High-amplitude impulsive sounds produced by pile driving and airguns may result in hearing damage in nearby seals. By swimming at the water surface, seals may reduce their exposure to underwater sound, as sound pressure levels (SPLs) are often lower just below the surface than deeper in the water column. 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 sounds is lower when swimming at the water surface (when hearing may be focused on aerial sounds) than when swimming at depth. To investigate this possibility, hearing thresholds of two female harbor seals were quantified psychophysically, while their heads were in the position normally adopted while swimming at the surface. The seals' hearing thresholds at the water surface were similar to each other and to previous measurements 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, to estimate the sound energy that reaches the seals' ears.
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