Irrawaddy dolphin

The Irrawaddy dolphin (Orcaella brevirostris) is a euryhaline species of oceanic dolphin found in scattered subpopulations near sea coasts and in estuaries and rivers in parts of the Bay of Bengal and Southeast Asia. It closely resembles the Australian snubfin dolphin (of the same genus, Orcaella), which was not described as a separate species until 2005. It has a slate blue to a slate gray color. Although found in much of the riverine and marine zones of South and Southeast Asia, the only concentrated lagoon populations are found in Chilika Lake in Odisha, India, and Songkhla Lake in southern Thailand.

The Irrawaddy dolphin is an oceanic dolphin that lives in brackish water near coasts, river mouths, and estuaries. It has established subpopulations in freshwater rivers, including the Ganges and the Mekong, as well as the Ayeyarwady River, from which it takes its name. Its range extends from the Bay of Bengal to New Guinea and the Philippines, although it does not appear to venture offshore. It is often seen in estuaries and bays of Borneo, with sightings from Sandakan in Sabah to most parts of Brunei and Sarawak, and another specimen was collected at Mahakam River in East Kalimantan. Its presence in Chinese, Taiwanese, and Hong Kong waters has been questioned, as the reported sightings have been considered unreliable. Bangladesh: ~3,500 in coastal waters of the Bay of Bengal and 451 (VU) in the brackish Sundarbans mangrove forest India: 156 in the brackish-water Chilika Lake, Odisha, as of 2021. Cambodia: ~105 in a 190-km (118-mi) freshwater stretch of the Mekong River as of 2024 Indonesia: ~70, in a 420-km (260-mi) stretch of the freshwater Mahakam River Philippines: ~35 in the brackish inner Malampaya Sound, Palawan, at least 20 in Quezon in southern Palawan, 30–40 in the waters of Pulupandan, Bago, Negros Occidental, Guimaras and Iloilo in the Western Visayas Region and Negros Island Region, and at least two in San Miguel Bay, Bicol, the easternmost population and the only population in the Pacific Ocean. Myanmar: ~58-72 in a 370-km (230-mi) freshwater stretch of the Ayeyarwady River Thailand: less than 50 in the brackish Songkhla Lake, perhaps just 14. It has been extinct in Laos since 2022.

Indonesia

There are plenty of food items that this dolphin feeds upon. They include fish, crustaceans, and cephalopods. During foraging periods, herds of about 7 dolphins will circle around prey and trap their victim. These prey entrapments occur slightly below the water surface level.

Communication is carried out with clicks, creaks, and buzzes at a dominant frequency of about 60 kilohertz, which is thought to be used for echolocation. Bony fish (including catfish) and fish eggs, cephalopods (such as squid, octopus, and cuttlefish), and crustaceans are taken as food. Observations of captive animals indicate food may be taken into the mouth by suction. Irrawaddy dolphins are capable of squirting streams of water that can reach up to 1.5 metres (4 ft 11 in); this distinct behaviour has been known for herding fish into a general area for hunting. They do this sometimes while spyhopping and 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 spyhopping on command. The Irrawaddy dolphin is a slow swimmer, but swimming speeds of 20–25 kilometres per hour (12–16 mph) were reported when dolphins were being chased in a boat. In a year, females can sometimes swim as far as an average of 45 kilometres (28 mi), and males can swim up to an average of 100 kilometres (62 mi). Most Irrawaddy dolphins are shy of boats, not known to bow-ride, and generally dive when alarmed. They are relatively slow-moving but can sometimes be seen spyhopping and rolling to one side while waving a flipper and occasionally breaching. They are generally found in groups of 2-3 animals, though sometimes as many as 25 individuals have been known to congregate in deep pools. Groups of fewer than six individuals are most common, but sometimes up to 15 dolphins are seen together. Traveling and staying in groups not only enables Irrawaddy dolphins to hunt, but it also creates and maintains social bonds and allows copulation to occur. There are a few groups of dolphins in Brunei Bay seen near the mouths of the Temburong and Aloh Besar rivers, which are tame and gather around fishermen's boats that catch fish during the fishing season from around December to February. 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 30 to 150 seconds to 12 minutes. When 277 group dives were timed (time of disappearance of the last dolphin in the group to the emergence of the first dolphin in the group) in Laos, the mean duration was 115.3 seconds with a range of 19 seconds to 7.18 minutes. Interspecific competition has been observed when Irrawaddy dolphins were forced inshore and excluded by more specialized dolphins. When captive humpback dolphins (Sousa 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.

longest observed 30 years.

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.

