Freshwater: lifeblood of the Greater Mekong

Mekong River

The Mekong River and several of its largest tributaries are at the heart of the freshwater ecosystems in the Greater Mekong, home to an estimated 1,300 species of fish.

Notable fish species include the giant catfish (endangered) and giant barb, each of which can weigh 250 kilogrammes or more. There also are freshwater sharks and stingrays. Some of the last surviving populations of globally endangered species such as the Irrawaddy dolphin and Siamese crocodile live in the basin.

The Mekong is also rich in freshwater turtles, mussels and snails. Wetlands in Cambodia also are of great importance to large water birds such as the sarus cane and giant Ibis.

Tonle Sap Lake

The Mekong River system includes a physical marvel. During the wet season, the level of the Mekong River rises, backing up the Tonle Sap River and causing it to flow northwest into the Tonle Sap Lake.

Tonle Sap Lake expands from 2,500 square kilometres to 13,000/16,000 square kilometres, and its maximum depth increases from about 2.2 metres to more than 10 metres. In the dry season, the Great Lake reverses its flow and slowly drains into the Mekong River. This input helps control salinity intrusion and conserve mangrove forests in the Mekong’s Delta.

When the Tonle Sap and other parts of the Mekong system flood into fields and forests, fish take advantage of the huge increase in the availability of food. Some fish spawn in the main river channels and eggs and larvae drift into the flooded areas. Other species spawn in the flooded areas. As the floodwaters recede, fish retreat to main river channels.

Fish migrations from the Tonle Sap help restock fisheries as far upstream as China and in many tributaries along the way. There also is evidence that many species that are important parts of the subsistence and commercial fisheries in Tonle Sap spawn in other parts of the river system.