Temporal changes in the diet composition of swordfish, Xiphias gladius (Linnaeus, 1758), in a Mediterranean foraging...

Temporal changes in the diet composition of swordfish (Xiphias gladius) were observed in one of the most important Mediterranean foraging and reproductive ground, the Strait of Messina and the surrounding areas. Qualitative and quantitative analyses of the diet composition were performed, and a description of the feeding strategy of this predator were collected. A total of 133 individuals with 100-220 cm of lower jaw fork length (LJFL) was collected in two different fishing seasons (i.e. 2008 and 2019). Overall, 55 prey taxa were identified, and the most abundant were teleosts (19 families and 34 species) and cephalopods (14 families and 19 species). The percentage index of relative importance (%IRI) revealed that the cephalopods Todarodes sagittatus (%IRI = 48.17), Ancistroteuthis lichensteinii (%IRI = 23.14), Heteroteuthis dispar (%IRI = 10.23) and Ommastrephes caroli (%IRI = 6.70) were the most important prey items. Furthermore, the analysis of feeding strategy confirmed that squids were dominant in the swordfish's diet in the study area. Our results highlighted temporal changes in prey composition and feeding activity of swordfish. Generally, Ommastrephidae, Onychoteuthidae, Sepiolidae and Paralepididae were the main families responsible for the dissimilarity in the diet composition of Xiphias gladius between sampling years (2008 and 2019). These dietary changes could be a consequence of the environmental variations occurred in the Mediterranean Sea. To explore this hypothesis, environmental data were derived and analysed from Copernicus Marine Environment Monitoring Service (CMEMS). The results showed how surface temperature, despite frequent upwelling events, has been gradually warming in the years 1998-2008 (0.0509 °C/year), which is slightly more pronounced in 2009-2019 (0.0656 °C/year) while primary production remains stable. The aforementioned environmental changes are reflected in the dietary composition of marine predators, such as swordfish, with variations in food resources and feeding habits being observed. These variations may have important consequences in terms of ecosystem resilience, since fluctuations of key-stone prey could produce bottom-up effects on predator populations or top-down effects on their prey.