Examining environmental factors behind the declining occurrence of dolphins in Hong Kong waters using multivariate statistical methods

The decline of Indo-Pacific humpback dolphins observed in Hong Kong is a microcosm of cetacean population changes resulting from broader global issues, including habitat degradation, pollution, and unsustainable fishing practices. Identifying and prioritizing the natural and anthropogenic pressures on the species is crucial for effective population conservation. The loss of coastal habitats due to reclamation projects, such as those in Hong Kong, is a significant concern as it disrupts the dolphins' natural environment and displaces them from their traditional feeding and breeding grounds. This habitat loss, coupled with other anthropogenic stressors, has contributed to population declines in various regions, including the Eastern Taiwan Strait, Xiamen, and the Pearl River Estuary. This study investigates the decline of Indo-Pacific humpback dolphins (Sousa chinensis) in Hong Kong, where their numbers have dropped from 158 in 2003 to 37 in 2020. By analyzing dolphin density per survey effort (DPSE) and various environmental variables from 1996 to 2020, we identified several key factors influencing dolphin presence. Temperature, salinity, fish production, suspended solids, nitrogen, E. coli, seawater clarity, and high-speed ferry traffic all play significant roles. Reclamation activities in Hong Kong, in particular, have a substantial impact on dolphin habitats and contribute to their abundance dropped by over 80 % in the past 17 years. Despite the establishment of new marine protected areas, it is insufficient to prevent dolphin decline. This study highlights the need for comprehensive conservation measures, including the preservation of critical habitats and the reduction of human-induced stressors, to ensure the survival of Indo-Pacific humpback dolphins in Hong Kong and beyond.