Irrawaddy dolphin

The Irrawaddy Dolphin (Orcaella brevirostris) is a species of dolphin found near coasts and in estuaries in parts of south-east Asia.

Taxonomy

The Irrawaddy Dolphin was identified by Sir Richard Owen in 1866 and is one of two species in its genus (the other being the newly discovered Australian Snubfin Dolphin). It is similar to the beluga in appearance. It has sometimes been listed in either a family containing just itself and in Monodontidae and Delphinapteridae. Nowadays there is widespread agreement to list it in the Delphinidae family, a family of which the name was simply produced by merging the words Monodontidae and Delphinapteridae.

Genetically the Irrawaddy Dolphin is closely related to the Orca. The species name brevirostris comes from the Latin meaning short-beaked. In 2005 genetic analysis showed that the Australian Snubfin Dolphin found at the coast of northern Australia forms a second species in the Orcaella genus.

Physical description

This species has a large melon and a blunt, rounded head. Its beak is indistinct. The dorsal fin is short, blunt and triangular. It is located about two-thirds of the way along the back. The flippers are long and broad. It is lightly coloured all over - slightly more white on the underside than the back. It appears whiter than it actually is when set against the background of a muddy river.

Length is about 1 m at birth and 2.3 m at full maturity. Birth weight is about 10 kg. Adult weight exceeds 130 kg. Lifespan is about 30 years.

The Irrawaddy Dolphin is a slow swimmer. It surfaces in a rolling fashion and lifts its tail fluke clear of the water for a deep dive only. Irrawaddy Dolphins spit streams of water from their mouths while spyhopping. Dolphins of the species kept in captivity have been trained to do this on demand.

Population and distribution

Although sometimes called the Irrawaddy River Dolphin, it is not a true river dolphin but an oceanic dolphin that lives near coasts and enters rivers, including the Ganges and the Mekong as well as the Ayeyarwady River (Irrawaddy River) from which it takes its name. Its range extends from the Bay of Bengal to New Guinea and northern Australia.

Human interaction

On account of their coastal nature Irrawaddy Dolphins are more susceptible to interference than other river dolphins. The most direct threat is the capturing of Irrawaddys for their oil. As an endangered species, they are legally protected from hunting, however, enforcement may be poor along tens of thousands of miles of coast line. Entanglements in gillnets and deaths injury due to explosives used in fishing are common in Vietnam and Thailand. All along its distribution population and other habitat degradation worries conservationists. Human influence such as nets crossing river channels restrict movement and isolate populations, causing them to decline. The population in India's Chilka Lake, which has suffered from entanglement in fisher's gill nets and drag nets, is believed to have dwindled to as few as 50 individuals.

Irrawaddys are also taken to perform in aquariums. Though this practice is less common than it used to be, it still has a significant local impact.

The IUCN lists several populations, including those in the Mahakam River and Malampaya Sound, as critically endangered.

References

Database entry includes a lengthy justification of why this species is listed as data deficient
National Audubon Society Guide to Marine Mammals of the World ISBN 0375411410
Encyclopedia of Marine Mammals ISBN 0125513402
Whales, Dolphins and Porpoises, Mark Carwardine, ISBN 0751327816
IUCN Red list entry for Irrawaddy Dolphins

External Links

WWF Irrawaddy page
ARKive - images and movies of the Irrawaddy dolphin (Orcaella brevirostris)
National geographic Gold mining, nets endanger dolphin (contains photo)

Toothed whales