Effects of anthropogenic noise on haul-out numbers of harbor seals (Phoca vitulina)

Haul-out sites of harbor seals, Phoca vitulina Linnaeus, 1758, include areas with high levels of anthropogenic noise. In some cases, seals haul out at night when there are lower in-air noise levels. However, it is unclear if there are additional responses to noise pollution. To determine potential impacts of anthropogenic noise on haul-out behavior, we compared numbers of hauled-out harbor seals relative to in-air noise levels at two sites in Washington State, USA —one close to human activities (Bellingham waterfront) and one more distant (Semiahmoo marina)— between July 2020 and August 2021. We used Generalized Linear Mixed Models to identify predictors of seal numbers. The marina had lower mean noise levels than the waterfront (39.7 ± SD 6.1 dB, n = 29 observations versus 51.2 ± SD 5.2 dB, n = 126 observations). The plotted model prediction showed a significantly negative association between noise and seals at the marina, and no association was found at the waterfront. Results indicate that in-air noise levels may influence seal numbers at sites where human activities are low. They also suggest that, besides hauling out at night, seals may become tolerant to in-air noise levels at sites where human activities are high.