Mapping total environmental sound exposure of bottlenose dolphins, tursiops truncatus, in the Wilmington, north Carolina intracoastal waterway

Abstract

The Intracoastal Waterway (ICW) in Wilmington, North Carolina is an important habitat for bottlenose dolphins year round. However, little is known about the levels of physical, biological, and anthropogenic noise to which dolphins are exposed in this habitat. We are utilizing a digital signal acquisition system linked to a moored hydrophone (-170.1 dB re 1V/uPa) in the ICW in order to characterize and quantify ambient sea noise and the factors affecting it. In particular, we are quantifying the contribution of anthropogenic noise from recreational boats and personal watercraft to total environmental sound levels. Surveillance video of the ICW is captured by a computer stored in a weatherproof container at the end of the Center for Marine Science pier, a structure that extends into the ICW. Audio output from the hydrophone is amplified and filtered before being digitized by an analog-to-digital converter, sampling at rate of 75 kHz. The digitized signal is acquired continuously in 30-second time-stamped AIFF files. Both audio and video files are transferred to a second computer located within the Center for Marine Science building via a fiber optic cable which runs the length of the dock. Video files are annotated for boat activity. Audio files are used to generate one spectrogram from each second of waveform data. Spectral data are stored on with raw waveform data and video data on 20 GB digital audio tapes. Using this innovative system, we will map levels of total sound exposure over time to determine if bottlenose dolphins are being subjected to acoustic harassment (both as that term has been defined in the literature, and how that term may soon be defined by the National Marine Fisheries Service) by exposure to high levels of anthropogenic noise, and if so, how often. Comparative spectra generated during different times of day and across seasons will provide a comprehensive picture of the acoustic environment experienced by this group of bottlenose dolphins. This study promises to provide one of the first quantitative assessments of the potential impacts of anthropogenic noise on bottlenose dolphins.