The Irrawaddy dolphin symbolizes the magnificence of the Mekong River and its continued high biodiversity. The latest population survey showed that only 92 Irrawaddy Dolphins inhabit the Mekong River. Even this low number would make the Mekong subpopulation the largest of only five remaining critically endangered freshwater populations of this species in the world.
The Irrawaddy dolphin (Orcaella brevirostris) inhabits a 190km stretch of the mainstream Mekong River between Kratie, Cambodia and Khone Falls on the border with Lao PDR.
The Irrawaddy is a shy, small dolphin that is dark grey in colour with a paler underside, a small rounded dorsal fin and a bluntly rounded head. It can reach lengths of 2.75m, weighs up to 150kg, and normally lives in groups of up to 6. The Irrawaddy dolphin is one of only 3 whale and dolphin species that occupy both fresh and marine waters.
Marine Dolphin populations inhabit coastal areas, particularly muddy and brackish waters, while freshwater populations prefer deeper areas of lakes and large rivers. The largest known marine numbers about 6000 animals and is found off the coast of Bangladesh. All five remaining freshwater populations number less than 100 individuals and are listed as critically endangered by the IUCN Red List.
Irrawaddy dolphins were once found throughout the Mekong from the Lao-Cambodia border down the Mekong to the delta in Vietnam and in Tonle Sap. The population and range of the dolphins has declined greatly in the last 39 years. During the dry season from January to May when water levels fall, the population is concentrated in nine deep water pools in a 190km stretch of the Mekong north of Kratie to the Lao border. While these pools provide safe habitat for dolphins to rest and forage, such congregations also make dolphins vulnerable to fishing activities. In the wet season dolphins are occasionally seen south of Kratie.
Irrawaddy dolphins spend most of their time foraging. They are neither particularly active nor acrobatic dolphins, but they do make low leaps on occasion. Irrawaddy dolphins typically dive for less than 2 minutes, but dive times are longer when animals are frightened. Life expectancy of Irrawaddy Dolphins is around 30 years, and while some individuals reach adult size at the age of four to six, the specific age of sexual maturity is unknown. Young Dolphins show impressive growth rates – born at around 1m and 12kg, calves increase by over 50cm and 33 kg in their first 7 months! Females give birth every 2-3 years; but in stressed populations mating may take place at an earlier age and calving at shorter intervals.
Historically, populations were decimated by hunting for meat and oil, accidental drowning in nets, explosives fishing, and even being used as target practice by various armies during Cambodia’s recent troubled periods. Today, accidental drowning in gillnets is the main threat to adult Irrawaddy dolphins in the Mekong, especially with fishing becoming increasing intensive and cheap nylon gillnets replacing thicker traditional nets that could be detected by dolphin echolocation. But many other possible threats exist: