Vocal Behaviour of a Bottlenose Dolphin Pod During a Deadly Bycatch Event in the Gulf of Catania, Ionian Sea

Cetacean bycatch is increasing worldwide and poses a threat to the conservation of several delphinids. The bottlenose dolphin (Tursiops truncatus) is frequently involved in bycatch incidents, due to its coastal distribution and opportunistic behaviour. The acoustic behaviour of cetaceans during death-related events is a largely unexplored topic. During an acoustic monitoring survey of the bottlenose dolphin population inhabiting the Gulf of Catania (Ionian Sea), we documented the entangling and subsequent death of a sub-adult male in a fishing net. Here, we provide a detailed analysis of the vocal behaviour of the pod of bottlenose dolphins during the event. We identified a total of 720 vocalisations in a timespan of 138 min, including 436 signature whistles, 51 non-signature whistles, and 233 burst pulses. We recorded high vocal activity, with a peak characterised by emission rates of 36 signature whistles per minute and 70 burst pulses per minute. Although future studies are required, our results show massive vocal activity during net entangling, characterised by a significant emission of signature whistles and burst pulses, including bray series of gulps and squeaks. Therefore, we suggest that developing tools for automatically detecting the peaks of these sound types could be helpful in recognising bycatch events, in order to better quantify the impact of professional fishing on small cetaceans.