Abstract
High amplitude impulsive sounds produced by pile driving and airguns may result in hearing damage in nearby seals. By swimming close to the surface, seals may reduce their exposure to underwater sound, as sound pressure levels (SPLs) are often lower just below the surface. There is evidence that seals can make physiological adjustments such that they can switch between having maximum sensitivity for either aerial or underwater sounds. This could mean that hearing sensitivity for underwater sound might be reduced when swimming at the water surface (when hearing might be focused on aerial sounds) compared to when swimming at depth, which - if confirmed - might reduce the perceived sound level still further. To investigate this possibility, hearing thresholds of two female harbor seals for 11 narrow-band sweeps (center frequency range: 0.125 – 63 kHz) were quantified psychophysically, while their heads were in the position normally adopted while swimming at the surface. The seals’ water surface hearing thresholds were similar to each other (mean difference = 2.3 ± 2.2 dB) and to previous measurements (mean difference of 3.5 ± 2.8 dB) made at 1 m depth. When calculating the cumulative sound exposure level for hearing damage assessment, the SPL just below the water surface needs to be measured, or modeled, and the proportion of time seals normally swim at the water surface needs to be estimated, in order to estimate the sound energy that reaches the seals’ ears.