By Lifting Their Heads Above Water, Harbor Seals (Phoca vitulina) Can Reduce Their Exposure to Underwater Sounds Below 8 kHz.

High-amplitude anthropogenic sounds may result in hearing damage in marine mammals that are under water and close to the sound source. Most energy in anthropogenic sound is < 4 kHz. In the wild, harbor seals (Phoca vitulina) often rest while "bottling" with their head and ears above the water surface; this behavior may also serve as a self-mitigation method to reduce sound pressure levels (SPLs) received by their ears. We quantified the hearing sensitivity of two harbor seals for underwater sounds when they were bottling, and we simulated sound propagation near a bottling seal. We also assessed the physiological mode of hearing during hearing tests and the pathways through which sound reaches the inner ears. The head-above-water (HAW) hearing thresholds of the two seals for underwater sounds were very similar. For hearing test signals between 0.031 and 6.3 kHz, the HAW hearing thresholds were 14 to 62 dB higher than the underwater hearing thresholds of the same seals for the same sound frequencies, showing that they were able to reduce their exposure to these underwater sounds by bottling. For signals between 8 and 80 kHz, the HAW and underwater hearing thresholds were much more similar, differing by only 0 to 8 dB. Numerical simulations (< 20 kHz) were consistent with exposure level differences at the ears between HAW and submerged positions, providing theoretical background for the observations. Between 0.1 and 4 kHz, the mean corresponding aerial SPLs radiated by the underwater signals at threshold levels were similar to the theoretical masked thresholds for harbor seals (i.e., background noise spectral density level + critical ratio). The aerial hearing thresholds at 0.031 kHz (84 dB re 20 µPa) and 0.063 kHz (80 dB re 20 µPa) can be added as data points to the existing unmasked aerial audiogram of harbor seals (0.1 to 32.5 kHz). Hearing test signals < 4 kHz were probably heard via the "air-outer ear-middle ear-inner ear" pathway (aerial hearing; unmasked between 0.031 and 0.063 kHz, and masked between 0.1 and 4 kHz), and signals > 8 kHz were heard via a "water-body tissue-cochlea" pathway (underwater hearing). When estimating the effect on harbor seal hearing of high-amplitude, long-duration, continuous underwater sound (e.g., from continuous active naval sonar, offshore vibratory pile driving, or marine vibroseis) or high-amplitude, repetitive impulsive underwater sound (e.g., from offshore percussion pile driving), the seals’ ability to self-mitigate their exposure to sounds < 8 kHz, at times when they are not diving or foraging, should be taken into account. [ABSTRACT FROM AUTHOR]