The Behavior and Ecology of the Indo-Pacific Humpback Dolphin (<I>Sousa chinensis</I>)

Abstract

The Indo-Pacific humpback dolphin (Sousa chinensis) can be divided into two morphological types: (1) west of India, plumbea-type humpback dolphins are dark gray or almost black in color, with a defined “hump” and (2) east of India, in Southeast Asia and Australia, chinensis-type humpback dolphins do not possess a “hump” and are often white or very light in color, with or without blue-gray spots and freckles. Plumbea-type humpback dolphins inhabit coastal waters, bays, and estuaries typically within 0.5 km of the coast, in waters less than 15 m deep. School sizes are small (< 25), although schools of up to 100 have been sighted off Oman. Diurnal patterns and seasonal and tide-related changes in behavior are observed, which have been attributed to changes in seawater temperature and, ultimately, the availability of prey. Feeding behavior tends to be correlated with rocky reefs and rocky shores. Social and sexual behavior, as well as births, occur year-round, but with seasonal peaks (October-May). The acoustic behavior of the plumbea-type humpback dolphin is little known although clicks of 20-25 kHz, “screams” from 3 to 20 kHz, and whistles from 3 to 25 kHz have been reported. Interactions between plumbea-type humpback dolphins and a variety of non-cetacean species have been reported, and in Zanzibar mixed groups of humpback and bottlenose dolphins are common. Plumbea-type humpback dolphins typically display aversive reactions to boat traffic. Chinensis-type humpback dolphins are primarily coastal and estuarine, almost exclusively estuarine in the northern parts of their range. Australian dolphins off the Great Barrier Reef were observed at considerable distances offshore (up to 55 km), but always close to shallow water. Inhabited water depth is usually less than 10 m. School sizes resemble those of plumbea-type humpback dolphins, although groups of up to 44 have been observed. The home ranges of individual animals are more compact and less coastal than plumbea-type humpback dolphins, varying both by season and year. Seasonal changes in distribution observed in Hong Kong are linked to changes in hydrography of the Pearl River. Diurnal and tide-related changes in behavior also have been noted. Feeding is the predominant behavior noted for chinensis-type humpback dolphins in Hong Kong, which is frequently associated with estuarine mixing zones and trawling activities. Social behavior occurs year-round, but peaks during the same period as calf conception. Calves primarily are born between January and August, with peaks in April/May and August. Epimeletic behavior has been reported in chinensis-type humpback dolphins. Chinensis-type humpback dolphins have been recorded producing whistles of between 1.2 and 16 kHz, and broadband harmonic pulses and low frequency, narrow band “grunts.” The spectra of broadband click pulses ranged from 30 to 200 kHz. The sounds produced by these humpback dolphins can be as low as 600 Hz and coincide with frequencies produced by many types of boat traffic. In Moreton Bay, Australia, humpback dolphins often are observed in mixed groups with bottlenose dolphins (Tursiops truncatus), although humpback dolphins do not associate with finless porpoises (Neophocaena phocaenoides) in Hong Kong. Associations with fishing trawlers have been noted in China and Australia. Increased dive durations as a result of increased shipping density and avoidance of high-speed vessels have been recorded in Hong Kong. In addition ship-strikes have been a documented cause of mortality in this area. Chinensis-type humpback dolphins often are present in areas of high shipping traffic densities and, thus, impacts of boat traffic on this species are a cause for concern. Despite some very detailed studies in discrete areas (e.g., South Africa and Hong Kong), little is known about the ecology and behavior of either