Irrawaddy Dolphin (Orcaella brevirostris)
Etymology and taxonomic history
The Irrawaddy dolphin was first described by Sir Richard Owen in 1866 based on a specimen found in 1852, in the harbour of Visakhapatnam on the east coast of India. It is one of two species in its genus. It has sometimes been listed variously in a family containing just itself and in Monodontidae and in Delphinapteridae. There is now widespread agreement to list it in the Delphinidae family.
Genetically, the Irrawaddy dolphin is closely related to the killer whale (orca). The species name brevirostris comes from the Latin meaning short-beaked. In 2005, genetic analysis showed the Australian snub fin dolphin found at the coast of northern Australia forms a second species in the Orcaella genus. Overall, the dolphins' colour is grey to dark slate blue, paler underneath, with no distinctive pattern. The dorsal fin is small and rounded behind the middle of the back. The forehead is high and rounded; the beak is lacking. The flippers are broad and rounded. The species found in Borneo, the finless porpoise, Neophocaena phocaenoides, is similar and has no back fin; the humpback dolphin, Sausa chinensis, is larger, has a longer beak and a larger dorsal fin.
Vernacular names for O. brevirostris include the following.
Chilika dialect: baslnyya magar or bhuasuni magar (lit. oil-yielding dolphin)
Khmer: ផ្សោត ph’sout
Oriya: khem or khera
Thai: pla loma hua bat ("alms-bowl dolphin", due to the shape of their head)
Irrawaddy dolphins are similar to the beluga in appearance,
though most closely related to the orca. They have a large melon and a blunt,
rounded head, and the beak is indistinct. The dorsal fin, located about
two-thirds posterior along the back, is short, blunt and triangular. The
flippers are long and broad. It is lightly coloured all over, but slightly more
white on the underside than the back. Adult weight exceeds 130 kg (290 lb) and
length is 2.3 m (7.5 ft) m at full maturity. Maximum recorded length is 2.75 m
(9.0 ft) of a male from Thailand.
Irrawaddy dolphins communicate with clicks, creaks and buzzes at a dominant frequency of about 60 kilohertz, which is thought to be used for echolocation. Bony fish and fish eggs, cephalopods, and crustaceans are taken as food. Observations of captive animals indicate food may be taken into the mouth by suction. Irrawaddy dolphins sometimes spit streams of water, sometimes while spy hopping, during feeding, apparently to expel water ingested during fish capture or possibly to herd fish. Some Irrawaddy dolphins kept in captivity have been trained to do spy hopping on command. The Irrawaddy dolphin is a slow swimmer, but swimming speeds of 20–25 km/hour were reported when dolphins were being chased in a boat. It surfaces in a rolling fashion and lifts its tail fluke clear of the water only for a deep dive. Deep dive times range from 70–150 seconds to 12 minutes. When 277 group dives were timed (time of disappearance of last dolphin in group to emergence of first dolphin in the group) in Laos, mean duration was 115.3 seconds with a range of 19 seconds to 7.18 minutes. They make only occasional low leaps and never bow-ride. Groups of fewer than six individuals are most common, but sometimes up to 15 dolphins are seen together. Intraspecific competition has been observed when Orcaella was forced inshore and excluded by more specialised dolphins. When captive humpback dolphins (Sonsa chinensis) and Irrawaddy dolphins were held together, reportedly the Irrawaddy dolphins were frequently chased and confined to a small portion of the tank by the dominant humpbacks. In Chilika Lake, local fishers say when Irrawaddy dolphins and bottlenose dolphins meet in the outer channel, the former get frightened and are forced to return toward the lake. These dolphins are thought to reach sexual maturity at seven to nine years. In the Northern Hemisphere, mating is reported from December to June. Its gestation period is 14 months; cows give birth to a single calf every two to three years. Length is about 1 m (3.3 ft) at birth. Birth weight is about 10 kg (22 lb). Weaning is after two years. Lifespan is about 30 years. Irrawaddy dolphins prefer coastal areas associated with the muddy, brackish waters at river mouths, ranging offshore as far as the extent of the freshwater plume – often only a few km but more than 60 km at the Meghna River mouth in Bangladesh (Smith et al. 2005). In rivers and mangrove channels, the species is most often observed at channel confluences and divergences and downstream of sharp meanders. They have been seen in the same area as finless porpoises in coastal waters of Bangladesh and Myanmar (Smith et al. 2005), and Ganges River dolphins in the waterways of the Sundarbans mangrove forest (Smith et al. 2006).
