Behavioral Responses of a Harbor Porpoise (Phocoena phocoena) Depend on the Frequency Content of Pile-Driving Sounds

Abstract

The loud, impulsive, broadband underwater sounds produced during offshore pile driving are known to have auditory and behavioral effects on harbor porpoises (Phocoena phocoena) in the areas around piling sites. Thresholds to prevent behavioral effects have not yet been set, and it is unclear whether or not auditory frequency weighting of piling sounds, as used for criteria to protect hearing (Southall et al., 2019), is also useful for predicting behavioral responses and therefore required to set safety criteria and develop mitigation measures. A harbor porpoise in a pool was exposed to playbacks of piling sounds, and her behavioral responses were quantified in comparison to baseline periods without piling sounds. The full-spectrum playback piling sound was recorded at 100 m from a piling site for an offshore wind turbine. For comparison, five low-pass filtered (6.3, 3.2, 1.5, 1.0, and 0.5 kHz) versions of the sound in which the bandwidth decreased were played back at the same duty cycle (46 strikes/min) and similar single-strike sound exposure levels (power average in the pool: 135 dB re 1 µPa²s; t90: 90 to 100 ms). As the bandwidth of the piling sounds decreased, the porpoise’s behavioral response became weaker. Although these results are based on only one porpoise, they indicate that harbor porpoises respond most strongly to the higher frequencies in piling sounds. Therefore, frequency weighting of the sound exposure level (SEL) will improve prediction of behavioral responses, and behavioral response threshold levels for criteria should also be expressed as weighted SELs. However, it is unclear whether the weighting for predicting auditory effects is also the best weighting to predict behavioral effects. Mitigation of the effects of piling sounds on harbor porpoise behavior should be focused on reducing the high-frequency part of the spectrum.