An unknown component of a well-known population: socio-demographic parameters of Indo-Pacific humpback dolphins ( Sousa chinensis ) at the western reaches of the Pearl River Delta region

Quantifying demographic parameters and patterns of social dynamics is fundamental to understanding the population ecology of group-living animals and carries considerable conservation implications. In the Pearl River Delta (PRD) region, one of the world’s largest, most urbanised and industrialised estuarine systems located on the southeast coast of China, Indo-Pacific humpback dolphins are under a multitude of anthropogenic pressures. While in Hong Kong, at the eastern perimeter of the region, these dolphins have been studied for many years, at the western reaches of the PRD they have received little research and conservation attention. In this study, with the application of mark-recapture and socio-demographic modelling techniques, we quantify population parameters, group dynamics and social structure, establishing socio-demographic baselines for this little known component of the longest-studied population of the genus Sousa . These dolphins live in a fluid fission–fusion society with markedly weak inter-individual affiliations. Individual ranging patterns and spatial preferences (e.g., foraging areas) appear to be among the key factors determining their grouping pattern and socio-spatial structure across the region, with several social clusters which, although discernible, especially at the peripheries of the region, frequently interact socially and overlap spatially. Currently, 914 dolphins inhabit waters of western PRD, but their estimated survival rates are below the previously estimated threshold needed for long-term survival as a viable demographic unit. These findings indicate high levels of environmental stress and raise conservation concerns, especially in the face of manifold and growing anthropogenic pressure. This study calls for comprehensive assessments of cumulative anthropogenic impacts and for a critical revision of present conservation measures.