Biosonar activity of the Indo-Pacific humpback dolphin (Sousa chinensis) near the tunnel section of the world's longest cross-sea bridge-the Hong Kong-Zhuhai-Macao Bridge-is negatively correlated with underwater noise

Underwater noise pollution from the world's longest cross-sea bridge-the Hong Kong-Zhuhai-Macao Bridge (HZMB)-which stretches across the Chinese White Dolphin National Nature Reserve (of the People's Republic of China, PRC) in the Pearl River Estuary may affect the distribution of local humpback dolphins. In this study, static passive acoustic monitoring was applied to monitor biosonar activity of humpback dolphins and underwater noise adjacent to the tunnel section of the HZMB for more than one year. During the monitoring period, dolphin biosonar signals were detected on 88.5% of days. A significant temporal pattern occurred in dolphin biosonar activity and in anthropogenic noise. Biosonar activity was significantly higher at night than during the day, whereas underwater noise occurred more in the day than at night. Significantly more echolocation signals were detected in winter-spring than in summer-autumn, and highest acoustic activity occurred significantly more during high tide than at other tidal periods. In addition, the negative correlation between elevated underwater noise and dolphin sonar activity in winter suggests that dolphins may avoid noisy waters for short periods, perhaps due to auditory stress, but fish prey movement cannot be ruled out. These findings facilitate understanding activity patterns of humpback dolphins in the Pearl River Estuary and may contribute to conservation efforts.