Systems: Freshwater; Marine
Irrawaddy dolphins have a discontinuous distribution in the
tropical and subtropical Indo-Pacific, almost exclusively in estuarine and fresh
waters (Stacey and Arnold 1999; Arnold 2002). They occur from Borneo and the
central islands of the Indonesian Archipelago north to Palawan, Philippines, and
west to the Bay of Bengal, including the Gulf of Thailand. There are freshwater
subpopulations in three large rivers: Ayeyarwady (up to 1,400 km upstream) in
Myanmar, Mahakam (up to 560 km upstream) in Indonesia, and Mekong (up to 690 km
upstream) in Viet Nam, Cambodia and Lao PDR, and two marine-appended brackish
water bodies or lakes: Chilika in India and Songkhla in Thailand. The
fine-scale range of the species is poorly documented throughout much of its
range in estuarine waters.
Bangladesh; Brunei Darussalam; Cambodia; India; Indonesia; Lao People's Democratic Republic; Malaysia; Myanmar; Philippines; Singapore; Thailand; Vietnam
Although sometimes called the Irrawaddy river dolphin, it is
not a true river dolphin, but an oceanic dolphin that lives in brackish water
near coasts, river mouths and in estuaries. It has established subpopulations in
freshwater rivers, including the Ganges and the Mekong, as well as the Irrawaddy
River from which it takes its name. Its range extends from the Bay of Bengal to
New Guinea and the Philippines.
It is often seen in estuaries and bays in Borneo Island, with sightings from Sandakan in Sabah, Malaysia, to most parts of Brunei and Sarawak, Malaysia. A specimen was collected at Mahakam River in East Kalimantan.
No range-wide survey has been conducted for this vulnerable species; however, the worldwide population appears to be over 7,000, with over 90% occurring in Bangladesh. Populations outside Bangladesh and India are classified as critically endangered. Known subpopulations of Irrawaddy dolphins are found in eight places, listed here in order of population, including conservation status.
Chilka Lake, Orissa, India, habitat of Irrawaddy dolphins
Bangladesh; 5,832 (VU) in coastal waters of the Bay of Bengal and 451 (VU) in the brackish Sundarbans mangrove forest
India; 138 (VU) in the brackish water Chilka Lake
Laos and Cambodia; 78-91 (CR) in a 190 km (118.1 mi) freshwater stretch of the Mekong River
Indonesia; (CR), in a 420 km (261.0 mi) stretch of the freshwater Mahakam River
Philippines; about 42 (CR) in the brackish inner Malampaya Sound. Researchers are studying the recent discovery of 30-40 dolphins sighted in the waters of Bago City and Pulupandan town in the province of Negros Occidental, in Western Visayas
Burma; about 58-72 (CR) in a 370 km (229.9 mi) freshwater stretch of the Ayeyarwady River
Thailand: less than 50 (CR) in the brackish Songkhla Lake.
No range-wide survey has been conducted for this species; nor is there a synoptic estimate of total numbers from local or regional surveys. Statistically rigorous abundance estimates are available for only a few portions of the range: 77 (CV = 27.4%) in Malampaya Sound, Philippines (Smith et al. 2004); at least 125 (95% CI = 114-152) in the Mekong River (Beasley et al. 2007); 70 (CV = 10%; 95% CL = 58-79) in the Mahakam River, Indonesia (Kreb et al. 2007); 58-72 in the Ayeyarwady River, Myanmar (Smith et al. 2007-a); 5,383 (CV=40%) in coastal waters of Bangladesh (Smith et al. 2005); and 451 (CV=9.6%) in the Sundarbans mangrove forest of Bangladesh (Smith et al,/i>. 2006).
The estuarine and freshwater occurrence of this species makes
it particularly vulnerable to threats from the human activities that occur in
these environments. Threats include direct mortality from fisheries interactions
(particularly gillnet entanglement), vessel strikes, and habitat loss and
degradation (e.g. declining or altered freshwater flows due to dam and
embankment construction, environmental contamination). Live captures for
aquarium display also have been a conservation issue in some local areas.
Irrawaddy dolphins have been hunted directly in the past, at least in the Mekong
and Mahakam Rivers, but are revered by local people in many areas of Asia.
