Acoustic Behaviour of Bottlenose Dolphins (Tursiops truncatus) During Interactions with Trawlers in the Gulf of Trieste

Dolphins rely on acoustic communication to share information about the environment and this communication channels are disrupted by anthropogenic underwater noise. Even if acoustic masking is also caused by bottom trawlers, various populations of bottlenose dolphins have developed an opportunistic feeding strategy based on following them; this strategy has also been observed in the bottlenose dolphin population in the Gulf of Trieste and adjacent waters in the northern Adriatic Sea. The aim of this study is to characterise whistle repertoires and see if there are any differences during interaction with trawlers or in their absence. Five hours and 54 minutes of recordings were collected in both conditions, from July to August 2022, using an omnidirectional hydrophone from a research vessel; dolphin behaviour and group size were recorded. Whistles were extracted from recordings and their spectro-temporal characteristics and contour shape were analysed for differences in their parameters between dolphins in interactions with trawlers or in their absence. Moreover, whistles were divided in signature (SW) and non-signature whistles (NSW), which convey different types of information. 177 whistles were considered fit for further analysis and were divided in 125 NSW and 52 SW. Further analysis were conducted on the overall whistles and NSW separately, while the SW were only described due to their small sample size. Statistically significant differences were observed in low frequencies both in whistles and in NSW, as well as in whistle duration. The whistle rate did not show any differences between the two conditions. Furthermore, different whistle contour types were produced during interaction and absence of trawlers, suggesting possible differences of information and behavioural context. These findings lay the basis providing guidelines for future study to add more information on possible changes in vocalizations due to man-made noise and plan proper mitigation measures.