From an intensive review of historical records and the examination of stranded cetaceans it was determined that 16 cetacean species have been reported from Hong Kong, including resident populations of the lndo-Pacific hump-backed dolphin (Sousa chinensis) and the finless porpoise (Neophocaena phocaenoides).
Behavioural surveys of both species were carried out from land-based vantage points in areas of high dolphin 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.
The diet of Hong Kong’s resident cetaceans was determined by post-mortem analysis of stomach contents. Both species primarily feed on demersal and small pelagic fish. Humpbacked dolphins primarily consumed sciaenids and mugilids. Finless porpoises predominantly ingested cephalopods and the fish species, Nemipterus japonicus.
Pollution may pose a risk to Hong Kong’s resident cetaceans. Concentrations of trace metals and organochlorines were determined from stranded cetacean tissues. Trace metal concentrations were generally low. Mercury levels in older animals were, however, shown to be high and possibly of toxicological significance. Potential prey species from areas of high dolphin abundance were also tested for trace metals and concentrations were shown to be high, particularly levels of lead and cadmium.
Concentrations of the organochlorine pesticides mirex, chlordane, dieldrin and cyclohexanes were low. Generally, PCB concentrations were low, but in older animals PCBs had accumulated to potentially health-threatening levels. The concentrations of DDT and its metabolites were considerable and amongst the highest recorded from small cetaceans world-wide. High concentrations of DDT were identified from cetacean calves and milk.
The number of cetacean strandings reported in Hong Kong’s· territorial waters has increased notably in recent years. This may be partially due to an increased awareness of the need to report strandings, but it may also reflect an increase in the mortality rate. Analysis of stranded cetacean carcasses has shown contamination by sewage-borne bacteria and that a high proportion of stranded animals died as the result of anthropogenic activities, i.e., collisions with boats and entanglements in fishing gear.
The majority of Hump-backed dolphin strandings occur during the summer monsoon, possibly from an influx of animals during this period. Over a third of Finless porpoise strandings are neonates. These strandings occur between November and April, defining this period as the Finless porpoise breeding season.
Hong Kong’s cetaceans face considerable threats and these undoubtedly work synergistically to reduce their viability. With the escalating amounts of anthropogenic pollutants and increasing environmental decrepitude their long-term survival cannot be envisaged unless action is taken to conserve Hong Kong’s cetaceans.