Socio-spatial ecology of Indo-Pacific humpback dolphins (Sousa chinensis) in Hong Kong and the Pearl River Estuary

Abstract

The Indo-Pacific humpback dolphin (Sousa chinensis) in the Pearl River Estuary (PRE) is threatened by intense and ever-growing human activities. Effective conservation measures for this locally endangered species are urgently needed. This study assesses the area utilisation pattern and the social dynamics of this species in Hong Kong and the eastern section of the PRE (EPRE) and provides data that may prove fundamental in making informed conservation decisions.

Boat-based surveys were conducted in Hong Kong in 2010–2015 and throughout the EPRE in 2011–2015. Analyses of the spatial distribution of dolphin encounters and behaviours suggest that humpback dolphins in the EPRE prefer natural, undisturbed, rocky shores. Spatial modelling identified six core areas within the EPRE. Core areas of primary importance in the EPRE were located around southwest Lantau Island and Lung Kwu Chau in Hong Kong and western Neilingding Island and Sanjiao Island in mainland China waters. The other two core areas, located around eastern Qi’ao Island in China and southern Macau, appeared to be of secondary usage in this region. Foraging appeared to be the key determinant of the dolphins’ overall distribution pattern, and foraging probability was affected by distance to shore, locations, season, group size and year.

Analyses of photo-identification data collected in Hong Kong revealed that humpback dolphins form multiple, closely interacting social clusters that have different core areas but overlapping ranges. The majority of dolphins in Hong Kong belong to three social clusters. Temporal association patterns among social clusters appeared to be similar, and associations between dolphins were found to be highly fluid with various levels of temporal stability. The social and spatial segregation and variability in associations between individuals are most likely driven by the fine-scale differences in the area utilisation pattern of individuals.

To investigate the behavioural responses of humpback dolphins to anthropogenic changes in the environment, this study focused on assessing the impacts of two types of human activities: construction of large-scale infrastructure and fisheries. During the construction of the Hong Kong–Zhuhai–Macau Bridge in Hong Kong, dolphins’ core areas and ranges shifted away from the construction site and towards the south of Hong Kong; some of the responses differed between social clusters. Such changes suggest the distribution and abundance of resources are key factors shaping dolphins' social structure. Fisheries’ practices were also found to influence social dynamics and movement of humpback dolphins. After the ban on trawling in Hong Kong, trawler-associating dolphins changed their residency pattern, and associations between trawler-associating and non-trawler-associating dolphins increased. These findings highlight humpback dolphins' behavioural and social plasticity in response to environmental change.

This study demonstrates the importance of understanding animal behaviour and socio-spatial ecology in formulating effective conservation measures. It reveals that the marine protected area (MPA) coverage of behaviourally important areas in Hong Kong and the EPRE are insufficient. Such inadequacy has left critical habitats vulnerable to anthropogenic disturbances. An immediate revision to the MPA design in the EPRE is crucial for the conservation of humpback dolphins in the PRE.