First use of triple-element stable isotope analysis of elasmobranch vertebrae reveals strong ontogenetic shifts in smooth hammerhead sharks

Shark populations are declining globally, yet fundamental knowledge about their ecology and critical habitats across life history remains scarce, often preventing more effective conservation measures. To address these knowledge gaps for the vulnerable smooth hammerhead shark ( Sphyrna zygaena ), we conducted triple-element (carbon, nitrogen, and sulphur) stable isotope analysis of vertebral growth layers from individuals from the Atlantic Ocean. This approach, including the first analysis of δ 34 S in elasmobranch vertebrae, produced individual lifetime records of assimilated feeding ecology and habitat use from birth to their maximum recorded age. We found consistent ontogenetic shifts with early juveniles characterized by coastal-associated isotope signatures followed by a transition around 2.5 years towards a progressively more oceanic-associated habitat and diet until adulthood. Isotopic variation was greatest in juveniles and females, reflecting pronounced interindividual variability that may arise from the use of isotopically distinct nurseries, niche partitioning, or both. Sulphur isotope data revealed many of the clearest patterns, underscoring the value of δ 34 S in elasmobranch vertebrae as a robust ontogenetic environmental tracer. Taken together, our results demonstrate, how time-resolved stable isotope records can inform life stage-specific ecological dynamics, highlighting the importance of integrating ontogenetic patterns into management efforts of highly mobile marine species.