A series of behavioural surveys were conducted in Hong Kong upon resident populations of Indo-Pacific hump-backed dolphins (Sousa chinensis) and finless porpoises (Neophocaena phocaenoides). The surveys were carried out from land-based vantage points in areas of known cetacean abundance. Hump-backed dolphins were present year-round in the waters to the north of Lantau Island and to the south during the summer monsoon season. Seasonal changes in abundance were significantly correlated with water temperature (positively) and salinity (negatively). Changes in the outflow of the Pearl River influence hump-backed dolphin distribution in the western waters of Hong Kong.
Finless porpoises were only sighted south of Lantau Island and were more frequently observed during the winter. Their abundance was correlated with water temperature (negatively) and salinity (positively) and also with the number of reported neonatal porpoise strandings. Seasonal distribution appears to be linked with reproductive cycles and hydrography. Diurnal patterns and tidal state affect the abundance of both species.