Diet and feeding habits of two skate species, Raja brachyura and Raja miraletus (Chondrichthyes, Rajidae) in Sardinian waters (central-western Mediterranean)

The diet and inter-and intraspecific interactions of two shallow-water skates, Raja brachyura and R. miraletus, caught at depths between 30 and 162 m during trawl surveys carried out in 2005 and 2006 in the seas surrounding Sardinia (central-western Mediterranean), were analysed. The low values of Levin and Shannon indexes (Bi=0.37 and 0.42, respectively, for the former, and H'=1.31 and 1.62, respectively, for the latter) showed that these two species are specialized predators. An ontogenetic analysis of the diets of three size groups showed that they adopted similar trophic habits. Young R. brachyura specimens (Bi=0.27; H'=1.05) feed mainly on Crustacea (% IRI=74) (mainly Mysidiacea and Amphipoda Gammaridea), which were replaced in the diet of medium-large specimens principally by Osteichthyes (mainly Gymnammodytes cicerelus). Multidimensional scaling analysis indicated a high level of intraspecific competition between medium and large individuals. In R. miraletus, Crustacea represented the main prey in all size groups (% IRI=99.5, 99.6, and 88, respectively). Amphipoda Gammaridea were the predominant prey for small individuals, and were replaced in medium and large specimens by decapods. Low levels of interspecific competition were generally found.