The effect of boat noise and depth of water on the frequency of echolocation of bottlenose dolphins (Tursiops truncatus) in Cardigan Bay

Liz Shaw BSc (Hons) Zoology,  Aberystwyth University

As visibility in the marine environment can be poor, cetaceans communicate, navigate and forage acoustically. Anthropogenic noise can propagate over long distances underwater in the marine environment and there is increasing evidence and concern that noise generated by boats in particular, may lead to signal masking in cetaceans. Here I investigate the effect of boat noise on the frequency of echolocation in bottlenose dolphins in Cardigan Bay. This study shows that the bottlenose dolphins (Tursiops truncatus) of Cardigan Bay show plasticity in the frequency of echolocation and are able to reduce this masking effect by altering the frequency of echolocation (Hz) when boats are present. Dolphins have significantly lower echolocation frequencies when boats are present. The number of clicks per second was also significantly lower when boats were present. In contrast, there was a significant negative correlation between distance from New Quay harbour, where peak levels of boat activity are found, and the frequency of echolocation, though there was no significant correlation between peak frequency and distance to shore. There was also a significant correlation between water depth and echolocation frequency. These findings illustrate that bottlenose dolphins in Cardigan Bay have the ability to change their acoustic signals in response to masking. Therefore, although human activity and increasing levels of boat noise may be a cause for concern, bottlenose dolphins seem to be able to find ways of avoiding signal masking.

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