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

Abstract

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.

Appendix

See Figs. A1, A2 and Table A1

Fig. A1

Photo credit: Stephen C.Y. Chan

Indo-Pacific humpback dolphin (Sousa chinensis) photographed in Hong Kong, at the eastern perimeter of the Pearl River Delta region. By volume, the estuarine system of the Pearl River is the second largest in China, and among the most productive with extensive coastal fishery; albeit also one of world’s most urbanised, industrialised, and most polluted. While the restricted inshore distribution of humpback dolphins makes them highly susceptible to the manifold anthropogenic stressors, the dolphins inhabiting the PRD waters represent the world’s largest population of this species. However, several recent studies indicate that the population is on a downward trajectory and—in the absence of effective conservation measures—unlikely to survive to the next century.

Table A1 The numbers of humpback dolphins (105 individuals seen ≥ 6 times) grouped into seven social clusters, and the mean values (and SD) of half-weight association indices (HWI)

Fig. A2

a Individual sighting frequency and b temporal distributions (throughout the study period, between 2012 and 2016) of the seven social clusters of humpback dolphins in western PRD. a Each grey bar denotes one individual, and each coloured bar represents the average of a social cluster. The mean (and SD) of the number of sightings is also listed. b Each dot represents a sighting of any individual in a social cluster along the timeline. The colour of bars and dots is the same as the node colour in Fig. 5