From hormones to habitat: A new framework for assessing maturity in the Central-Eastern Mediterranean blue shark (Prionace glauca) population

The blue shark (Prionace glauca) is a widely distributed pelagic species, yet its Mediterranean population remains understudied despite being classified as critically endangered in the region. This study provides new insights into the reproductive features, physiological status, and spatial distribution of blue sharks at different stages of maturity in the Central-Eastern Mediterranean, specifically in the Southern Adriatic Sea. First, we evaluated the reproductive anatomy of 77 blue sharks and demonstrated significant changes across maturity stages, highlighting the importance of considering such anatomical shifts to improve maturity assessments. We then examined the potential of combining morphological, hormonal and lipid-based indicators as a non-lethal method for maturity classification. Principal component analysis revealed distinct morphological/physiological profiles between maturity stages in both sexes, and a supervised machine learning model (random forest classifier) achieved high classification accuracy (100 % for females and 75 % for males), confirming the predictive strength of these indicators. Converting the model outputs into a continuous maturity index enabled us to spatialize maturity patterns for males using geographic coordinates and environmental covariates in a generalised additive model. The results revealed a clear ontogenetic shift: mature males were predominantly located offshore, while immature individuals were found closer to the shore. This integrative approach supports the use of blood-based physiological profiling combined with morphological traits as a promising non-lethal method for assessing maturity and identifying potential geographic structuring between maturity stages in the Adriatic Sea. By linking physiology, maturity and environment, this study establishes a framework for future research and monitoring of vulnerable elasmobranch populations.