Stable isotope analysis of juvenile white sharks inside a nursery area reveal foraging in demersal-inshore habitats and trophic overlap with sympatric sharks

Knowledge about the trophic ecology of top predators is crucial for defining their role in ecosystems, understanding habitat preferences, characterizing stage-specific feeding habits, and evaluating their interaction with fisheries. In the northeastern Pacific, white sharks occupy coastal habitats during early life stages, including those of Vizcaíno Bay (VB) in Mexico, considered a nursery area. Although VB presumably provides high food abundance, the trophic ecology of juvenile white sharks (JWS) is poorly understood. Hence, carbon and nitrogen bulk stable isotope analyses were used to explore the trophic relationship of JWS with their potential prey and to infer dietary overlap with sympatric shark species. Muscle samples from JWS and six inshore demersal prey commonly found in stomach contents (Myliobatis californica, Hypanus dipterurus, Gymnura marmorata, Pseudobatos productus, Atractoscion nobilis, and Paralichthys californicus) were sampled. Demersal prey and literature-derived isotope ratios for pelagic and offshore species were incorporated into mixing models with a Bayesian framework to estimate their contribution to JWS isotope ratios. Nearshore demersal prey had the highest contribution (45%), consistent with the known interactions with local fisheries targeting these demersal species and local JWS movements patterns. Offshore (30%) and pelagic (25%) contributions were smaller and indicate potential periodic changes in foraging grounds. Isotope ratios for the juveniles of the three sympatric sharks (smooth hammerhead Sphyrna zygaena, copper Carcharhinus brachyurus, and dusky C. obscurus sharks) were used to estimate the overlap in isotopic niche space by calculating standard ellipse areas. JWS had the smallest isotopic niche, while the highest was estimated for carcharhinid sharks (copper and dusky). Overlap was higher between JWS and hammerhead (~50%), while overlap with carcharhinids was small (<20%), suggesting limited competition for prey. These results suggest some resource overlap between shark species, and the importance of coastal demersal prey heavily targeted by local fisheries, in support of the importance of VB as a nursery habitat for JWS.