Irrawaddy dolphin

(Orcaella brevirostris- freshwater populations)

Common and local names: Ayeyarwady river dolphin, Lin Paing in Myanmar, Mahakam river dolphin and Pesut in Indonesia, and Mekong dolphin in Cambodia.


IUCN Red List Conservation status: Critically Endangered

The Irrawaddy dolphin is found in marine, brackish, and freshwater environments throughout southeast Asia. The name is derived from the first specimens that were described from the Ayeyarwady river in Myanmar. Although the species as a whole is considered endangered in every habitat throughout its range, the freshwater populations are considered to be at greatest risk.  It is the three riverine populations that are focused on here.

Distribution and habitat

Small isolated freshwater populations of Irrawaddy dolphins are found in the Ayeyarwady (Myanmar), Mahakam (Indonesia) and the Mekong (Cambodia) rivers. In the Mahakam, the species is found in the area from about 180km above the river mouth to 600km upstream, ranging into some tributaries and lakes1. In the Mekong, dolphins inhabit a 190km long stretch from Kratie (about 500km upstream of the river mouth in Vietnam) to slightly upstream of the Laos/Cambodia border at Khone Falls (or Lee Pee). In Myanmar, the dolphins are also restricted to an inland segment of the Ayeyarwady River distant from the rivermouth itself. Where habitat use has been studied, dolphins are associated with deep (seasonal) pools at confluences and above and below rapids 1, 2.

Ayeyarwady (Myanmar)

Mahakam (Indonesia)

Mekong (Cambodia)


There are fewer than 250 freshwater Irrawaddy dolphins left in the three river basins where they are found. The Mekong population declined from an estimated 200 dolphins in 19973 to 127 in 2005, 93 in 20074, 85 in 20105 and 80 in 20156. Since 2007, populations have appeared to stabilise; the most recent estimate in 2020 is 89 individuals (with 95% confidence interval: 78-102 individuals)7. The number of Irrawaddy dolphins in the Ayeyarwady river in Myanmar has been estimated to be between 59 and 72 individuals8. The population trend is not known, but 42 deaths were recorded between 2002 and 20169. The Mahakam River population was estimated at 79-81 individuals in 201922, and was therefore stable in relation to previous estimates from 200810.


River guard with illegal fishing net in the Mekong river. © Thomas Cristofoletti / WWF-UK

Entanglement in gillnets poses the most serious threat to all three riverine populations, and illegal fishing methods such as electrofishing are also documented in these rivers11, as is contamination due to gold mining, particularly in the Ayeyarwady River. In the Mahakam River, gillnets were determined to be responsible for 66% of deaths with a known cause, while boat collisions were thought to be responsible for 9% of mortalities. Calf mortality is high in the Mekong population, although the cause has not been identified. Long-term monitoring of the population shows that 73% of the individuals in the Mekong population are more than 20 years old, attributed to the high calf mortality7.

Habitat degradation and population fragmentation due to dam development upstream in Laos has reduced water flow in the Mekong and restricted dolphins to nine pools, with populations there ranging from 2,212 to only three individuals. Further threats to the species are caused by land use changes for palm oil plantations.  For example, the plantations have caused increased sediment load in the Mahakam, restricting the Mahakam dolphin population to the middle section of the river.  The plantations have also reduced available fish spawning areas, and caused pollution from pesticides, herbicides and fertilisers13. Mercury poisoning from gold mining and hydraulic blasting and dredging in Myanmar also pose a serious threat to the  Ayeyarwady population14.

Biology and ecology

Irrawaddy dolphins have a charismatic appeal. Their moveable lips give them expressive faces, and flexible necks allow them to move their heads in all directions. They are grey all over but lighter on the belly. Their flippers are long and paddle-shaped, with curved leading edges and rounded tips. Their tails are also large in comparison to their body length. Irrawaddy dolphins can weigh 90-200 kg and can reach lengths 180-275 cm15. Like the other river dolphins, they rely on echolocation for communication, navigation and hunting, with  Irrawaddy dolphins having a rich vocal repertoire23.  In riverine portions of the Sundarbans they produced broadband echolocation clicks with a mean source level of 195 dB (max 203 dB) re 1 µPa16. Foraging behaviour was characterised by the high movement of the animals in various directions with no obvious pattern and frequent deep dives17.

Irrawaddy dolphin. Illustration by Uko Gorter

Reproduction and growth

Female Irrawaddy dolphins are thought to become sexually mature at 7–9 years and to mate and give birth throughout the year. The gestation period lasts for about 14 months. Females usually give birth to a single calf every 2–3 years, with reports that in the Mahakam this may be longer. The estimated lifespan of Irrawaddy dolphins is estimated to be around 30 years18.


Irrawaddy dolphins eat any type of fish that are available, including prey on the bottom of riverbeds. However, they are partial to bony fish and fish eggs. The species can spit water up to 1.5 metres away in order to herd fish while hunting. They rely on a diet based primarily on fish and secondarily on crustaceans19.


Irrawaddy Dolphin (Orcaella brevirostris) spitting water. © / Roland Seitre / WWF

Irrawaddy dolphins are slow swimmers in comparison to other dolphin species ,and generally avoid boats. These dolphins typically occur in small groups of about seven individuals but can form larger groups of up to 30 individuals. They are relatively social, producing a wide range of vocalisations to communicate. Although shy around boats, they can display interesting social behaviours, including tail slaps, fin waves, breaching, and sideways swimming. They are also known to shoot jets of water out of their mouths, a behaviour unique to Irrawaddy dolphins. The ‘water-spitting’ is thought to have feeding and sexual courtship purposes15.

