Feeding and biodiversity study by stomach contents of marine predators in the northwestern Mediterranean Sea

The Mediterranean Sea, with more than 8,500 species of macroscopic marine organisms, is a biodiversity hotspot. Despite being considered an oligotrophic sea, some areas of the basin are important for primary production. The intense biological activity generated in these areas influences the development of plankton populations and consequently on marine trophic chains, up to the top predators, such as cetaceans and elasmobranchs: this is the case of the northwestern Mediterranean. As a result of great frequency and abundance of marine mammals, and consequently of biodiversity, the Pelagos Sanctuary was established in this area of the basin, which aims to promote mitigation actions and protection of cetaceans and their environment. The focus of this PhD project is to collect information on feeding habits of some predator species which occur in the southern Ligurian and northern Tyrrhenian Sea (north-western Mediterranean Sea), through the study of stomach content analysis (SCA). This technique is one of most used and inexpensive methods which allows us to obtain multiple information simultaneously about the ingested prey (e.g. identification, abundance, size, biomass, importance), in order to study the diet of marine vertebrates. Therefore, another objective of this PhD Thesis is to give greater importance to the SCA, so that it can be used as indirect monitoring technique for the study of biodiversity. Stomachs of four cetaceans and a bony fish species, stranded or captured, were collected in Tuscan waters, between 2005 and 2022, and stomach contents (SCs) were investigated in order to evaluate the feeding habits of the predator. As a result of the greater availability of stomachs, for the bottlenose dolphin (Tursiops truncatus) it was possible to investigate the diet of this species, obtaining important results regarding its ecology, such as the weaning and sexual intraspecific differences in the diet of these dolphins, but also direct and indirect opportunistic interaction with fishing activities and information on coastal biodiversity. Despite the small samples of sperm whales (Physeter macrocephalus), the SCA provided important information on the feeding behavior based on sperm whales size and on the possibility that this predator interacts opportunistically with fishing activities also in the Mediterranean Sea; furthermore, important information on deep-sea cephalopods abundances were provided. Concerning Cuvier’s beaked whale (Ziphius cavirostris) and Risso’s dolphin (Grampus griseus), from the SCA of three specimens of these cetaceans it was possible to provide the list and abundance of identified prey items. Moreover, significant results on the presence/abundance of cephalopods considered rare, such as grater argonaut (Argonauta argo), have been obtained. Finally, the first detailed results regarding the faunal occurrence of prey items, including rare cephalopods, identified in SCs of rare opah (Lampris guttatus) are obtained. However, further studies will be necessary to improve ecological knowledge of predators, of their prey and marine biodiversity, but this can only be done by increasing and implementing a continuous and careful monitoring process prone to collect samples for the SCA.