One of the earliest recorded descriptions of the Irrawaddy dolphin was 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 the Monodontidae and Delphinidae. Widespread agreement now exists to list it in the family Delphinidae. The Irrawaddy dolphin's colour is grey to dark slate blue, paler underneath, without a 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 front of its snout is blunt. The flippers are broad and rounded. The finless porpoise (Neophocaena phocaenoides) is similar and has no back fin; the humpback dolphin (Sousa chinensis) is larger and has a longer beak and a larger dorsal fin. It ranges in weight from 90 to 200 kg (200 to 440 lb) with a length of 2.3 m (7.5 ft) at full maturity. The maximum recorded length is 2.75 m (9.0 ft) of a male in Thailand. The Irrawaddy dolphin is similar to the beluga in appearance, though most closely related to the killer whale. It has a large melon and a blunt, rounded head, and the beak is indistinct. Its dorsal fin, located about two-thirds posterior along the back, is short, blunt, and triangular. Dorsal fin shapes differ from one Irrawaddy dolphin to another. The flippers are long and broad. These dolphins are usually two-toned, with the back and sides being gray to bluish-gray and the belly lighter. Unlike any other dolphin, the Irrawaddy's U-shaped blowhole is on the left of the midline and opens towards the front of the dolphin. Its short beak appears very different from those of other dolphins, and its mouth is known for having 12-19 peg-like teeth on each side of the jaws.

Irrawaddy dolphins are more susceptible to human conflict than most other dolphins that live farther out in the ocean. Drowning in gillnets is the main threat to them throughout their range. Between 1995 and 2001, 38 deaths were reported, and 74% died as a result of entanglement in gillnets with large mesh sizes. The majority of reported dolphin deaths in all subpopulations are due to accidental capture and drowning in gillnets and dragnets, and in the Philippines, bottom-set crab nets. In Burma, electrofishing, gold mining, and dam building are also serious and continuing threats. Though most fishers are sympathetic to the dolphins' plight, abandoning their traditional livelihood is difficult for them. Another identified threat towards the Irrawaddy dolphins was noise pollution from high-speed vessels. This caused the dolphins to dive significantly longer than usual. The Irrawaddy dolphins always changed directions when they encountered these large vessels. In recent years, Laotians and Cambodians developed techniques of using explosives for fishing. The government of Laos has made the use of such tactics illegal, but few regulations have been made in neighboring Cambodia, where explosives are sold in local markets and the practice of using fishnets has been abandoned. The practice of using explosives instead has become very popular and led to a steady decline of populations of fish, and especially the dolphins swimming in the area. Although Laotians may not use explosives, they do use nylon gillnets, which pose another large threat to the survival of the Irrawaddy. Some dolphins accidentally become entangled in the net. Poor fishermen refuse to cut and destroy their nets because it would result in too great an economic loss to save one Irrawaddy dolphin. In Laos, a dam across the Mekong River is planned. This could threaten the existence of the endangered Irrawaddy dolphins in downstream Cambodia. Laos's government decision is to forge the dam upstream of the core habitat of the Irrawaddy dolphins. This could precipitate the extinction of this specific species in the Mekong River. The dam builders propose to use explosives to dig out the tons of rock. This will create strong sound waves that could kill the Irrawaddy dolphins due to their highly sensitive hearing structures. In several Asian countries, Irrawaddy dolphins have been captured and trained to perform in public aquaria. Their charismatic appearance and unique behaviors, including spitting water, spyhopping, and fluke-slapping, make them very popular for shows in dolphinariums. The commercial motivation for using this dolphin species is high because it can live in freshwater tanks, and the high cost of marine aquarium systems is avoided. The region within and near the species' range has developed economically; theme parks, casinos, and other entertainment venues that include dolphin shows have increased. In 2002, there were more than 80 dolphinariums in at least nine Asian countries. Collateral deaths of dolphins due to blast fishing were once common in Vietnam and Thailand. In the past, the most direct threat was killing them for their oil. The IUCN lists five of the seven subpopulations as endangered, primarily due to drowning in fish nets. For example, the Malampaya population, first discovered and described in 1986, at the time consisted of 77 individuals. Due to anthropogenic activities, this number dwindled to 47 dolphins in 2007, which further dwindled to 35 by 2013. In the Mahakam River in Borneo, 73% of dolphin deaths are related to entanglement in gillnets, due to heavy fishing and boat traffic.

The Irrawaddy dolphin's proximity to developing communities makes the effort for conservation difficult. Entanglement in fishnets and degradation of habitats are the main threats to Irrawaddy dolphins. Conservation efforts are being made at international and national levels to alleviate these threats.

The species' name, brevirostris, is from the Latin meaning "short beaked".