Orcaella brevirostris (Gray,
English: Irrawaddy dolphin
German: Irrawadi Delphin
Spanish: Delfín del Irrawaddy
Orcaella brevirostris ©Wurtz-Artescienza (see links).
The Irrawaddy dolphin resembles the beluga whale Delphinapterus
leucas in general appearance and certain anatomical features.
However, recent morphological and genetic studies consistently place
it in the family Delphinidae, and its closest relative might be
the killer whale Orcinus
orca (Arnold, 2002). Rice (1998), pointed out that O.
brevirostris shares more morphological similarities with the
other Delphinidae than with the Monodontidae, based on morphological
features, isozyme and immunological distance studies, by studies
of satellite DNA, and by sequencing the cytochrome b gene. Recently,
a previously considered sub-population of the Irrawaddy dolphin
found in Australia and Southern Papua New Guinea was re-classified
as a different species of the same genus, O.
heinsohni, based on differences in coloration, morphology
and genetics (Beasley et al. 2005).
The mobile head of the Irrawaddy dolphin is broadly rounded, and
there is no sign of a beak. A shallow dorsal groove extends to the
dorsal fin, which is small; the flippers are broad, paddle-like,
with a convex leading edge and highly mobile. The colour pattern
varies regionally between dark grey to light grey. Maximum recorded
length is 275 cm, but average length is only 210 cm. Adult body
mass is 115-130 kg (Arnold, 2002).
Irrawaddy dolphins are discontinuously distributed mostly in the
coastal, shallow, brackish, or fresh turbid waters at the mouths
of rivers in southeastern Asia. On the Asian mainland they range
from Vishakhapatnam, Andhra Pradesh, India, around the Bay of Bengal
to the Strait of Malacca and the Gulf of Thailand. There are freshwater
populations in the tributaries at the mouths of the Ganges, in the
Irrawaddy as far as 2,300km upstream to Bhamo, and in the Mekong
and Sekong Rivers as well as in the Ayeyarwady River (Marsh et al.
1989; Jefferson et al. 1993; Rice, 1998; Baird and Mounsouphom,1997,
Smith et al. 1997a, 1997b).
Distribution of Orcaella brevirostris:
warm coastal waters and rivers from the Bay of Bengal to
western Sulawesi, Indonesia (Reeves et al. 2008; © IUCN; enlarge
The presence of the species has not been fully confirmed
in China, but it is likely to occur there (Reyes, 1991 and refs.
therein). The species occurs in Malampaya Sound, Palawan, in the
Philippines (Reeves et al, 2008). It occurs on the Sunda shelf:
it is known from the Sungai Belawan Deli in northeastern Sumatra;
Belitung; north coast of Jawa Timur (East Java); south coast of
Jawa Tengah (Central Java); Kepulauan Bunguran (Natuna Islands);
river mouths along the coast of Sarawak, Brunei, and Sabah; the
Seruyan and Mahakam river systems, including Semayang, Melintang,
and Jempang lakes, in Kalimantan Timur (East Kalimantan); Sungai
Kumai in Kali-mantan Tengah (Central Kalimantan); south-western
Sulawesi (mod. From Rice, 1998).
Several authors were unable to find differences between populations
in the Irrawaddy, the Mekong, and marine waters (Marsh et al. 1989).
3. Population size
Various populations have recently been assessed from India to Indonesia.
Most of these are very small (< 100 individuals) with the exception
of the population off Bangladesh. These are discussed here moving
from west to east.
In Chilika lake, Orissa, India, which forms the largest brackish
water lagoon in Asia, there are about 80-90 Irrawaddy dolphins (Sarkar,
The largest population occurs in the nearshore waters of Bangladesh.
A vessel-based line-transect survey conducted in 2004 along 1,018
km of systematic trackline resulted in an abundance estimate of
5,383 (CV=39.5)(Smith et al. 2008). In waterways of the Sundarban
mangrove forest of Bangladesh, Smith et al. (2006) estimated an
abundance of 452 (CV = 10%) individuals.
Off the Mergui (Myeik) Archipelago of southern Myanmar a vessel-based
line-transect survey of nearshore waters (to a depth of 40-60m)
conducted in 2005 searching along 955 km of trackline resulted in
30 cetacean sightings, only one of which was an Irrawaddy dolphin
(Smith and Tun, 2008).
