Life of Irrawaddy Mekong Dolphin

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.

Threats to the Mekong's dolphin population 
The Irrawaddy Dolphin’s habitat preferences for freshwater and near shore marine environments make the species especially vulnerable to intensive human use and abuse. Living in very confined areas, freshwater populations are in particularly serious decline in terms of range and size.

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:

• Over fishing with small-meshed nylon nets that depletes fish stocks and threatens future livelihoods of humans and dolphins. Illegal fishing methods using electricity or explosives can even kill dolphins directly.

• Dams and irrigation systems reduce habitat availability for fish and dolphins, particularly in the crucial dry season, change the water flow and block fish and dolphin migration.

• The hydropower-dams that are planned along the Mekong-mainstream could have disastrous consequences for dolphins, many other species, and the people that depend on it, as they cause large-scale shifts in habitat size, water flow, sedimentation, and animal mobility.

• Pesticides, heavy metals, plastic particles and other contaminants from industry, agriculture, and towns have been implicated in dolphin mortality and, since many of these pollutants are persistent, will affect local biodiversity for many years to come.

• Boat traffic on the Mekong is increasing with rising human populations, development and tourism. This can disturb dolphins, lead to stress and illness, and even result in collision-associated deaths of dolphins through contact with the propeller.

  Source: WWF-Cambodia