A dolphin feeding on fish

Since 2004, we have been analysing the partially digested stomach contents of dead dolphins to identify their prey. Bottlenose dolphins are generally opportunistic feeders consuming fish and cephalopods (e.g., squid). The analysis detects the presence of the beaks of cephalopods and fish sagittal otolith (ear bones). These structures are digested slowly. The shape of the otolith is specific to each fish species and is continuously formed throughout the lifetime of the fish by accreting layers of calcium carbonate and gelatinous matrix, creating “winter” and “summer” rings, these can be used to determine size and age. Otoliths are coloured red using a fibre tip pen to aid identification. Similarly, the lower beaks of cephalopods are used for species classification. During the study, a reference catalogue was made of more than 50 local fish species. From 2000 to 2008, 25 stranded dolphin stomachs were analysed. The results showed that bottlenose dolphins in this area mostly feed on demersal fish from the Sparidae family, hake (Merluccius merluccius) and horse mackerel (Trachurus sp.). The most abundant Cephalopod order found in the stomachs was Teuthoidea– squid. Continuous monitoring of feeding habits is important because any shift in preference prey species could imply changes, not just in fish stocks, but also increasing competition with fishery in the area.

Dolphins following a bottom trawler

Identifying pray through analysis of stomach contents have some limitations and does not show full range of prey consumed by dolphins or potential shifts or individual behaviour of dolphins. In 2011, we applied new methodology of pray identification through analysing of stable isotopes content of nitrogen and carbon in skin and blubber biopsy samples taken from wild animals. Through analysis of the content of stable isotopes, we identified previously unknown facts about feeding behaviour of bottlenose dolphins. Namely, the analysis revealed that bottlenose dolphins can completely shift their feeding strategies depending on the most available prey. We found out that although bottlenose dolphins in Vis area (like Lošinj dolphins) were feeding on demersal prey through most of the year, due to the high abundance of small pelagic fish during winter and early spring, dolphins shifted to feeding almost exclusively on sardines and other schooling fish.