Irrawaddy dolphin (Orcaella brevirostris)
Facts
| Also known as: | orcelle |
|---|
| Kingdom | Animalia |
|---|---|
| Phylum | Chordata |
| Class | Mammalia |
| Order | Cetacea |
| Family | Delphinidae |
| Genus | Orcaella (1) |
| Size |
Length: 200 – 275 cm (2) Length at birth: 96 cm (2) Weight at birth: 12.3 kg (2) |
| Weight |
90 – 200 kg (2) |
Status
The Irrawaddy dolphin is classified as Vulnerable (VU) on the IUCN Red List (1) and is listed on Appendix I of CITES (3). Geographically isolated populations in the Ayeyarwady River (Myanmar), Mahakam River (Indonesia), Malampaya Sound (Philippines), Mekong River (Lao PDR, Cambodia and Vietnam) and Songkhla Lake (India) are classified as Critically Endangered on the IUCN Red List (1).
Description
A particularly distinctive dolphin, the Irrawaddy has a rounded head with no beak and a flexible neck, causing visible creases behind the head (2). Although most closely related to the orca (4), the Irrawaddy dolphin is similar in body form to the beluga whale, but darker in colour, with a pale to dark grey back and a light underside. The dorsal fin is small, triangular and rounded, and the flippers are long and broad (2).
Range
The Irrawaddy dolphin has a patchy distribution in the shallow, coastal waters of the Indo-Pacific from the Philippines to northeastern India. Freshwater subpopulations occur in three river systems: the Mahakam of Indonesia, the Ayeyarwady (previously Irrawaddy) of Burma, and the Mekong of Laos, Cambodia and Viet Nam. It is also found in completely or partially isolated brackish water bodies such as Chika Lake in India and Songkhla Lake in Thailand (1).
Habitat
Inhabits coastal, brackish and freshwaters of the tropical and sub-tropical Indo-Pacific (2).
Biology
Believed to be reincarnated humans by some of the people of Laos (2), Irrawaddy dolphins are less active than many other dolphins with only the uppermost dorsal surface of the animal becomes visible during a slow rolling dive; they make only occasional low leaps and never bow-ride. Feeding together in groups of usually less than six, but as many as 15 (5), the Irrawaddy dolphin can dive for up to 12 minutes to feed on bony fish, crustaceans, cephalopods and fish eggs. Irrawaddy dolphins are known to spit water to herd fish, and have even been reported to stun large fish with a blow from the lower jaw, only to play with them before casting them aside (2). In some areas of Asia, fishermen consider the Irrawaddy dolphin to be a competitor for fish, but in other areas the fishermen attract them to the boat and encourage them to drive fish into the nets where the dolphins also benefit by preying on fish whose movements are confused by the nets and those that are momentarily trapped around the edges or in the mud (6). Irrawaddy dolphins communicate with clicks, creaks and buzzes (7) at a dominant frequency of about 60 kilohertz which is thought to be used for echolocation (8).
Little is known about the reproductive biology of the Irrawaddy dolphin, but it is thought to breed between April and June in the Mahakam River, and gestation is estimated at 14 months and weaning after two years (2).
This dolphin species is known to carry out daily migrations from the Semayang Lake in eastern Borneo to the Mahakam River, returning to the lake in the evening. In Indonesia, Irrawaddy dolphins move into tributaries at high water and into the main river during low water (4).
Threats
The major threat to the Irrawaddy dolphin is incidental entanglement in gillnets (5) (9), but it is not believed to be at risk of imminent extinction (2). In the Ayeyarwady River it is also at risk from electrocution and prey depletion from electric fishing (10). Other issues affecting this dolphin include increasing pollution, construction of dams and the build-up of silt and sedimentation following severe erosion (2). Fishing with explosives also results in dolphin casualties (4). Most live captures are for the oceanarium trade in Asia (4), and hunting of this species is rare, occurring only in parts of India to harvest oil for the treatment of rheumatism (2).
