Animal Spotlight: Irrawaddy Dolphin

Taxonomy:1913850146

  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Mammalia
  • Order: Cetartiodactyla
  • Family: Delphinidae
  • Genus: Orcaella
  • Species: Brevirostris

 

  • Weight: 250-290 lbs (114-143 kg)
  • Length: 4.9-9 ft (1.5-2.75 m)
  • Oldest Known Age: estimate of 28 years old

Irrawaddy dolphins’ skin coloration varies from slate-blue to slate-gray with a lighter underside. As a marine mammal, they are warm-blooded and homoiothermic, having a constant body temperature regardless of its surroundings.

The Irrawaddy dolphin, a river dolphin species, can be found in estuaries and mangrove areas throughout much of the tropical and subtropical Indo-Pacific region. They prefer coastal areas, particularly muddy, brackish waters at river mouths and deltas, and do not venture far offshore. Their range includes the coasts of India, Bangladesh, Myanmar, Thailand, Cambodia, Lao Peoples’ Democratic Republic, Socialist Republic of Vietnam, Malaysia, Singapore, Brunei Darrusalam, and Indonesia. Some populations are restricted to the freshwater rivers of Ayeyarwady in Myanmar, Mahakam in Indonesia, and Mekong in Vietnam, Cambodia, and Lao PDR.

Due to lack of information, scientists do not know the estimated lifespan of the Irrawaddy dolphin. Several individuals have been found dead, entangled in nets, so it is believed that Irrawaddy dolphins can live longer. However, due to small population size and difficulty in tracking the species, no further information is known regarding their average lifespan.

Physical Characteristics

The Irrawaddy dolphin is identified by a bulging forehead and a short beak. Like other river dolphins, their neck is highly flexible, which enables them to forage in shallow waters. They have 12 to 19 narrow, pointed, peg-like teeth on each side of its jaws.

The dolphin has a broadly triangular and large pectoral fins, or its flippers. It also has a small, blunt, rounded, triangular dorsal fin on the posterior end of the back. Their U-shaped blowhole is located to the left of the midline on their dorsal, upper, side and opens to the front, unlike other species.

o20brevirostris202009_wurtz
drawing by Maurizio Würtz

Irrawaddy dolphins have two anatomically unique features.

The first is that they lack a cardiac sphincter. The esophagus is surrounded at the top and bottom by two muscular rings, known as the upper esophageal sphincter, and the lower esophageal sphincter. They close the esophagus when food is not being swallowed. The lower esophageal sphincter, also called the cardiac or cardioesophageal sphincter, is the junction between the esophagus and the stomach. Without the cardiac sphincter, Irrawaddy dolphins are unable to control the food when it goes down into its stomach and feed constantly.

The second is that their stomach is subdivided into three compartments. Connected with narrow orifices or openings, the compartments are able to digest the large amounts of prey the dolphins feed on.

Diet

Irrawaddy dolphins eat a diet of fish, fish eggs, crustaceans, and cephalopods. It is believed that two species of cyprinid fish, Cirrihiunus siamensis and Paralaubuca typus, are important food sources for subpopulations in northeastern Cambodia and Laos. Carp is also a primary species that is consumed by other subpopulations.

Irrawaddy dolphins display a unique behavior of expelling a stream of water up to 1.5 m. It is believed to be used to herd fish.

Conservation/Subpopulations

orcaella_brevirostris_range_mapOnce listed as data deficient in 1996, a great deal of new information has become available of the species. The International Union for Conservation of Nature (IUCN) lists the Irrawaddy dolphin, as a whole, as vulnerable. However, it also identifies five subpopulations of the species as critically endangered; the only exception is the Bangladesh subpopulation. There is no estimated worldwide count.

The Ayeyarwady River subpopulation is estimated to have about 59 individuals. A quantitative estimate of the population trend cannot be made from data from various surveys, but there is clear evidence that there was a major reduction of occurrence and range. It appears to be that the population has declined by nearly 60% since the 19th century. The earliest reference to these dolphins in the Ayeyarwady River is from the New T’ang History which mentions trade in ‘river pigs’ among the Pyu people.

