The effect of vessel noise on humpback whale, Megaptera novaeangliae, communication behaviour

Abstract

The modification of communication signals in response to a changing noise environment has been demonstrated in many terrestrial species, although there are comparatively fewer studies in marine mammals. It is rare, even in terrestrial studies, that the response to an anthropogenic source of noise is compared with a natural source of noise in order to evaluate how animals cope with anthropogenic noise. Humpback whales produce two different types of communication signal other than song: vocal sounds and nonvocal sounds from surface-generated behaviours (breaches, pectoral slapping, tail slapping). In this study, the response of humpback whales to noise dominated by wind (a natural source of noise from breaking waves) was compared with the response to noise dominated by a passing vessel (anthropogenic noise). There were no significant changes in frequency or duration of two common vocal sounds in response to increases in either wind or vessel noise. As with previous studies, humpback whale groups, in response to increasing wind-dominated noise, increased their vocal source level and switched from using primarily vocal sounds to surface-generated sounds in their repertoire. In response to an increase in vessel noise, groups did neither. The proportion of nonvocal sounds in their repertoire was significantly correlated with wind speed (and therefore wind noise) regardless of vessel noise suggesting this response is specific to increased wind noise. However, vocal source levels were lower than expected in vessel noise, suggesting a potential for signal masking in these groups. This comparison of response to two different sources of noise shows that humpback whales may not be able to cope with an increase in anthropogenic noise in the same way they cope with an increase in natural noise. The results highlight the benefits of using the response to a natural source to assess the potential effects of anthropogenic noise on animal communication.