Indo-Pacific humpback dolphin
Indo-Pacific humpback dolphin (Sousa chinensis)
The Genus Sousa can be divided into several species or sub-species, depending on author (Ross et al., 1994; Rice, 1998; Jefferson, 2000; Jefferson & Karczmarski, 2001). These distinctions are based on a variety of factors including number of teeth, number of vertebrae, dorsal fin shape and body colour. Originally, Sousa was divided into three species: the Atlantic humpback (S. teuszi), the Indian humpback (S. plumbea) and the Pacific humpback (S. chinensis). Other authors considered the Indian and Pacific dolphins to be part of the same species (the Indo-Pacific humpback, S. chinensis), and this is the view presently adopted by the International Whaling Commission.
The Indo-Pacific humpback can be further divided into two morphological types depending on location. Those west of India towards South Africa have a defined hump and are dark grey in colour while those east of India, through southeast Asia towards Australia are often much paler in colour (often white or pink) and may have blue-grey spots. Their hump is also less pronounced than in those found to the west of India (Jefferson & Karczmarski 2001; Ross et al., 1995).
Photo by Dr Thomas A.Jefferson taken in Hong Kong
The humpback dolphins around Singapore are part of the S. chinensis species and they are pink. When born they are dark grey in colour and look very like a bottlenose dolphin but as they mature their colour changes and the hump on the back becomes more prominent. The dorsal fin is small and triangular and positioned in the centre of the back. They grow to about 2.5-2.8m long and can weigh up to 260kg.
S. Chinensis are found throughout coastal waters of the western Pacific including southern China, Taiwan, Thailand, Borneo, Singapore, Philippines, Indonesia, Vietnam and Australia (Rice 1998; Jefferson & Karczmarski, 2001).
Distribution of S. chinensis, produced by CMS/GROMS, modified from Jefferson et al, 1993 & Rice, 1998.
S. teuszii ranges on the coast of West Africa from Dakhla in Western Sahara south to the Arquipélago dos Bijagós in Guinea-Bissau, and also in Nigeria and Cameroon (Culik 2004; published assertions that it ranges to Angola are purely conjectural; Rice, 1998).
Humpback dolphins are quite shy and they tend to avoid boats, often re-appearing some distance away if approached (Carwardine, 1995; Ross et al., 1994). Unlike with bottlenose dolphins, the long beak is often exposed when the animal surfaces and they strongly arch the back and raise the tail flukes when diving. They are sometimes associated with bottlenose dolphins and to a lesser extent, finless porpoise and spinner dolphins.
They are rarely found more than a few kilometres from shore and prefer coastlines containing mangrove swamps, lagoons and estuaries as well as reefs, sand and mud banks. They sometimes enter rivers, although usually stay within the tidal range. They prefer waters less than 25m deep and are typically found within the surf zone (Carwardine, 1995; Ross, 2002; Parsons, 2004). Not surprisingly, their prey tends to be coastal, reef dwelling or estuarine species (Parsons 2004). Their diet consists mainly of fish with some cephalopods and crustaceans taken as well. In India and Hong Kong, feeding has been observed in the area where fresh and salt-water mix and they have been known to follow fishing trawlers in pursuit of food (Parsons 1998; Jefferson, 2000).
Humpback dolphin school size tends to be small. Those recorded from around South Africa generally contain around a dozen animals and single animals are often sighted (Karczmarski 1999; Parsons 2004). Sightings to the west of India tend to be of single animals or pairs (Huang & Chou, 1995; Jefferson, 2000; Parsons 1998). Humpback dolphins are reported around Hong Kong throughout the year but their abundance varies with season, with larger numbers being sighted during the summer (Parsons 1998). This coincided with changes in water temperature and salinity. Larger group sizes (50 plus) seem to be restricted to the Arabian region (Parsons, 2004).
Humpback dolphins have been highlighted as being at risk by several international organizations, including the International Union for the Conservation of Nature and Natural Resources (IUCN) (Perrin, 1989; Reeves & Leatherwood, 1994; Reeves et al., 2003) and the International Whaling Commission (IWC: 2003).
Illustration Jefferson, T.A., Webber, M.A. & Pitman, R.L. (2008). Marine Mammals of the World: A Comprehensive Guide to their Identification. (1st Ed). Academic Press.