Uncovering an unknown habitat hotspot for a well-studied population: Integrating real-time acoustic and visual surveys for Indo-Pacific humpback dolphins in the Pearl River Estuary by Quan Xie instant download

Uncovering an unknown habitat hotspot for a well-studied population: Integrating real-time acoustic and visual surveys for Indo-Pacific humpback dolphins in the Pearl River Estuary by Quan Xie instant download

Estuaries are among the most productive marine ecosystems, characterized by complex oceanographic processes that often determine the distribution and habitat use of marine mammals. The Pearl River Estuary (PRE) hosts the largest known population of Indo-Pacific humpback dolphins (Sousa chinensis) worldwide. Despite urgent conservation concerns, effective survey methods for this wide-spread population are still lacking due to the limitations of conventional visual observations, while ship-based passive acoustic monitoring (PAM) is hindered by the shallow water and hydrodynamic environment. This study explores the use of integrating real-time towed PAM with visual surveys to investigate the habitat use pattern of S. chinensis along the West Shoal of the PRE. Over 12 surveys conducted between 2022 and 2024, primarily during the dry season, 52 encounters were recorded: 36.5 % were detected acoustically, 9.6 % visually, and 53.8 % by both methods. A total of 7853 acoustic detections were documented with a 94 % accuracy, classified as 7118 click trains, 406 whistles, 258 buzzes, and 71 burst-pulsed sounds. Compared to visual methods, the towed real-time PAM system detected encounters earlier and more frequently, identifying significantly more foraging and socializing behaviors defined by different types of acoustic signals. Spatial analyses of the combined visual and acoustic data revealed that foraging and socializing habitats are concentrated along waterways near Qi'ao Island, with its complex topography and oceanographic features. These findings highlight the feasibility and necessity of implementing towed real-time PAM for detecting and monitoring underwater dolphin behaviors in nearshore waters, offering novel insights into the habitat use of PRE dolphins during the dry season.