The Mahakam River subpopulation is estimated to have about 31-42 mature individuals. A 2005 survey best estimates a total population size between 67 to 70 dolphins. Very little is known about this subpopulation.

The Malampaya Sound subpopulation is the only known subpopulation in the Philippines and the nearest other known subpopulation is in northern Borneo. Surveys conducted in 2001 estimated that at best the population consists of 77 individuals. However, it is plausible that numbers may be down to just 45. This species occurs only in the inner portion of the sound, an area of about 134 km2

The Mekong River subpopulation is estimated to be of 69 individuals, based off a 2003 survey. They inhabit a 190 km stretch of the Mekong River between Cambodia/Lao PDR border at Khone Falls and Kratie, Vietnam. Dolphins previously inhabited Tonle Sap but have been extirpated and driven out to the Mekong through a channel connecting the lake to the river. The first published record of the species in the Mekong was from a diary of a 19th century explorer, Mouhout.

The Songkhla Lake subpopulation in Thailand is estimated to be fewer than 50 mature individuals. Surveys conducted in 2001 and 2002 reported extremely low numbers of groups detected. In 2003 however, there was an absence of sightings. These dolphins were first recorded by Pilleri and Gihr (1974) who examined three stranded specimens.

The Bangladesh subpopulation in the Sundarbans mangrove forest near Bay of Bengal was recently discovered in 2009, giving hope to conservationists that the species could recover. Approximately 6,000 individuals were found and is the largest subpopulation found.

Threats

Though the Irrawaddy dolphin is not hunted, it is indirectly threatened due to bycatch, the accidental capture of aquatic animals in fishing gear, and habitat degradation. There have been observations and reports of deaths in drifting gillnets. At least 30% reduction in the range-wide population size is suspected over a period of 45 to 48 years. This is based on increasing levels of bycatch and habitat degradation in recent years.

The threat of gill net entanglement occurs primarily during the dry season (December to May) when Irrawaddy dolphins settle in deep water pools.

large_t_1353001733

Deforestation and gold, sand, and gravel mining cause major changes to the geomorphologic and hydraulic features of rivers and lakes where the dolphins reside. Increased sedimentation resulting from deforestation of surrounding watersheds has resulted in declining water depths in Songkhla, Chilika, and Semayang Lakes.

Overfishing is also a concern for the dolphins’ survival. Populations in Chilka Lake in India is said to be declining because of reduction in food supply. Reduction of fish populations in Indonesian rivers by illegal fishing methods is a serious threat to the dolphin population there. In Myanmar, there are constant reports that fishermen deplete the fish populations in recent years due to illegal electric fishing. It was only recently that illegal fishing methods such as dynamite and electric fishing are eliminated.

Other threats are collisions with boats, injuries from boat propellers, and uncontrolled tourism interfering with dolphins’ feeding and social activities.

Interesting Facts

  • Genetically, the Irrawaddy dolphin is closely related to the killer whale/orca
  • The scientific name brevirostris comes from Latin meaning “short-beaked”
  • Their teeth are only 1 centimeters long
  • In 2005, genetic research separated the Irrawaddy dolphin from the Australian Snub-fin dolphin (Orcaella heinsohni)
  • Irrawaddy dolphins are not notably active as they are never observed to bow-ride, or leap from the water. Their tails are rarely seen.
  • They can breathe at intervals of 70 to 150 seconds.
  • No more than 10 animals to a group usually, but solitary individuals are rarely seen
  • Some fishermen are known to tap on the side of the boat to get the dolphins’ attention. Then the dolphins will swim in circles to gather up the schools of fish, allowing fishermen to capture the fish while the dolphins feed on the stragglers.
  • Fishermen have reported seeing the dolphins stun large fish and play with them before eating
  • The Irrawaddy dolphin is regarded as a sacred animal and revered by local people in many areas of Asia including the Khmer and Lao people

 

References + For More Reading

WWF: Irawaddy Dolphins

Preventing Bycatch of Irrawaddy Dolphins in the Mekong River

Irrawaddy Dolphins

Orcaella brevirostris: Irrawaddy dolphin

Arkive: Irrawaddy Dolphins

IUCN: Orcaella brevirostris IUCN: Orcaella brevirostris

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