Does exceeding the legally permissible number of vessels impact feeding and socialising in bottlenose and common dolphins off southern Portugal?

Alicia Quirin, Julia Rentsch, Miguel P. Martins, Francisco O. Borges, André Cid, Joana Castro

The presence of maritime touristic vessels is known to affect the behaviour of multiple cetacean species. Therefore, different countries have implemented legislations to regulate the number of boats in proximity of cetaceans. This study examines if exceeding the legally permissible number of boats affects the feeding and socialising behaviour of bottlenose (Tursiops truncatus) and common dolphins (Delphinus delphis) off southern Portugal, an area with high dolphin-watching boat pressure. Between 2012 and 2023 opportunistic, as well as dedicated boat-based marine mammal surveys were conducted. Data on the species, maximum number of boats, and initial and general behaviour of the dolphins were collected. When the initial behaviour was feeding and/or socialising we compared it to the general behaviour to determine if there was a change. Using Fisher’s Exact Test, we analysed if feeding and socialising of either species ceased more often when the number of boats exceeded the legal maximum. A total of 967 sightings were analysed, 373 for bottlenose and 594 for common dolphins. In 29.58% of all sightings the legal number of boats was exceeded (N = 286), 45.31% for bottlenose (N =169) and 19.70% for common dolphins (N = 117). Neither, feeding nor socialising in either species significantly changed when more than the permissible number of boats were present. For any number of boats, both species seemed generally more prone to cease feeding, rather than socialising. Specifically, bottlenose dolphins appeared to stop socialising more frequently (16.95%) and feeding less frequently (19.59%) compared to common dolphins (11.61%, 28.86% respectively). These preliminary analyses highlight the lack of enforcing current measures and indicate potential differences in vessel exposure of both species. Further studies are needed to examine if surpassing the legal number of vessels affect other aspects of the behaviour of bottlenose and common dolphins off southern Portugal.


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