Abstract
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.
The ecological trade‐offs required by nursery dependence are poorly understood. Using multi‐tissue stable isotope analysis, this research provides evidence of constrained resource use in juvenile great hammerheads (
Sphyrna mokarran
) associated with year‐round dependence on the Biscayne Bay nursery during their first two years of life. Subadult hammerheads shift to coastal foraging but seasonally increase bay foraging, potentially to reduce interactions with adult conspecifics which are more abundant in those months. Specialized use of bay habitats by immature individuals suggests elevated vulnerability to anthropogenic disturbance, while their expansion into coastal foraging habitats necessitates inclusion of those areas in nursery boundaries.