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

Résumé fourni par la source

Two experiments were conducted to quantify the behavioral response of harbor seals (Phoca vitulina) to impulsive underwater sounds as produced during impact pile driving for offshore wind turbines. In Experiment 1 (dose-response relationship), 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 relationship as expressed by the harbor seals’ position 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 behavioral responses to pile-driving sounds should be frequency-weighted to reflect hearing sensitivity. The seals were exposed for 15 min to the minimally 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 behavioral response than unweighted SELss. The results indicate that the design of noise mitigation measures for harbor seals should focus on the reduction of the high-frequency components of impact pile-driving sounds.

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