Isotopic Assessment of Ontogenetic and Sex-based Niche Segregation in Blue Sharks (Prionace glauca)

The blue shark (Prionace glauca) plays a key ecological role in pelagic ecosystems due to its broad distribution and diverse feeding strategies. To better understand its trophic dynamics in the Northeastern Pacific, we examined the feeding ontogeny of individuals sampled off Baja California Sur (Mexico), a key area within this region, using carbon and nitrogen stable isotope analysis (SIA) from muscle and vertebral tissues. δ¹⁵N values from both tissues indicated broadly overlapping trophic positions across maturity stages, although a slight but significant increase was observed in adults. In contrast, δ¹³C values revealed clear ontogenetic shifts in habitat use: adults occupied more coastal-influenced zones, while juveniles were associated with oceanic waters. Isotopic niche overlap between maturity stages (I-II vs. III) and between sexes was low (<0.25), indicating marked ecological segregation. Significant δ¹⁵N differences linked to maturity stage and sex may reflect female-biased migratory behaviour and broader thermal tolerance. Overall, our results highlight pronounced variation in trophic niches associated with sex and ontogeny, likely driven by sea surface temperature gradients and spatial differences in prey availability. These findings provide new insight into the ecological drivers shaping blue shark feeding strategies in a dynamic subtropical ecosystem.