Orcelle
The Irrawaddy dolphin (Orcaella brevirostris) is a euryhaline species of oceanic dolphin found in scattered subpopulations near sea coasts of the Bay of Bengal and Southeast Asia. It closely resembles the Australian snubfin dolphin and was not described as a separate species until 2005.
The Irrawaddy dolphin's color 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 Irrawaddy dolphin is similar to the beluga in appearance, though most closely related to the killer whale. 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.
The species is found in scattered areas throughout the Indo-Pacific region, occurring from the Philippines to northeastern India. The Irrawaddy dolphin can be found in the following freshwater river systems: Mahakam (Indonesia); the Ayeyarwady, previously called Irrawaddy (Burma); and finally, the Mekong (Laos, Cambodia, and Vietnam). It is also known to live in Chika Lake in India as well as Songkhla Lake in Thailand. The Irrawaddy dolphin generally inhabits coastal areas and is not found in offshore waters. It prefers muddy, brackish waters at river mouths and deltas.
Irrawaddy dolphins are highly social animals. However, compared to pods of other dolphins, groups of this species are smaller, usually containing 3-6 individuals. Coming to the surface, these curious animals are known to look around and explore the surrounding area. As opposed to other dolphins, they are not very active, swimming slowly with sluggish movements. As they come to the surface to breathe, only the top of the head is seen; once appearing on the surface, they immediately dive back. As a matter of fact, only 14% of all surfacings include playful behavior such as rolling, splashing, limb waving, and slapping. They usually surface twice before diving. When disturbed, Irrawaddy dolphins, living in Chilka Lake swim onto a sand bar and roll around. Every day, these diurnal dolphins travel from Semayang Lake in eastern Borneo to the Mahakam River, coming back to the lake in the evening. Irrawaddy dolphins communicate with each other through a wide variety of clicks and buzzing calls, helping them identify individuals.
Irrawaddy dolphins are carnivores (piscivores and molluscivores). Their diet includes fish, cephalopods, and crustaceans.
These dolphins have a polygynous mating system, where each male mates with a number of females. Male dolphins compete with each other for their mating rights. There is very scarce information on the reproductive biology of this species. However, they are likely to breed in December-June. The gestation period lasts for about 9 months, though data from two captive births has shown 14 months of gestation. Usually, females yield a single baby with an interval of 2-3 years. During the first seven months of its life, the offspring feeds exclusively on the nutrition from its mother. The calf learns to prey on fish, observing the behavior of its mother as well as other members of the pod. By the age of 6 months, young dolphins begin eating fish, being completely weaned at 2 years old. They are thought to become reproductively mature between 7 and 9 years old.
The main threat to the population of this species is drowning in gill nets, which most often happens in December-May, during the dry season, when these animals remain in deep water bodies. On the other hand, electrocution and prey depletion from electric fishing poses a real threat to these animals, particularly to those found in the Ayeyarwady River. The dolphin also suffers from pollution, the construction of dams as well as the build-up of silt and sedimentation, which leads to a high degree of erosion. In addition, the animal currently faces dangers such as overfishing, collisions with boats, and resulting injuries, which can sharply reduce the population of this dolphin.
According to the IUCN Red List, the total population size of the Irrawaddy dolphin is unknown, but there are estimates available for only a few portions of the range: in Malampaya Sound (Philippines) - 77 dolphins; in the Mekong River - 125 dolphins; in the Mahakam River (Indonesia) - 70 dolphins; in the Ayeyarwady River (Myanmar) - 58-72 dolphins; in coastal waters of Bangladesh - 5,383 dolphins; in the Sundarbans mangrove forest of Bangladesh - 451 dolphins. Currently, this species is classified as Endangered (EN) on the IUCN Red List, and its numbers today are decreasing.
Irrawaddy dolphins act as top ecosystem predators, feeding on and regulating populations of fish, cephalopods, and crustaceans.