In the South China Sea, Smith et al. (1997b) reported that they
had only four cetacean sightings during 1,121 km of transect. There
were no sightings during 224 km of search effort in the Mekong River.
However, a subsequent survey in the Mekong River (Baird and Beasley,
2005) reports a 'best' estimate of 40 animals. These authors confirm
that the Mekong River dolphin population is apparently declining
In eastern Borneo's Semayang Lake, Pela River and adjacent Mahakam
River, a survey undertaken in the late 1970s reported between 100
and 150 dolphins. (Reyes, 1991, and refs. there-in). A 1999-2002
survey in the Mahakam River (East Kalimantan) estimated a total
population size of 33-55 dolphins (95% confidence limits 31-76);
no changes in abundance >8% were detected over 2.5 years (Kreb
and Budiono, 2005).
A geographically isolated population of Irrawaddy dolphins recently
discovered in Malampaya Sound, Palawan, Philippines was estimated
at 77 (CV = 27.4%) individuals. This is the only known population
of the species in the Philippines; the nearest known other population
is in northern Borneo, some 550 km to the south (Smith et al. 2004).
4. Biology and Behaviour
Habitat: lrrawaddy dolphins seem to prefer coastal areas,
particularly the muddy, brackish waters at river mouths, and do
not appear to venture far offshore, since all sightings have been
made within only a few kilometres of the coastline. Some populations
are apparently restricted to fresh water, e.g. Chilika Lake, India
and Songhkla, Thailand (Reyes, 1991 and refs. therein).
In the Mekong River these dolphins are often observed near sand
banks where streams flow into lakes (Reyes, 1991 and refs. therein).
During the dry season, they are generally confined to sections of
the river with water levels >8-10 m (Baird and Beasley, 2005).
In the Mekong River of Laos, sightings were most common in the morning
and decreased throughout the day. This could indicate diurnal feeding,
given that foraging is suggested by repeated direction changes,
lack of through travel, and observed fish consumption. Habitat use
was most intense off a tributary mouth and adjacent Sandy Island.
Mean water depth at the study site was 18.4 m, current speed of
the main channel was 0.15 m/s, and water temperature was 31°C.
Mean dive duration of dolphins was 115.3 s and similar for all group
sizes (Stacey and Hvenegaard 2002).
In the nearshore waters of Bangladesh they prefer low salinity and
shallow depth (Smith et al. 2008). In Malampaya Sound, Palawan,
Philippines mean water temperature was 30.2 °C, depth 6.5 m,
salinity 28.3 ppt and turbidity 2.2 NTUs (Smith et al. 2004). However,
salinity preferences seem to reflect ecological (prey availability)
rather than physiological constraints (Smith, 2009).
Schooling: Groups of fewer than 6 individuals are most common,
but sometimes up to 15 dolphins are seen together (Marsh et al.
1989; Jefferson et al. 1993; Stacey and Hvenegaard 2002). They have
been seen in the same area as bottlenose and lndo-Pacific humpback
dolphins (Jefferson et al. 1993). Surfacing is quite inconspicuous,
with only the uppermost part of the back becoming visible in slow
rolling dives. Leaps are infrequent, as are spy-hopping, tail slaps
and body rubbing (Smith, 2009).
Food: Fish, cephalopods, and crustaceans are taken as food.
Irrawaddy dolphins sometimes spit water while feeding, apparently
to herd fish (Marsh et al. 1989; Reyes, 1991; Jefferson et al. 1993).
In the upper reaches of the Ayeyawady River (formerly known as Irrawaddy
River), Myanmar, fishermen practice castnet fishing with the help
of Irrawaddy dolphins. Dolphins and fishermen communicate by audio
and visual signals during fishing (Tun, 2008). Catch per cast, defined
by the number of fish, their weight and economic value, was higher
while the fishermen were cooperating with dolphins, the differences
being primarily explained by the much higher frequency of zero catches
in non-cooperative casts (Smith et al. 2009a).
Reproduction: The calving season is not well known. Some
calves appear to have been born from June to August, but 1 captive
female gave birth in December (Jefferson et al. 1993). In the Northern
Hemisphere, mating is reported from December to June (Arnold, 2002).
