Orcaella brevirostris, Irrawaddy Dolphin [+]

Kingdom Animalia Phylum Chordata Class Mammalia Order Cetacea Family Platanistidae Genus Orcaella species: Orcaella brevirostris [+ITIS +WoRMS]

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Description & Behavior

Irrawaddy dolphins, Orcaella brevirostris (Owen in Gray, 1866), aka Mekong or Mahakam River dolphin, reach 2-2.75 m in length. The color of the Irrawaddy dolphin is dark blue to dark gray ranging to pale gray with a pale ventral side. They have a high rounded forehead and no beak. The U-shaped blowhole is located to the left of the midline on the dorsal side and opens to the front, unlike other species. They have a small, blunt, rounded triangular dorsal fin and large flippers. Like other river dolphins the neck is highly flexible, which is likely to enable them to forage in shallow water. They have narrow, pointed, peg-like teeth about 1 cm in length in both the upper and lower jaw. Two unique anatomical features of the Irrawaddy dolphin are that it does not have a cardiac sphincter and the stomach is subdivided into compartments. They are not acrobatic dolphins and are not known to bowride; however they have been observed leaping from the water.

Irrawaddy dolphins dive for an average of 30-60 seconds but longer dives have been recorded up to 12 minutes. Although they are generally slow swimmers, a swim rate of 25 kph was recorded for one individual. This species is usually seen in small groups consisting of <6 animals, but groups of between 10-15 have been reported. This species spends most of its time feeding. They display a unique behavior of expelling a stream of water up to 1.5 m thought to be used to herd fish. Fishermen have reported Irrawaddy river dolphins stunning large fish then playing with them before the fish is consumed.

Irrawaddy dolphins are not thought to communicate through vocal signals.

World Range & Habitat

The Irrawaddy dolphin, Orcaella brevirostris, is found in the coastal waters and large rivers of Southeast Asia, Northern Australia, and Papua New Guinea. They prefer coastal, brackish, and fresh water of the tropical and sub-tropical Indo-Pacific.

» GBIF occurrence data in Google Earth [Tips] | Ocean Biogeographic Information System (OBIS) (World Distribution) [about]

Feeding Behavior (Ecology)

Irrawaddy dolphins, Orcaella brevirostris, feed on fishes, crustaceans, cephalopods, and fish eggs. Two species of cyprinid fish, Cirrihinus siamensis and Paralaubuca typus, are thought to be important food sources for Irrawaddy dolphins found in northeastern Cambodia and Laos. Carp is the primary species consumed in other areas.

Life History

Little is known about the reproductive habits of Irrawaddy dolphins. It is thought that sexual maturity is reach around 4-6 years old. The mating season is believed to occur between April-June in the Semayang Lake/Mahakam River area of Kalimantan, Indonesia based on the birth of calves born in captivity in Jakarta between July-December. The gestation period is estimated at 14 months. Newborns measured following birth in captivity measured 96 cm long and weighed 12.3 kg. During its first 7 months, the calf increased in length by 57 cm (59%) and in weight by 32.7 kg (266%). The calf was nursed for about 2 years, although it began consuming fish at 6 months.

Conservation Status/Additional Comments

Irrawaddy dolphins are reported to work with fishermen by herding fish into their nets. In Burma, Irrawaddy dolphins were observed responding to fishermen who tapped on the sides of their boats signaling the dolphins to begin herding fish into the nets. In return, the fisherman share their catch with the dolphins.

Irrawaddy dolphins are listed generally as Vulnerable (VU A4cd) and are not thought to be threatened or endangered except for the Ayeyarwady River, Mahakam River, Malampaya Sound, Mekong River, and Songkhla Lake subpopulations which are listed as Critically Endangered (CR); however additional research to gather baseline population data is needed throughout it's range. Like the Indus river dolphin, the Irrawaddy dolphin is affected by human activity including pollution and the construction of barrages or dams, and silt build up. They have also been caught in anti-shark gillnets in Australia and have become entangled in fishing gear in other areas.

Vanishing Act- Is it too late for the Irrawaddy dolphin? - The Plankton Forums (July 9, 2008)