In Myanmar, the Irrawaddy dolphins help fishermen cast their nets. Fishermen let the dolphins know they are ready to fish by tapping the side of their boats. One or two dolphins then swim in smaller and smaller semicircles, herding the fish towards the fishermen. They then signal to the fishermen when to throw the net as the fish approach the boat. The dolphins often dive deeply just after the net is thrown and create turbulence under the surface around the outside of the net. The dolphins eat the fish whose movements are confused by the sinking net and those that are momentarily trapped around the edges of the lead line or stuck in the mud at the bottom just after the net is pulled up. This phenomenon is called “cooperative fishing” and it takes years of skill and trust building between humans and dolphins20, 21.


  1. Smith BD, Shore RG, Lopez A, eds. Status and conservation of freshwater populations of Irrawaddy dolphins: WCS Working Paper No. 31. Wildlife Conservation Society; 2007.
  2. Minton AG, smith BD, Braulik G, Kreb D, Sutaria D, Reeves R. Orcaella brevirostris. The IUCN Red List of Threatened Species 2017. e.T15419A50367860. Downloaded on 10 December 2017.; 2017.
  3. Baird IG, Beasley IL. Irrawaddy dolphin Orcaella brevirostris in the Cambodian Mekong River: an initial survey. Oryx 2005;39:301-10.
  4. Beasley I, Pollock K, Jefferson TA, et al. Likely future extirpation of another Asian river dolphin: The critically endangered population of the Irrawaddy dolphin in the Mekong River is small and declining. Marine Mammal Science 2013;29:E226-E52.
  5. Ryan GE, Dove V, Trujillo F, Doherty Jr. PF. Irrawaddy dolphin demography in the Mekong River: an application of mark-resight models. Ecosphere 2011;2:1-15.
  6. Phan C, Hang S, Tan S, Lor K, Phay S. Population Monitoring of the Critically Endangered Mekong Dolphin Based on Mark-Resight Models: WWF Cambodia; 2018.
  7. Eam S, Phay S, Hang S, et al. The Monitoring of Irrawaddy Dolphin Population in the Mekong River: The Long-Term Population Monitoring based on Mark-Resight Models. . FiA/WWF- Cambodia Technical Report 2020:19.
  8. Smith BD, Tun MT. Review of the status and conservation of Irrawaddy dolphins Orcaella brevirostris in the Ayeyarwady River of Myanmar. In: Smith BD, Shore RG, Lopez A, eds. Status and Conservation of Freshwater populations of Irrawaddy dolphins: Wildlife Conservation Society; 2007:21-40.
  9. Thomas P, Gulland F. Report of the International Workshop on the Conservation of Irrawaddy Dolphins in the Mekong River. 2017; Kratie, Cambodia, January 2017. p. 36.
  10. Kreb D, Susanti I. PESUT MAHAKAM CONSERVATION PROGRAM TECHNICAL REPORT: Abundance and threats monitoring surveys during medium to low water levels, August/September & November 2007. Samarinda: Yayasan Konservasi Rasi; 2008.
  11. Thomas PO, Gulland FMD, Reeves RR, et al. Electrofishing as a potential threat to freshwater cetaceans. Endangered Species Research 2019;39:207-20.
  12. Ryan GE. Last chance for dolphins in Laos: a review of the history, threats, and status. Hanoi: WWF-Greater Mekong Programme; 2012.
  13. Kreb D, Sustani I. Abundance and threats monitoring surveys during medium to high water levels September & October/November 2010. Pesut Mahakam Conservation Programme: Technical Report 2010:24.
  14. Smith BD. Conservation Status of the Irrawaddy Dolphin (Orcaella brevirostris). Bonn, Germany: Convention on the Conservation of Migratory Species of Wild Animals; 2007. Report No.: CMS/ScC14/Doc.8.
  15. Smith BD. Irrawaddy dolphin, Orcaella brevirostris. In: Würsig B, Thewissen JGM, Kovacs KM, eds. Encyclopedia of Marine Mammals. San Diego: Academic Press, Elsevier; 2018.
  16. Jensen FH, Rocco A, Mansur RM, Smith BD, Janik VM, Madsen PT. Clicking in Shallow Rivers: Short-Range Echolocation of Irrawaddy and Ganges River Dolphins in a Shallow, Acoustically Complex Habitat. PLoS ONE 2013;8:e59284.
  17. Muhamad HM, Xu X, Zhang X, et al. Echolocation Clicks of Irrawaddy Dolphins (Orcaella brevirostris) During Foraging in the Bay of Brunei, Malaysia. Acoustics Australia 2020;48:201-10.
  18. Stacey P, Arnold PW. Orcaella brevirostris. Mammalian Species 1999;616:1-8.
  19. Jackson-Ricketts J, Ruiz-Cooley RI, Junchompoo C, et al. Ontogenetic variation in diet and habitat of Irrawaddy dolphins (Orcaella brevirostris) in the Gulf of Thailand and the Andaman Sea. Marine Mammal Science 2019;35:492-521.
  20. Onishi S. Mutualistic fishing between fishermen and Irrawaddy dolphins in Myanmar. Tiger paper 2008;35:1-8.
  21. Smith BD, Mya TT, Aung MC, Hang W, Thida M. Catch composition and conservation management of a human-dolphin cooperative cast-net fishery in the Ayeyarwady River, Myanmar. Biological Conservation 2009;142:1042 – 9.
  22. Kreb, D. & Budiono.  Laporan Teknis Monitoring Pesut  Mahakam dan Kualitas Air – Augustus 2018- Mai 2019. 2019.
  23. Kreb, D. Facultative river dolphins: conservation and social ecology of freshwater and coastal Irrawaddy dolphins in Indonesia. 2004

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