Irrawaddy dolphins are caught accidentally in fishing nets in almost all areas where they have been studied (Smith et al. 2007-b). The most detailed information on bycatch comes from the Mekong River where, of 15 confirmed human-caused deaths 2001-2005, 13 (87%) were due to gillnet entanglement (Beasley et al. 2007). Based on reports from local fishermen and the retrieval of eight carcasses along the Mahakam River between 1995 and 2005, Kreb et al. (2007) documented 48 deaths, 66% of them from entanglement in large-mesh (10 –17.5 cm) gillnets. Mortality also has been recorded in drift gillnets targeting elasmobranches in coastal waters of Bangladesh (Smith et al. 2005) and bottom-set gillnets targeting crabs in Malampaya Sound (Smith et al. 2004). Fishermen in some areas report the dolphins are released if found still alive (Smith and Hobbs 2002, Kreb et al. 2007), but in the case of drowned animals, the oil may be used for medicinal purposes or the flesh eaten (Smith et al. 2004).
There have been no systematic observer schemes in freshwater or coastal regions, but evidence of bycatch and increased use of gillnets is cause for concern (IWC 2000). Fishing with electricity is considered a dire threat to the Ayeyarwady subpopulation (Smith et al. 2007-a).
Many dams have been proposed that are likely to degrade the channels inhabited by Irrawaddy dolphins in the Mekong River Basin. Of greatest concern are the large run-of-the-river dams (dams without a reservoir that generally preserve a relatively natural flow regime) proposed for the Mekong main stem near Stung Treng and Sambor (Perrin et al. 1996; Mekong Secretariat 1995). Dam projects in Lao PDR, Cambodia and Viet Nam threaten not only dolphins but also fisheries and therefore human livelihoods (Smith et al. 2007-b). A recent report of a high dam planned for the headwaters of the Ayeyarwady River, Myanmar, in Myitsone just below the confluence of the Mali Hka and N’Mai Hka tributaries, provides reason for concern about its effects on the population of Irrawaddy dolphins downstream (Anon. 2007).
Deforestation and gold, sand and gravel mining are causing major changes to the geomorphologic and hydraulic features of rivers and marine-appended lakes where Irrawaddy dolphins occur (Smith et al. 2007-b). Increased sedimentation resulting from deforestation in surrounding watersheds has resulted in declining water depths in Songkhla, Chilika and Semayang Lakes. The last of these water bodies is appended to the Mahakam River and previously supported dolphins throughout most of its breadth. Now it contains suitable habitat only in a small area near the channel connecting it with the main stem (Kreb et al. 2007). Between 1992 and 1997 the maximum depth of Chilika Lake declined from 3.4 to 1.4 meters and the accumulation of sediments led to shrinkage of the opening channel and a dramatic decline in salinity. A new channel dredged in the northern portion of the lake in 2000 apparently has mitigated at least some of the problems caused by sedimentation (Pattnaik et al. 2007).
Habitat loss and population fragmentation in several areas have resulted from the proliferation of fixed fishing gears. In the middle and southern portions of Songkhla Lake about 27,000 Sai nong or sitting traps and 13,000 Sang sai or barrier traps create more than 8000 linear km of barrier in multiple rows. These fishing structures are left in place year-round and restrict dolphin movements such that their habitat is substantially reduced and the potential for demographic interaction with individuals in the Gulf of Thailand is eliminated (Smith et al. 2004).
The species is listed in Appendix I of CITES.
The Action Plan for the Conservation of Freshwater Populations of Irrawaddy Dolphins (Smith et al. 2007-c) notes that multiple-use protected areas will play a key role for conserving freshwater populations of Irrawaddy dolphins. Protected areas could be a particularly effective conservation tool due to the fidelity of the species in freshwater systems to relatively circumscribed areas, as this can facilitate management. The Action Plan also provided details on strategies for mitigating bycatch that included establishing core conservation areas where gillnetting is banned or severely restricted; promoting net attendance rules and providing training on the safe release of entangled dolphins; initiating a program to compensate fishermen for damage caused to their nets by entangled dolphins that are safely released; providing alternative or diversified employment options for gillnet fishermen; encouraging the use of fishing gears that do not harm dolphins by altering or establishing fee structures for fishing permits to make gillnetting more expensive while decreasing the fees for non-destructive gears; and experimenting with acoustical deterrents and reflective nets.