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Boat traffic creates daily noise for dolphins

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Boat Traffic Noise High Enough in Intracoastal Waterways to Raise Concerns About Hearing Damage and Making; Dolphins Seem to Avoid Weekends
Haviland-Howell, Frankel, Powel, Bocconcelli, Herman, Sayigh. Recreational boating traffic: A chronic source of anthropogenic noise in Wilmington, North Carolina Intracoastal Waterway. J. Acoust. Soc. Am. 122 (1), July 2007, p.151-160.
 This study analyzed recordings of ocean noise during the summer season (June 21-September 1) in a 100m-wide intracoastal waterway off North Carolina. The mean received sound levels (measured as RMS re 1uPa, at frequencies averaged in steps over 0-37.5kHz) ranged from 109dB at 6am to 118dB in early afternoon and back down to 111dB by 10pm. Not surprisingly, weekends and holidays were 1-3dB louder than weekdays. The frequency spectrum shows that low frequencies (below 1kHz) dominate, with significant energy remaining up to 5kHz, and lesser but still perceptible noise up to the 37.5kHz limit of the study. Dolphin observations showed a clear reduction in numbers of dolphins observed on weekends and holidays, relative to weekdays; less than half as many dolphins were observed on weekends. Noise levels in the range of 5-25kHz, the primary range of dolphin social whistles, was of particular concern.
┬áThe researchers note that “mean hourly RLs exceeded 116 dB nearly every day surveyed, indicating bottlenose dolphins in the ICW near Wilmington, North Carolina could be at risk for noise exposure on a daily basis. High mean RLs were often recorded over consecutive hours, making high sound levels the rule in this area during the summer, not the exception.” Further, they note that since bottlenose dolphins feed mainly on soniferous fishes (fish that make noise), and that fish vocalizations are primarily below 1kHz, the range most dominated by boat sounds, dolphins may well find it more difficult to hear and find prey.

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