Dietary habits and trophic ecology of elasmobranchs from Amvrakikos Gulf, Greece.
Supervisors: Ioannis Giovos Martina Ciprian (ISEA, Greece)
The role of elasmobranchs as mesopredators has received less focus in comparison with the studies realized in high predators like sharks. The aim of this study was to describe qualitatively and quantitively the dietary habits of the elasmobranchs inhabiting Amvrakikos Gulf, as well as determine their trophic position and its relationship with heavy metal concentration in tissue samples from these elasmobranchs. A total of 537 stomachs were analyzed belonging to 10 species: Aetomylaeus bovinus (n=72), Bathytoshia lata (n=2), Dasyatis marmorata (n=117), Dasyatis pastinaca (n=58), Dasyatis tortonesei (n=76), Gymnura altavela (n=48), Mustelus mustelus (n=54), Myliobatis aquila (n=35), Torpedo marmorata (n=3) and Torpedo torpedo (n=72). The most recurrent taxa found among all the species were Crustaceans and Osteichthyes. Six species of elasmobranchs were found to have specialist diets, while the other two had a generalist diet. Elasmobranchs from the gulf exhibit continuous and intermittent feeding behaviour. A niche overlap was found among D. pastinaca and D. marmorata, and between D. pastinaca and D. tortonesei, as they share similar food items in terms of composition and quantity. Trophic level positions ranged between 2.89 (D. pastinaca) and 3.25 (A. bovinus), suggesting that elasmobranchs from Amvrakikos Gulf are mesopredators. Heavy metal concentrations were found at low levels, however a negative correlation between the trophic level position and concentration values of Hg and Cu was found. The composition found in diets corresponds to the ones reported in other parts of the Mediterranean, except for M. aquila, which preference for crustaceans in Amvrakikos might respond to their high plasticity to adapt to the availability of prey in different habitats. The display of high specialistic diets might be the reason behind the low niche overlapping. Specialist consumers could be impacted by the changes in prey population structure as a result of overfishing or habitat degradation, which added to the ecosystem fragility of Amvrakikos Gulf, increases the vulnerability of sharks and rays.