Covering a quarter of a million hectares, the Tonle Sap Lake is usually just about one metre deep for much of the year.

But every June there comes a day when the slow flow of water out of the lake reverses, and its waters begin to rise as snowmelt from the mountains of Tibet and monsoon rains swell the Mekong River system and push their way into the lake.

The Tonle Sap's depth increases by up to nine metres while its surface area expands by a factor of five to cover 1.25 million hectares.

The flow starts to reverse again in October, when the monsoons end, and the waters drain until low level is reached and the cycle starts again.

The months of flooding are a catalyst that sparks a huge boom in numbers of fish and aquatic organisms, making the Tonle Sap ecosystem one of the most productive inland fisheries in the world. It provides Cambodia with 200 000 tonnes of fish each year - 60 percent of its total fish catch.

The importance of fish

Catching, processing and selling of captured fish are the most important economic activities on the lake, followed by cage culture of catfish and snakeheads.

For families who practice aquaculture, the fish "are like a bank account," says FAO specialist Patrick Evans, who has worked on the Tonle Sap for eight years. "They make the investment, put in some fish, raise them, and as the fish grow and multiply their value compounds, like interest. When people need money, they harvest some of the fish."

Land bordering lake extremely fertile

And when the flood waters recede, the alluvial soil surrounding the lake is perfect for growing rice, with productivity up to six times greater than the national average, Evans adds.