The number of humpback dolphins in Hong Kong waters was estimated to be about 250 (Agriculture, Fisheries and Conservation Department, 2000a; Jefferson, 2000) representing a subgroup of a larger population, probably more than 1000, which resided in the Pearl River Estuary (Jefferson & Leatherwood, 1997). Probably because of feeding demand, they seldom go beyond the boundary of the brackish water (Huang, Wen, & Tang, 1978)