Trophic Ecology of Juvenile Swordfish (Xiphias gladius) in the Northwestern Mediterranean Sea: The Role of Cephalopods and Gelatinous Plankton

Fisheries Society of the British Isles Annual Symposium (FSBI 2023), 24-28 July 2023, Essex

Understanding the interactions between fish and the wider food web is essential for effective marine ecosystem management. The study of the diet of large pelagic fish that perform long vertical migrations and contribute to the demersal-pelagic coupling of the ecosystem, such as swordfish (Xiphias gladius), provides an opportunity to document the structure and functioning of marine ecosystems through characterizing diversity, distribution, and availability of prey. Here, we examined the diet of juvenile swordfish in the north-western Mediterranean Sea in 2012 and 2020, using two complementary methods: stomach content analysis (SCA) and mass-balance isotopic mixing models, which allowed us to explore the consumption of organisms that are not usually detected in SCA due to fast degradation. SCA showed an increase in cephalopod consumption between the two periods, and an increase in the relative importance of bathypelagic fish with size. This was supported by the mixing models’ results. Although gelatinous prey were only found in one stomach, the models pointed to higher consumption in 2012, when several gelatinous blooms were reported. Further studies are needed to inform whether the consumption of low-energetic prey may affect the physiology of this species. By using SCA results as informative priors in the stable isotope mixing models, our study highlights the importance of combining multiple and complementary approaches to better reconstruct the diet of generalist species. Our findings suggest that swordfish in the Mediterranean may have an opportunistic diet that is responding to changes in the ecosystem, being useful indicators for monitoring marine ecosystems