Effects of vessels on the surface and vocal behaviour of bottlenose dolphins in Doubtful Sound, New Zealand

Abstract

Marine mammals in coastal areas are increasingly exposed to boats and noise as nature tourism grows. It is now well known that boat-based tourism has a wide range of detrimental effects on the surface behaviour of cetaceans, although effects on their acoustic behaviour are poorly understood. Measuring these impacts is critical for appropriate management, but translation of research findings into effective management often lags far behind research itself. The impacts of tourism on the endangered population of bottlenose dolphins (Tursiops truncatus) in Doubtful Sound, New Zealand, were first studied in 1999, demonstrating behaviour disruption due to boat presence. A code of conduct was implemented in 2008 to mitigate those impacts, establishing guidelines to leave dolphin encounters to chance and restricting vessel traffic in areas of critical habitat. The present study assessed the current impacts of tour boats and the observing research boat on the surface and vocal behaviour of dolphins, and evaluated the effectiveness of management guidelines in reducing interactions. An information-theoretic approach indicated that groups with mother-calf pairs were especially sensitive, showing significantly less cohesion and coordination when tour boats were audible, and were more vocal when boats were close and while moving away, presumably to re-establish group structure. Furthermore, groups with calves increased their whistle rates when tour boats were faster (i.e., louder) while groups without calves became quieter. These results suggest that elevated noise impairs communication and higher repetition rates are used to increase communication success. In a noisy environment, the need for vocal contact when calves are present seems to outweigh the costs of whistling more often. In addition, different whistle modifications were used in response to increased noise levels from tour boats: groups without calves produced longer whistles shifted towards lower frequencies, while groups with calves produced shorter and higher-frequency whistles. Although whistle rates were unaffected by the research vessel, movement patterns and whistle parameters were mildly affected, highlighting the importance of accounting for observer effects in studies of tourism impacts. The extent of dolphin-boat interactions in Doubtful Sound has decreased substantially since the previous study (1999 - 2002). This decline appears to be in response to the management guidelines established in 2008, showing that science-based management can mitigate potential long-term impacts of tourism. Nevertheless, because the Doubtful Sound population is small and has a history of low calf survival, and because groups with calves are particularly vulnerable to boats, it is crucial to further minimise current anthropogenic impacts. The present study provides recommendations for improving current management.