Effect of Impact Pile-Driving Playback Sound on Harbor Seal (Phoca vitulina) Behavior: Dose-Response Relationship and Frequency Weighting

AbstractTwo experiments were conducted to quantify the behavioral response of harbor seals (Phoca vitu-lina) to impulsive underwater sounds as produced during impact pile driving for offshore wind tur-bines. In Experiment 1 (dose-response relation-ship), two female harbor seals in a quiet pool were exposed to playbacks of minimally filtered pile-driving sounds (46 strikes/min) recorded at one location in the North Sea, at seven unweighted broadband single-strike sound exposure levels (SELss) at 6 dB steps between 125 and 161 dB re 1 μPa2s. Considering the dose-response rela-tionship as expressed by the harbor seals’ posi-tion and their jumps out of the water, the onset of behavioral response occurred at 131 dB re 1 μPa2s in seal F01 and at 137 dB re 1 μPa2s in seal F02. The response was very clear in both harbor seals ≥ 143 dB re 1 μPa2s. Experiment 2 (effect of weighting) assessed whether sound levels used in predictions of harbor seal behav-ioral responses to pile-driving sounds should be frequency-weighted to reflect hearing sensitivity. The seals were exposed for 15 min to the mini-mally filtered pile-driving playback sound (PS), a low-pass filtered version (LP, filtered at 0.5 kHz), and a high-pass filtered version (HP, filtered at 4 kHz), with the same mean received unweighted broadband SELss (161 dB re 1 μPa2s). With the auditory weighting function for Phocidae in water applied, SELss were 156 (PS), 151 (LP), and 161 (HP) dB re 1 μPa2s. Both seals responded to all three pile-driving sounds but were most affected by the PS and HP sounds, showing that the high-frequency components of pile-driving playback sounds caused most of the behavioral effects. The second experiment showed that weighting of SELss is useful when setting underwater sound criteria for behavioral responses in Phocidae, as weighted SELss was a better predictor of behav-ioral response than unweighted SELss. The results indicate that the design of noise mitigation mea-sures for harbor seals should focus on the reduc-tion of the high-frequency components of impact pile-driving sounds.