Assessing the Impacts of Watercraft Noise on the Acoustic Behavior of Bottlenose Dolphins
by Kara Buckstaff, MSc

Watercraft may provide the greatest source of anthropogenic noise for bottlenose dolphins living in coastal waters. In 2002, 41,558 recreational watercraft were registered in Sarasota and Manatee counties combined, accounting for 95% of total registered vessels (the other 5% being commercial vessels). Previous studies have shown that due to the high level of watercraft traffic in the study area these dolphins are exposed to a vessel passing within 100 meters approximately every six minutes during daylight hours. Energy of boat noise ranges between 0.1-10 kHz. This frequency range overlaps that of dolphin whistles (4-20 kHz), an important mode of communication among individuals. Recent studies indicate that the direction and type of acoustic behavioral responses are variable among species, but typical responses may enhance signal detectability and group cohesion in a noisy environment.

For my Master’s research (completed in June 2003), I investigated the circumstances under which watercraft may impact the acoustic behavior of this community, specifically looking for short-term changes in whistle frequency range, duration, and rate of production. Signature whistle duration and frequency range did not change significantly when boats approached, however, dolphins whistled significantly more often before the presence of a vessel compared to during and after approaches. Whistling rate was also significantly greater at the onset of a vessel approach than when no vessels were present (see figure). Increased whistle repetition as watercraft approach may simply reflect heightened arousal, an increased motivation for animals to come closer together, with whistles functioning to promote reunions. It may also be an effective way to reduce signal masking and enhance communication in a noisy environment. Funding for this research was provided by the Disney Wildlife Conservation Fund, the NOAA Fisheries, the Earl and Ethyl B. Myers Oceanographic Trust, the American Cetacean Society, and the University of California-Santa Cruz Ocean Sciences Department.