Preliminary application of integrated visual and passive acoustic techniques in behavioral monitoring of Indo-Pacific humpback dolphin (Sousa chinensis)

Cetaceans are important indicator species of aquatic ecosystem health and stability, and the establishment of an integrated, multi-method monitoring framework is essential for their scientific study and conservation. In this study, Indo-Pacific humpback dolphins (Sousa chinensis) in the Jiangmen coastal waters were investigated using a combination of boat-based visual surveys and passive acoustic monitoring to examine their behavioral patterns from multiple perspectives. Four survey cruises were conducted in January, March, July, and September 2025, during which a total of 21 dolphin groups were recorded. Group sizes ranged from 2 to 40 individuals, and dolphins were observed in waters with depths of 3.9–15.2 m. Visual observations indicate that surface behaviors included foraging, socializing, resting at the surface, fast traveling, porpoising, breaching with splashing, and other behaviors (e.g., foraging in association with fishing vessels), with foraging being the most frequently recorded behavior (14 occurrences). Passive acoustic monitoring successfully identified four types of vocalizations: clicks, buzzes, burst pulses, and whistles, among which clicks predominated (949 detections, accounting for 68.2% of all signals). Comparative analyses show that visual behaviors and acoustic signal usage exhibit consistent seasonal patterns: foraging dominates in winter, remains prevalent in spring and summer with increased social activity, and social behaviors become more prominent in autumn. These patterns indicate that the Jiangmen coastal waters provide integrated ecological functions supporting foraging, reproduction, and social communication of humpback dolphins. Fisher's exact tests revealed no significant associations between surface behaviors and underwater acoustic signal characteristics (p>0.05), highlighting the complementary nature of visual and acoustic approaches in capturing different dimensions of dolphin activity. This study demonstrates that the integration of visual surveys and passive acoustic monitoring not only provides a more comprehensive and multidimensional understanding of cetacean behavioral ecology, but also offers an effective framework for cetacean monitoring and conservation practices.