A lifetime perspective on resource use: spatial and ontogenetic trophic shifts in the blackspotted smooth-hound...

Elasmobranchs are facing severe threats worldwide from overfishing and related high bycatch, and particularly so in the Mediterranean Sea, where 65% of sharks and rays are now at risk of extinction. Understanding their biology and ecological roles is crucial for the development of effective conservation strategies. This study analysed the trophic ecology of the vulnerable blackspotted smooth-hound (Mustelus punctulatus) in three sub-areas of the northwestern Adriatic Sea, combining stomach content and stable isotope analyses. We hypothesized that feeding strategies vary in response to differences in prey availability driven by local environmental conditions, as well as to ontogenetic changes in resource requirements, using shark size as a proxy for developmental stage. Stomach content analysis revealed that specimens from the northern areas had more diverse diets, including higher contributions of cephalopods and fish, while those from the southern area mostly preyed on decapod crustaceans. Stable isotope analyses on muscle and vertebrae tissues supported these findings and further revealed ontogenetic shifts in resource and habitat use. δ¹⁵N values varied among the different sub-areas but were independent of shark size. Values of δ¹⁵N decreased southward, in agreement with the higher contribution of crustaceans in the diet. Values of δ¹³C did not vary among sub-areas and are affected by the influence of freshwater contribution of the Po River. Furthermore, the use of vertebral core and edge isotopic values provided evidence of changes in habitat and resource use over the individual lifespan, suggesting a trophic shift towards higher trophic level prey in mature females (based on δ¹⁵N values) and an offshore migration after the early life stages (based on δ¹³C values). These results highlighted the importance of considering both spatial and ontogenetic variation in the trophic ecology of M. punctulatus and provide valuable information to support conservation and management strategies.