Behavioral Responses of Harbor Seals (Phoca vitulina) to FaunaGuard Seal Module Sounds at Two Background Noise Levels

Abstract: To prevent permanent hearing impairment in seals, SEAMARCO and Van Oord Dredging and Marine Contractors have developed the FaunaGuard Seal Module (FG-SM), which is intended to deter seals to safe distances from high-amplitude impulsive sound sources such as offshore pile driving operations. As a first step towards testing and validating the FG-SM, a study with captive harbor seals is presented. The effects of 16 sounds (200 Hz to 20 kHz, with random inter-sound intervals of 3 to 10 s, mean interval 6.5 s, and duty cycle ~60%) produced by the FG-SM on the behavior of two harbor seals were quantified in a pool. The overall behavioral response threshold for these sounds was determined by transmitting the sounds at four sound pressure levels (SPLs) at two background noise levels resembling those occurring during Beaufort Sea States 0 and 4. Behavioral responses ranged from no reaction to increased time spent with the head above the water surface, more frequent hauling out, and increased numbers of jumps. The seals differed in their responses to the sounds: whereas seal 01 increased the time she spent with her head above the water surface as the SPL increased, seal 02 hauled out more often. Based on “jump” behaviors specifically, the mean received behavioral threshold SPL of the two seals in both background noise conditions appeared to be between 136 and 148 dB re 1 μPa (for the effect calculation, 142 dB re 1 μPa was used). No effect of the ambient noise level was observed; the level of the ambient noise at both Sea States was too low to mask the sounds of the FG-SM at the average levels the animals were exposed to in the pool. Based on the source level of the FG-SM, the mean behavioral response threshold SPL found in the present study for jumps, and two generic propagation models, the deterring effect range of the FG-SM is estimated to vary between 100 m (propagation model: 20log R) and 500 m (propagation model: 15log R). In most cases in the shallow North Sea, permanent hearing threshold shift (PTS) in harbor seals would be prevented if they moved 100 to 200 m away from the source of pile driving sounds, and, thus, the FG-SM is considered a good mitigation device.
Key Words: ADD (Acoustic Deterrent Device), AHD (Acoustic Harassment Device), and AMD (Acoustic Mitigation Device). acoustic disturbance, anthropogenic noise, FaunaGuard Seal Module, offshore pile driving, Phocid, harbor seal, Phoca vitilina, PTS
Document Type: Research Article
Page Numbers: 347-363

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