Species:

Indo-Pacific Humpback Dolphin

Indian Ocean Bottlenose Dolphin

Irrawaddy Dolphin

Finless Porpoise

Dugong

 

 

 

Irrawaddy Dolphin (Orcaella brevirostris)

Description

The Irrawaddy is slightly smaller than the humpback dolphin, growing to about 2.6m in length (130kg). They are grey in colour with a white underside. The head is rounded and they don’t have a prominent beak like many other dolphin species. There is an indentation behind the head forming a distinct neck crease and they are able to rotate their head. The dorsal fin is small and positioned about two-thirds along the back. They resemble the beluga whale in appearance but morphological and genetic studies confirm they are a member of the dolphin (delphinidae) family and its been suggested the killer whale may be its closest relative (Arnold, 2002).

Distribution:

Mostly found in the coastal waters of Southeast Asia and Australasia including India, Bay of Bengal, Thailand, Philippines, Sabah and parts of Australia. They are known to inhabit both salt and fresh waters, with populations found in the Ganges, Mekong and Sekong rivers ( Marsh et al. 1989; Jefferson et al. 1993; Rice, 1998; Baird and Mounsouphom, 1997, Smith et al. 1997 ) . There are freshwater populations found 2,300 km upstream in the Irrawaddy River.

Distribution of O. brevirostris, produced by CMS/GROMS, Modified from Amano 2002.

 

Behaviour:

Surfacing in unobtrusive and they perform a slow, low roll showing little of the back. This combined with the small dorsal fin makes them very difficult to spot in the wild. They may raise their flukes and slap the water and they have been known to spyhop. They are a slow swimmer but have been observed breaching on occasion although they are not know to bow-ride (Jefferson et al 1993). They are occasionally seen in the same areas as bottlenose dolphins and Indo-Pacific humpback dolphins.

They seem to prefer coastal waters, particularly river mouths and do not appear to venture offshore with the majority of sightings occurring within a few kilometres of the coast. Some populations appear to be restricted to fresh water.

Group sizes of less than 6 are common although groups of up to 15 have been reported on occasion (Marsh et al., 1989; Jefferson et al., 1993). Irrawaddy dolphins have a generalized diet, feeding mainly on fish but they also like squid and crustaceans. They have been observed spitting water while feeding, apparently to herd fish (Marsh et al 1989; Reyes 1991; Jefferson et al 1993) and cooperative hunting has been reported in the Irrawaddy River.

All coastal species are subject to pressure from human activities but riverine populations are particularly sensitive to development. The Irrawaddy dolphin is listed as “data deficient” by the IUCN and the IWC have stated that more research is needed on the behaviour and distribution of this species.