Coastal construction projects and fishery practices affect socio-spatial behaviour of Indo-Pacific humpback dolphins (Sousa chinensis) in Hong Kong

Abstract

The ranging pattern and socio-behavioural strategies of coastal dolphins have long been suggested to be influenced by resource distribution and predictability. In Hong Kong, these are under severe anthropogenic pressure and have been so for decades. However, whilst the amount of coastal development and volume of human activities have been continuously increasing in the region, their impacts on the social dynamics of coastal dolphins have never been empirically documented. In this study, we focused on assessing the impacts of two types of human activities: a large-scale coastal construction project and coastal fisheries. Our 6-year photo-identification study indicates that in Hong Kong waters, Indo-Pacific humpback dolphins form multiple, closely interacting social clusters that have different core areas but overlapping ranges. Temporal association patterns are similar among clusters, with various levels of temporal stability and generally fluid structure. The socio-spatial differences between individuals and clusters are most likely driven by fine-scale differences in the individual pattern of habitat choice and range use. During the construction of the Hong Kong–Zhuhai–Macau Bridge, the dolphin range use pattern and their core areas in Hong Kong have shifted southwards, away from the construction site. Their pattern of movement and social grouping has also changed, with individual clusters responding differently. In response to changes in fishery practices (the territory-wide ban on trawling), trawler-associating dolphins changed their residency pattern, and the associations between trawler-associating and non-trawler-associating dolphins increased. As both types of anthropogenic impacts discussed here affect the distribution and abundance of dolphin prey, our findings support the hypothesis of habitat/resource-driven social structure of coastal/inshore dolphins. Furthermore, our data highlights the susceptibility of humpback dolphins to anthropogenic pressure. With their high-specificity to shallow-water inshore habitats and not yet fully understood but likely restricted degree of socio-behavioural plasticity, their ecological adaptability to environmental change is likely limited.