Habitat utilization and recommendations for conservation of Indo-Pacific humpback dolphin along northern Hong Kong-Zhuhai-Macao Bridge

To understand the utilization of the habitat in the waters along the Hong Kong-Zhuhai-Macao Bridge by Indo-Pacific humpback dolphin (Sousa chinensis) and the potential migration pathways for S. chinensis to cross the bridge. We set four fixed passive acoustic monitoring points (A, B, C, D) along the northern waters of the Hong Kong-Zhuhai-Macao Bridge, and recorded a total of 2 947 hours of acoustic data in November 2020 and March 2021, respectively. The probabilities of detecting acoustic events in S. chinensis at four monitoring points are as follows: 15.31% (A), 17.30% (B), 8.97% (C), 5.85% (D), which indicates that the waters along the northern of the bridge is an important habitat for S. chinensis with spatiotemporal differences. There is no significant diurnal and nocturnal difference in acoustic events of S. chinensis observed at different monitoring sites. The probabilities of detecting S. chinensis at low tide levels of monitoring sites A, B, and C on bridge structures were 1.56%, 1.82% and 0.78%, respectively, all higher than high tide levels (0.94%, 1.56% and 0.26%). The probability of discovering S. chinensis during the high tide and low tide stages at monitoring sites A and B was similar, but that during the high tide stage at points C and D was 4.55% and 3.64%, both higher than during the low tide stage (3.12% and 1.30%), indicating that tidal changes have a certain impact on the habitat selection of S. chinensis in bridge structure waters with spatial differences. The result also indicates that S. chinensis may prefer the area between A and B sites to cross the bridge than the other area. Inspection and management of its passageway should be strengthened.