In Chilika Lake, Irrawaddy dolphins have a very low rate of breeding,
producing only one young in three years, with a gestation period
of nine months (Sarkar, 2007).
In Semayang Lake, eastern Borneo, Irrawaddy dolphins perform daily
migrations from the lake to the Mahakam River (Est Kalimantan, Indonesia),
returning to the lake in the evening. They may be found at distances
up to 1,300 km upstream in major rivers, an indication of movements
of considerable extent (Reyes, 1991). The distribution changes seasonally
and is influenced by water levels and presumably variation in prey
availability. Dolphins move into tributaries during high water and
back into the main river when water levels recede. Most sightings
were made at confluences and river bends (Kreb in IWC, 2000).
Direct catch: Some small-scale hunting by local people probably
occurs in many areas of the range (Jefferson et al. 1993). In some
parts of Kampuchea and India, they are taken for food, but in most
of the range they are protected by local beliefs (Marsh et al. 1989;
Reyes, 1991 and refs. therein). Khmer and Vietnamese fishermen regard
Orcaella as sacred animals and release them if they become
entangled in fishing nets. By contrast, Khmer-Islam fishermen kill
them for food. The dolphins are reputed to have learnt to distinguish
between the languages of these different communities, and are much
more cautious about approaching the Khmer-Islam fishermen (Marsh,
1989 and refs. therein). Kreb and Beasley (in IWC, 2000) informed
the IWC sub-committee that live captures have occurred for the oceanarium
trade in the Mahakam River and coastal regions of Indo-Malaysia.
In both these areas there are also reports of direct killing.
Incidental catch: Irrawaddy dolphins are accidentally caught
in fishing nets in Bangladesh, India, and the Gulf of Papua New
Guinea (IWC, 2000; Smith et al. 2008). In some areas animals are
released, but in the case of drowned dolphins, the oil may be used
for medicinal purposes. Because of their presence in coastal and
riverine areas, incidental catches in fishing nets are likely to
occur elsewhere in the range (Reyes, 1991; Jefferson et al. 1993).
There have been no systematic observer schemes in freshwater or
coastal regions, but evidence of bycatch and the increase in the
use of gillnets are causes for concern. In addition, fishing with
explosives may adversely affect this species in some areas (IWC,
Recent data suggests that the threats are ongoing: In Malampaya
Inner Sound (Philippines) 29 deaths due to by-catch were recorded
between 2001 and 2006. The distribution of fishing gear shows that
almost all of the Inner Sound is harvested and that there is almost
complete overlap with preferred dolphin habitat. Irrawaddy meat
is consumed by the community members after accidental death, but
the dolphins are not hunted for food. It is expected that this population
will continue to decline and will be lost within 7 years if current
fishing practices continue (Gonzales and Matillano, 2008; Smith
et al. 2004). Similarly, the concentrations of gill netters/long
liners of the Mergui (Myeik) Archipelago of southern Myanmar were
particularly high in shallow nearshore waters and at least 150 were
operating in the bay where the only sightings of Irrawaddy dolphins
were made (Smith and Tun, 2008).
In the Mekong River anthropogenic mortality is also high, and there
is considerable risk that the dolphin population will become locally
extinct in the near future. The establishment of community-managed
deep water Fish Conservation Zones with government support may represent
the best opportunity for reducing dry season dolphin mortality from
large-meshed gillnet entanglement (Baird and Beasley, 2005).
In the Mahakam River in East Kalimantan, Borneo, dolphins die mainly
from entanglement in gillnets (73% of deaths). Their main habitats
are also important fishing grounds and subject to intensive motorized
boat traffic. Sixty-four percent of deaths (1995-2001) with known
location (n = 36) occurred in these areas. Primary conservation
strategies would require the introduction of alternative fishing
techniques (Kreb and Budiono, 2005).
Habitat degradation: Habitat degradation may limit the distribution
and abundance of Irrawaddy dolphins, particularly in fresh water.