Conservation
Very few provisions have been made to conserve the dolphins or their habitat. The Irrawaddy dolphin is protected by law in Laos, Vietnam and Cambodia. Direct taking of cetaceans is prohibited in Bangladesh, India, Laos, Malaysia, Myanmar and Thailand. (9). In December 2005, the Department of Fisheries in Myanmar, established a protected area for Irrawaddy dolphins along a 74 kilometre segment of the Ayeyarwady River. Protective measures in the area include requiring fishermen to immediately release dolphins if found alive and entangled in their nets, and prohibiting the trade and catching or killing of dolphins and the use of electricity fishing and gill nets that obstruct the water-course, are more than 300 feet long, or spaced less than 600 feet apart (11). The problem of live captures for the aquarium trade has largely been solved by the uplisting of the species to Appendix I of the Convention on International Trade in Endangered Species (CITES) which prohibits international trade. Some captive breeding of this species has been successful (2).
Further Information
For further information on the conservation of the Irrawaddy dolphin see:
- Mekong Dolphin Conservation Project:
www.mekongdolphin.org - WDCS:
www.wdcs.org
Authentication
This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk
Beak: The bill of a bird. In cetacea (whales and dolphins): the elongated forward part of the head, comprising the lower jaw and upper jaw or ‘rostrum'.
Cephalopoda: From the Greek for ‘head-foot', a class of molluscs that occur only in marine habitats. All species have grasping tentacles, and either an internal or external shell. Includes nautiloids, cuttlefish, squids, octopuses, and extinct ammonites and belemnites.
Cetacean: Cetaceans are a group comprising all whale species, therefore including dolphins and porpoises.
Crustacea: Diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.
Dorsal fin: The unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans.
Echolocation: Detecting objects by reflected sound. Used by bats and odontocete cetaceans (toothed whales, dolphins and porpoises) for orientation and to detect and locate prey.
Gestation: The state of being pregnant; the period from conception to birth.
References
- IUCN Red List (October, 2008)
http://www.iucnredlist.org - Animal Diversity Web (December, 2004)
http://animaldiversity.ummz.umich.edu/site/accounts/information/Orcaella_brevirostris.html - CITES (December, 2004)
http://www.cites.org - Convention on Migratory Species (December, 2004)
http://www.cms.int/reports/small_cetaceans/data/O_brevirostris/O_brevirostris.htm - Kreb, D., Budiono, X. and Syachraini, X. (2007) Status and conservation of Irrawaddy dolphins Orcaella brevirostris in the Mahakam River of Indonesia. In: Smith, B.D., Shore, R.G. and Lopez, A. Eds. Status and Conservation of Freshwater Populations of Irrawaddy Dolphins. Wildlife Conservation Society, Bronx, New York.
- Smith, B.D., Thant, U.H., Lwin, J.M. and Shaw, C.D. (1997) Investigations of cetaceans in the Ayeyarwady River and northern coastal waters of Myanmar. Asian Marine Biology, 14: 173 - 194.
- Van Parijs, S.M., Parra, G.J. and Corkeron, P.J. (2000) Sounds produced by Australian Irrawaddy dolphins, Orcaella brevirostris. Journal of the Acoustical Society of America, 108 (4): 1938 - 1940.
- Kamminga, C., Wiersma, H. and Dudok van Heel, W.H. (1983) Sonar sounds in Orcaella brevirostris of the Mahakam River, East Kalimantan, Indonesia; the first descriptions of the acoustic behaviour. Aquatic Mammals, 10: 125 - 136.
- Beasley, I., Phay, S., Gilbert, M., Phothitay, C., Yim, S., Lor, K.S. and Kim, S. (2007) Status and conservation of Irrawaddy dolphins Orcaella brevirostris in the Mekong River of Vietnam, Cambodia and Laos. In: Smith, B.D., Shore, R.G. and Lopez, A. Eds. Status and Conservation of Freshwater Populations of Irrawaddy Dolphins. Wildlife Conservation Society, Bronx, New York.
- Zhang, X., Wang, D., Liu, R., Wei, Z., Hua, Y., Wang, Y., Chen, Z. and Wang, L. (2003) The Yangtze River dolphin or baiji (Lipotes vexillifer): population status and conservation issues in the Yangtze River, China. Aquatic Conservation: Marine and Freshwater Ecosystems, 13: 51 - 64.
- Smith, B.D. and Mya, T.T. (2007) Status and conservation of Irrawaddy dolphins Orcaella brevirostris in the Ayeyarwady River of Myanmar. In: Smith, B.D., Shore, R.G. and Lopez, A. Eds. Status and Conservation of Freshwater Populations of Irrawaddy Dolphins, WCS Working Paper Series 31. Wildlife Conservation Society, Bronx, NY.
