Frequency and amplitude shifts in the whistle vocalizations of bottlenose dolphins in response to anthropogenic noise

Abstract

Anthropogenic noise can have a number of negative effects on cetaceans including the masking of biologically important sounds. Although many observational studies are found in the literature, few data have been published on the effects of low frequency anthropogenic noise on odontocetes. The purpose of this thesis is to examine the effect of shipping and industrial noise on the whistle vocalizations of bottlenose dolphins (Tursiops truncatus) living in Galveston Bay, Texas. Dolphin whistles were recorded in different types of noise environments. Vocalization characteristics such as whistle mean frequency, frequency of peak energy, and sound pressure level were examined as functions of total background noise. Findings indicated that whistle mean frequencies were significantly lower in noisier areas. Whistles were also significantly louder in noisier environments. Combined frequency and amplitude results indicate that lower whistle frequencies had higher sound pressure levels. It may be, that given a specific amount of energy available for vocalization, a dolphin can produce a louder whistle at a lower frequency. Postulating a situation in which a dolphin has the option of more than one behavioral adaptation to avoid masking in a noisy site, the animal may implement the behavior that would require the least amount of energy. Energetic studies are needed to further investigate the relationship between vocalization frequency and amplitude. In addition, since large gaps in our knowledge of the effects of noise pollution still exist, more quantified, long-term data on a variety of species in different acoustic environments is needed.