The Development of Tonle Sap Basin

The reversal of the Tonlé sap river's flow also acts as a safety valve to prevent flooding further downstream. During the dry season (December to April) the Tonlé sap Lake provides around 50% of the flow to the Mekong Delta in Vietnam.

The lake occupies a depression created due to the geological stress induced by the collision of the Indian subcontinent with Asia. In recent years, there have been concerns from scientists about the building of high dams and other changed hydrological parameters in southern China and Laos that has threatened the strength and volume of the reverse flow into Tonle sap, which in turn decreases nesting, breeding, spawning, and feeding habitats in floodplain, which results in adverse impacts on fish productivity and overall biodiversity.

The river is home to at least 149 species of fish, eleven globally threatened species, and six near-threatened species. These species include the spot-billed pelican, greater adjutant, Bengal florican, darter, grey-headed fish eagle, and the Manchurian reed warbler. Specifically, the large colonies of unique birds constitute the Preak Toal Bird sanctuary. In addition, the Tonlé sap also supports significant reptile populations including nearly extinct Siamese crocodiles and the world’s largest population of freshwater snakes. Although the area around the lake has been modified for settlement and farming, about 200 species of plants have been recorded.

One of the most legendary species living in the Tonlé sap is the Mekong giant catfish, the largest freshwater fish in the world. The fish is 2.4 to 3.0 metres (8 to 10 ft) long and can weigh anywhere between 110 and 230 kilograms (250 and 500 lb). The largest of these catfish ever caught weighed 306 kilograms (674 lb). Despite its massive physical characteristics, the Mekong catfish is especially vulnerable to chemical changes, which is beneficial in alerting authorities of trouble in the river ecosystem early on. The population of these fish has been steadily declining since the Khmer Rouge era, led by Pol Pot, and in 2005, fisherman reported that on average only one giant catfish was caught per day. Currently, it is illegal for fishermen to catch and keep these fish with the exception of a few retained by fisheries for research. It also cannot be used in any form of trade in fear of the economic exploitation.

In 1997, the United Nations Educational, Scientific and Cultural Organization, more commonly known as UNESCO, deemed Tonlé sap an ecological hotspot. As a result, in 2001, by royal Decree issued by the government of Cambodia, the lake and its surrounding provinces became the Tonle sap Biosphere Reserve. There are nine provinces that are part of the Tonle sap Biosphere Reserve. These are Banteay Meanchey, Battambang, Kampong Chhnang, Kampong Thom, Preah Vihear, Pursat, Siem Reap, Otdar Meanchey and Pailin.

The government is responsible for fulfilling three functions:

1. a conservation function to contribute to the conservation of biological diversity, landscapes, and ecosystem, including genetic resources, plant, fishery and animal species, and to the restoration of the essential character of the environment and habitat of biodiversity;
2. a development function to foster sustainable development of ecology, environment, economy, society, and culture;
3. a logistic function to provide support for demonstration projects, environmental education and training, research and monitoring of environment related to the local, national and global issues of conservation and sustainable development.

Additionally, the Tonlé sap Biosphere Reserve established three zones: a core zone, a buffer zone, and a transition zone. Formally, the core area of a Biosphere Reserve is defined as an area devoted to biological resources, landscapes, and ecosystems. The core zone includes practices that protect sites for conserving biodiversity, monitoring minimally disturbed ecosystems and undertaking non-destructive research and related activities. As of today, the three zones are Prek Toal, Boeng Chhmar, and Stung Sen.

Despite this government protection, illegal fishing, poaching, and cutting of the forest for farmland are all still major problems. Because people living around the lake are extremely poor and depend on the lake for their survival, it is likely that this unsustainable living will continue. During recent years, the amount of fish caught has been steadily declining, which means peasants must also work harder to provide for their families. The government is working on supporting and educating these people to break this cycle of poverty and unsustainability. Finding a balance between survival and conservation seems to be the major question for the future.

The area is home to many ethnic Vietnamese and numerous Cham communities, living in floating villages around the lake. Approximately 1.2 million people living in the greater Tonle sap make their living by fishing on the local waters. Cambodia produces about 400,000 tonnes of freshwater fish per year, the majority of which comes from Tonle sap. These fisheries account for 16 percent of national GDP, making the fish industry not only essential to the diet of local populations but to the Cambodian economy as a whole.