The Tonle Sap Great Lake is a unique ecosystem of enormous ecological value.. It is a key breeding ground for fish, and is home to globally important colonies of endangered water birds, as well as reptiles, mammals and water snakes. It is the largest seasonally flooded freshwater swamp forest habitat in Southeast Asia.
The Tonle Sap Great Lake is the largest natural freshwater lake in Southeast Asia. Linked to the Mekong River by the 120 kilometre-long Tonle Sap River, it is a vital component of the Mekong Basin ecosystem. When water levels in the Mekong River rise during the wet season, the flow in the Tonle Sap tributary is reversed and water is pushed into the Tonle Sap Lake, increasing its surface area from around 2,500 km2 to 10,000-16,000 km2. The depth of the Tonle Sap Great Lake varies from 0.5 metres in the dry season to a maximum of nine metres during the wet season.
The lake's ecosystem is highly productive, with this productivity in large part dependent on a 'flood pulse' from the Mekong, which replenishes nutrients in the lake and enriches the floodplain. The flooded forests which border the main body of the lake offer ideal feeding and rearing conditions for fish, while the forest and shrub lands of the floodplain also provide important habitat, food, shelter and other resources for fish.
The annual fish catch of the Tonle Sap is estimated to be between 180,000 tonnes and 250,000 tonnes per year. At least 149 species of fish have been recorded on the lake, as well as globally important colonies of endangered waterbirds - Prek Toal is the only remaining breeding site in South-east Asia for two Globally Threatened species - Spot-billed Pelican Pelecanus philippensis and Milky Stork Mycteria cinerea - and the largest remaining site for five more Globally Threatened or Near-threatened species-Oriental Darter Anhinga melanogaster, Lesser Adjutant Leptoptilus javanicus, Greater Adjutant Leptoptilus dubius, Black-headed Ibis Threskiornis melanocephalus and Painted Stork Mycteria leucocephala. The lake also supports Siamese Crocodiles and the world's largest water snake harvest, with 6.9 million water snakes harvested annually, including at least 11 species of aquatic and semi-aquatic snakes.
The Tonle Sap Great Lake floodplain supports more than half of the world population of a Critically Endangered bird, the Bengal Florican. There are also many other threatened or important species including Sarus Crane, White-shouldered Ibis, and Greater Adjutant.