Orcaella brevirostris
Irrawaddy dolphin

Coastal waters and large rivers of S.E. Asia, Northern Australia, and Papua New Guinea Burton and Pearson (1987), Honacki et al. (1982), Jefferson et al. (1994)

Irrawaddy dolphins inhabit coastal, brackish, and fresh waters (major river systems) of the tropical and sub-tropical Indo-Pacific. They have been found as far as 1440 km upstream and can live permanently in freshwater. Jefferson et al. (1994), Nowak (1991), Ridgway and Harrison (1989)

Length: 2.00 to 2.75 m The color of the irrawaddy dolphin has been described as dark bluish grey, slate grey, battleship grey, or pale grey. The underside is usually paler than the dorsal surface. Orcaella brevirostris has a high, anteriorly convex forehead which overhangs the mouth. It does not have a beak, and its U-shaped blowhole is to the left of the midline. Unlike the condition in most dolphin species, the blowhole opens toward the front. Irawaddy dolphins resemble the finless porpoise, but unlike that species, they have a small, triangular, and bluntly rounded dorsal fin (with a barely concave rear margin) set just behind the midback. The flippers are relatively large (about one-sixth as long as the body) and have a great breadth with a gently curved leading edge. The mouthline is straight, and there may be a visible neck crease. The neck is unusually flexible because only the first two cervical vertebrae are fused. The tail is also quite flexible. Orcaella brevirostris has homodont, narrow, pointed, and peg-like teeth with slightly expanded crowns. The teeth are about 1 cm in length, and tooth counts are 17 to 20 (upper) and 15 to 18 (lower) in each quadrant. The skull is characterized by its globular shape, short rostrum, and broad facial region. The Irrawaddy dolphin does not have a cardiac sphincter; the stomach is subdivided into compartments communicating through narrow orifices.

Collins (1997), Jefferson et al. (1994), Nowak (1991), Ridgway and Harrison (1989)

Irrawaddy dolphins are not particularly active, but they do make low leaps on occasion. They are usually seen while surfacing slowly and exposing their blowhole. The animal then usually continues forward with a smooth slow roll. They are not known to bowride.

Nothing is known of the depths to which Irrawaddy dolphins dive, but considering their coastal and riverine distribution, it is unlikely for them to dive to considerable depths. It has been reported that O. brevirostris typically respires three times in rapid succession and then dives for 30-60 seconds (dive times are longer when the animal is frightened). The maximum dive time recorded is 12 minutes, and the maximum swimming rate recorded is 25 km/hr (recorded while a O. brevirostris was being chased by a boat). Irrawaddy dolphins are usually seen in small groups, which usually consist of less than six individuals but which may contain as many as 10-15 animals. There is no information on the population dynamics of this species. The behavior of O. brevirostris suggests that they spend most of their time feeding. Irrawaddy dolphins sometimes spit water while feeding (they can expel water from their mouths for distances of up to 1.5 m), apparently to herd fish. Fishermen also allege that the dolphins sometimes catch large fish for sport by stunning them with a blow from their lower jaw. The dolphins then play with the fish like a cat with a mouse before discarding them.

The vocalizations of Irrawaddy dolphins are short time-duration bandwith signals of about 25-30 microsecond duration. The main sonar signal consists of only a few cycles of a dominant frequency of around 60 kHz. Pulse trains are rather regular in nature. No audible whistles or pure tones have been recorded.

Baird et al. (1994), Jefferson et al. (1994), Ridgway and Harrison (1989)

Orcaella brevirostris is a generalist feeder, taking food both from within the water column and from the bottom. Bony fishes seem to be the main food of irrawaddy dolphins, but they have also been observed to eat crustraceans, cephalopods, and fish eggs. Stomachs of ten O. brevirostris from coastal waters off Townsville, Australia all contained bony fishes (from 16 orders and 13 families). Nine of the ten stomachs contained crustaceans (five with shrimps, two with isopods, and four with unidentified crustaceans), and all stomachs contained cephalopod remains (ten with squid, three with cuttlefish, and two with octopods). Two species of cyprinid fish, Cirrihinus siamensis and Paralaubuca typus, are believed to be an especially important food source for irrawaddy dolphins in northeastern Cambodia and Lao PDR. Carp seem to be the most important food source for irrawaddy dolphins in Semanyang Lake (Kalimantan). Baird et al. (1994), Jefferson et al. (1994), Nowak (1991), Ridgway and Harrison (1989)

In some areas of Asia, in contrast to above, irrawaddy dolphins are seen as competitors because they eat the same fish humans try to catch. Also, irrawady dolphins can become entangled in fish nets and damage them. Ridgway and Harrison (1989)

Reports from various parts of Asia suggest that irrawaddy dolphins regularly assist fisherman by driving fish into their nets. In one report, fisherman in Burma were observed to attract irrawaddy dolphins by tapping the sides of their boats with oars. The dolphins swim around the boats in ever-diminishing circles, therby forcing fish into nets. The fisherman share their catch with the dolphins and consider them friends that are not to be harmed. The oil of the irrawaddy dolphin has reportedly been used as a remedy for rheumatism in parts of India.

Nowak (1991), Ridgway and Harrison (1989)

IUCN Status: Insufficiently known Irrawaddy dolphins are considered by some authorities to be of conservation concern, but they are not believed to be threatened with imminent extinction. There is simply insufficient information for the irrawaddy dolphin to be placed in the endangered or threatened category. More research is needed to aquire a detailed knowledge of the irrawaddy dolphin's abundance, key habitats, and forage resources. The irrawaddy dolphin is likely affected by increasing pollution, the construction of barrages such as dams, and build-ups of silt. Also, shark gillnets in Australia and fish traps and other types of nets throughout the range are known to take away some irrawaddy dolphins. Furthermore, some small-scale hunting by local people probably occurs in many areas of its range (nonetheless, the irrawaddy dolphin is generally unexploited), and the irrawaddy dolphin inhabits some of the most vulnerable of aquatic habitats, those being tropical, riverine, estuarine, and coastal habitats.

Orcaella brevirostris is protected by law in Lao PDR; hunting, capturing, and trading dolphins is illegal. In general, the people of Lao PDR respect the dolphins and they are not usually hunted. In fact, many villagers believe that irrawaddy dolphins are reincarnated humans and that bad luck results from killing them. Orcaella brevirostris has been kept in captivity at the Cairns Oceanarium (Australia), the Jaya Ancol Oceanarium (Jakarta), and the Surabaya Zoo (indonesia). The program at the Jaya Ancol Oceanarium has been particularly successful, two of the captive females having conceived and given birth to live healthy young.

Baird et al. (1994), Burton and Pearson (1987), Jefferson et al. (1994), Ridgway and Harrison (1989)

The irrawaddy dolphin is also known as pesut mahakam, and it is the only species in the genus Orcaella. The irrawaddy dolphin was named after one of the rivers it inhabits, the Irrawaddy river.

Ridgway and Harrison (1989)