Dams (Baird and Mounsouphom, 1997), gold mining using mercury abstraction
techniques, increased sedimentation as a result of deforestation
and other changes in river catchments, overfishing, harmful fishing
techniques (poison and electrofishing), vessel traffic and noise
pollution are all potential threats to this species. Coastal development
with concomitant eutrophication is also cause for concern (IWC,
2000; Smith et al. 2009b). Most reports come from the eastern distributional
range of the species and show ongoing habitat reduction caused by
In Indonesia, Irrawaddy dolphins were formerly observed in the Makam
River up to Tengagarong and Samararinda. Since the 1980's, probably
due to the intense activity related to the timber industry, they
are no longer observed near these towns but only above Muarakamen
(Reyes, 1991 and refs. therein). In East Kalimantan, Indonesia,
Irrawaddy dolphins reacted to boats and surfaced less in the presence
of speed boats and tugs (Kreb and Rahadi, 2004).
In Lao People's Democratic Republic, large hydro-electric dams
planned for the Sekong River sub-basin and the mainstream of the
Mekong River are a threat to the dolphins, fish populations, and
local people (Baird and Mounsouphom, 1997). Stacey and Leatherwood
(1997) concluded that the apparent low abundance and recent declines
in numbers of the Irrawaddy dolphin are cause for serious concern.
Deep pools existing in some of the major tributaries of the Mekong
and in the Mekong itself are refuges during the dry season. In some
tributaries, where dams have been constructed, some deep pool habitats
have been affected by siltation as a result of changed hydrological
conditions. In some areas, the pool habitats and the fishes they
sustain have virtually disappeared. (Poulsen et al. 2002).
Overfishing: The population inhabiting Chilka Lake in India
is said to be declining because of reduction in food supply and
silting of the lake due to agricultural development. Reduction of
fish populations in Indonesian rivers by illegal fishing methods
is a serious threat. (Reyes, 1991 and refs. therein). Cast-net fishermen
in the Ayeyarwady River, Myanmar consistently reported dramatically
depleted catches in recent years due to illegal electric fishing.
Elimination of electric fishing in a recently established protected
area will be crucial for conserving the dolphins and the cooperative
cast-net fishery (Smith et al. 2009a).
Tourism: In Chilka Lake, India the population of O. brevirostris
(locally known as 'Bhuasuni Magar') is threatened due to the "plying
of mechanised boats in the lake" (Sarkar 2007).
Pollution: Since Irrawaddy dolphins are found in rivers,
they are likely to be affected by pollution and other habitat encroachment
associated with the development of their tropical habitat (Reyes,
1991 and refs. therein). In Chilika Lake, India, dichlorodiphenyltrichloroethane
and its metabolites (DDTs) were the predominant contaminants, the
highest concentration found was 10,000 ng/g lipid weight in blubber.
Hexachlorocyclohexanes (HCHs) were the second-most prevalent contaminants
in dolphin tissues. Efforts should be made to decrease the sources
of these contaminants (Kannan et al. 2005).
Range states (Reeves et al. 2008) :
Bangladesh; Brunei Darussalam; Cambodia; India; Indonesia; Lao People's
Democratic Republic; Malaysia; Myanmar; Philippines; Singapore;
Thailand; Viet Nam.
Listed in Appendix I and II of CMS. Listed in Appendix I of CITES.
The World Wide Fund for Nature (WWF) considers O. brevirostris
as one of the most endangered small cetaceans world wide (WWF, 2009).
The IUCN categorizes O. brevirostris as "Vulnerable"
(Reeves et al. 2008). The justification is that wherever there is
available data, subpopulations are low, ranging in the tens to one-hundreds,
except for Bangladesh; There have been significant range declines;
threats including by-catch and habitat degradation are well documented,
are persisting and cause unsustainable mortality resulting in an
estimated 30% reduction in population size over the next 3 generations.
Given the vast area and complexity of coastline inhabited by this
species, it is unlikely that a more quantitative assessment of the
global population will be feasible in the near future (Reeves et
The IUCN marine mammal specialists group considers the populations
of the Ayeyarwady (formerly Irrawaddy) of Myanmar (formerly Burma),
with 59 individuals (2003 data; Smith, 2004); the Mahakam River,
East Kalimantan, Indonesia, with 59-79 individuals (Jefferson et
al. 2008); Malampaya Sound, Palawan, Philippines, with 77 individuals
(Smith and Beasley, 2004); Mekong River (Viet Nam, Laos, Cambodia),
with 69 individuals (Smith and Beasley, 2004); and Songkhla Lake
in Thailand, with less than 50 mature individuals (Smith and Beasley,
2004) as Critically Endangered (Reeves et al. 2008). And although
a few areas where the species occurs have been designated as protected,
little has been done to conserve dolphin habitat (Smith, 2009).
