of predation, potentially resulting in constrained habitat and resource use that elevates their susceptibility to anthropogenic disturbance. We investigated trends in ontogenetic and seasonal resource use in Critically Endangered great hammerhead sharks (Sphyrna mokarran) during and after association with an urbanized nursery area in Biscayne Bay, FL, using analysis of carbon and nitrogen isotopes in muscle and plasma. We found that great hammerheads ranging from 63 to 287 cm fork length (FL) showed a partial ontogenetic shift from bay to coastal resources, predicted to occur around ~125 cm FL (or ~2 years old). Bayesian models suggested dependence on bay batoids supplemented by bay teleosts during nursery association, before transitioning to a coastal teleost-dominated diet. Significantly larger isotopic niches and greater trophic diversity were observed among subadults and adults relative to juveniles, with some also incorporating coastal shark prey alongside teleosts and others continuing to primarily exploit bay prey groups. Subadults transitioning away from year-round nursery dependence also exhibited clear seasonal variation in foraging, switching from coastal foraging during the dry season to bay foraging during the wet season, potentially due to a seasonal increase in the abundance of adult hammerheads in the area from the late dry to the early wet season. Overall, results suggest that the resource use of immature great hammerheads may be constrained by competition, predation, and specialization, potentially driving vulnerability to anthropogenic disturbance of critical nursery prey and habitats.