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(Osbeck, 1765) - Indo-Pacific hump-backed dolphin

Distinctive Characteristics

Dolphins of this highly variable species are characterized by a robust body with a long, well-defined beak. In most populations (especially those off southern Africa), the dorsal fin sits on a hump, or ridge, in the middle of the animal's back. In others the ridge appears to be absent, or less well-developed. In most areas, there also appear to be well-developed ridges on the tail stock. Males are larger and have more exaggerated ridges on the back and tail stock.

The colour pattern varies with age and area. In most regions, light coloured calves darkenwith age to become dark lead grey above and light grey below. However, off Malaysia and northern Australia calves and adults are nearly white. In the western Indian Ocean and off China dark calves lighten with age. In the latter case, adults are pinkish white with spots and blotches.

There are 29 to 38 teeth in each tooth row.

Can be confused with

Hump-backed dolphins are most likely to be confused with bottlenose dolphins. Differences in dorsal-fin shape (including presence of the hump on many hump-backed dolphins), head shape, and colour can be used to distinguish between the two. Also, humpbacks tend to surface differently, pausing at the top of their roll.

Size

Maximum known body sizes are 3.2 m (males) and 2.5 m (females). Weights of up to 284 kg have been recorded. Newborns appear to be around 1 m in length.

Geographical Distribution

Indo-Pacific hump-backed dolphins are found from northern Australia and southern China in the east, through Indonesia, and around the coastal rim of the Indian Ocean to southern Africa. They are inhabitants of tropical to warm temperate coastal waters and they enter rivers, estuaries, and mangroves.

Biology and Behaviour

Groups tend to contain fewer than 10 individuals, thoughsome contain up to 30. Group structure has been studied using photo-identification techniques and off South Africa, some herds are stable. Also, off South Africa, where these dolphins have been most thoroughly studied, groups preferentially use sandy bays for resting and socializing, and open rocky coastline for foraging. Herds often patrol slowly parallel to shore. They are moderately acrobatic, but do not often bowride.

Mating and calving occur all year, at least in South Africa, but there appears to be a calving peak in summer.

Feeding is primarily on nearshore, estuarine, and reef fish.

Exploitation

In the northwest Indian Ocean, at least, some direct catches for human consumption and for oil are known. Incidental captures in fishing nets are known or suspected throughout the range. Catches occur in antishark gillnets off southeast Africa and off eastern Australia. Mortality off South Africa has been implicated as a population threat there. Mangrove habitat degradation may also present a threat to this species.

IUCN Status

Insufficiently known.

Indo-Pacific hump-backed dolphin (Sousa chinensis)