Ontogenetic variability in the feeding ecology of the spiny dogfish (Squalus acanthias, Squalidae) in the Northern Adriatic Sea

Abstract

Despite their crucial role in shaping and regulating marine ecosystems, sharks are among the most endangered animal groups worldwide. Therefore, further research into their biology and ecology is urgently needed to improve current management policies. Spiny dogfish (Squalus acanthias) is among the most commercially exploited shark species worldwide, including in the heavily fished northern Adriatic Sea. In this study, the stomach contents of 249 samples captured between 2018 and 2022 off the Po River delta in the northern Adriatic Sea were analysed to determine their diet composition. On the basis of the values for the prey-specific index of relative importance, the prey taxa were mostly demersal: teleost fish formed the bulk of the diet (47.0%), followed by crustaceans (22.6%) and molluscs (14.9%). The mean stomach fullness percentage was relatively low and highly variable (2.9 ± 2.3%), indicating a probable intermittent feeding strategy. The diversity (Shannon–Weaver index, H’) and evenness (Pielou index, J’) indices were also low (0.27 ± 0.36 and 0.29 ± 0.38, respectively). No significant differences in diet were found between males and females. The diets of immature and mature individuals differed significantly, suggesting the occurrence of ontogenetic changes. Teleosts and cephalopods increased in importance as individuals grew but crustaceans decreased. The diet of maturing individuals was characterized by a high consumption of salps, especially during a particular sampling season, which coincided with a bloom event of this prey species, suggesting ontogenetic shifts in habitat use between juveniles and adults. These findings describe S. acanthias as an opportunistic and generalist predator, in accordance with past research.