As pointed out by Rosel and Reeves (2000), genetic and demographic
consequences associated with very small population size can result
in extinction even when effective measures are in place to protect
the animals and their habitat. This is explained by low genetic
variation, genetic drift and inbreeding and lower fitness.
Possible strategies for conserving the population include that:
(1) socioeconomic alternatives be developed to reduce dolphin mortality
by fishing gear; (2) fishery free zones be established in core areas
of dolphin distribution; (3) Irrawaddy dolphins be promoted as a
flagship species of environmental health in their habitat; (4) a
long-term programme be established to monitor local dolphin populations;
and (5) additional investigations be conducted to determine other
areas of occurrence in the respective range stated (mod. From Smith
et al. 2004). For additional recommendations, see also Perrin
et al. (1996).
· Arnold PW (2002) Irrawaddy dolphin - Orcaella brevirostris.
In: Encyclopedia of marine mammals (Perrin WF, Würsig B, Thewissen
JGM, eds.) Academic Press, San Diego, pp. 652-654.
· Baird IG, Beasley IL (2005) Irrawaddy dolphin Orcaella
brevirostris in the Cambodian Mekong River: an initial survey.
Oryx 39: 301-310
· Baird IG, Mounsouphom B (1994) Irrawaddy dolphins (Orcaella
brevirostris) in southern Lao PDR and northeastern Cambodia.
Nat Hist Bull Siam Soc 42: 159-175.
· Baird IG, Mounsouphom B (1997) Distribution mortality,
diet and conservation of Irrawaddy dolphins (Orcaella brevirostris
Gray) in Lao, PDR. Asian Mar Biol 14: 41-48.
· Beasley I, Roberston KM, Arnold P (2005) Description of
a new dolphin, the Australian snubfin dolphin Orcaella heinsohni
sp. n. (Cetacea, Delphinidae). Mar Mamm Sci 21: 365-400.
· Gonzales B-J, Matillano M-V (2008) Irrawaddy dolphin conservation
in the fisheries of Malampaya inner sound, Palawan, Philippines.
Mem Fac Fish Kagoshima Univ. Special Issue, pp. 16-25.
· IWC (2000) Annex K: Report of the sub-committee on small
cetaceans. Rep Int Whal Comm, Cambridge, UK.
· Jefferson TA, Karczmarski L, Kreb D, Laidre K, O'Corry-Crowe
G, Reeves RR, Rojas-Bracho L, Secchi E, Slooten E, Smith BD, Wang
JY, Zhou K (2008) Orcaella brevirostris (Mahakam River subpopulation).
In: IUCN 2009. IUCN Red List of Threatened Species. Version 2009.2.
· Jefferson TA, Leatherwood S, Webber MA (1993) FAO Species
identification guide. Marine mammals of the world. UNEP/FAO, Rome,
· Kannan K, Ramu K, Kajiwara N, Sinha RK, Tanabe S (2005)
Organochlorine pesticides, polychlorinated biphenyls, and polybrominated
diphenyl ethers in Irrawaddy dolphins from India. Arch Environ Contam
Toxicol 49: 415-420
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core areas: key to survival of a Critically Endangered population
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in a coastal and riverine environment in Indonesia. Aquat Mamm 30:
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dolphin - Orcaella brevirostris (Gray, 1866) In: Handbook
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on the Biology and Conservation of Small Cetaceans and Dugongs of
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(2002) Deep pools as dry season fish habitats in the Mekong River
basin. MRC Tech Pap 4, 24 pp.
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K, O'Corry-Crowe G, Rojas-Bracho L, Secchi ER, Slooten E, Smith
BD, Wang JY, Zhou K (2008) Orcaella Brevirostris. In: IUCN
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· Reyes JC (1991) The conservation of small cetaceans: a
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· Sarkar SK (2007) Dolphins in Chilika Lake - Endangered
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© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.