Troubled Waters.(Tonle Sap lake in Cambodia)

Each spring the waters of Tonle Sap lake rise and inundate the Cambodian village of Chhnok Trou, covering roads and fields and all but the tops of trees. The villagers don't mind. They build their houses on stilts. And besides, the flooding brings with it a bounty. When the rainy season subsides in November and the lake contracts, its burgeoning population of fish become easy prey for fishermen with handmade nets.

Hoy Ho, 45, has spent his entire life plying the waters of Chhnok Trou in his dilapidated fishing boat. His biggest worry has always been the authorities. Local policemen demand bribes as high as 10 times his daily profit of a few dollars, and fisheries inspectors "fine" him because he hasn't registered his boat (he can't afford to). But these days the fishing is also problematic: Hoy and his colleagues seem to pull in fewer and fewer each year. Now they worry that if the trend continues, they won't be able to feed their families. "There are less fish than last year, and less than the year before," he says. "I don't understand why."

Neither do many of the other 4 million people who depend on Tonle Sap for their living. Each year fishermen haul in about 230,000 tons of fish from its waters--half Cambodia's annual production. The lake's abundance, ecologists say, is showing signs of waning. Many species are no longer attaining their full size, a sign of pressure due to overfishing. The problem is most acute for the larger migratory species, such as the Mekong great catfish, formerly among the lake's most plentiful. A depletion of the fish stock, now a distinct possibility, would devastate the country. A staggering 60 percent of the population of 11 million get their daily protein from Tonle Sap. "If Tonle Sap dies, Cambodia will die, too," says May Sam Oeun, an official in the Ministry of Agriculture.

Tonle Sap, or Great Lake, is Southeast Asia's biggest freshwater lake, and it is something of a natural wonder. In spring, when snow melts in the Himalayan mountains and the rains fall in the lowlands, the waters of the Mekong River, Asia's longest, rise. The river gathers strength as it wends through China, Burma, Laos and Thailand, then back through Laos and down the Cambodian border. Near Phnom Penh, the current accumulates enough force to change the direction of Tonle Sap River, pushing water into the Great Lake and expanding its boundaries to more than 16,000 square kilometers--five times its original area. By storing water, the lake saves the Mekong Delta from flooding, and then releases the water downstream in the fall. "It is so crucial because it helps Cambodian areas for dry-season cropping, and in Vietnam for dry-season irrigation," says Sok Saing Im, a hydrologist at the Mekong River Commission, a group that promotes regional cooperation in development and conservation.

Tonle Sap's unique contribution to the Mekong River system also leaves it open to other countries' problems. The construction of hydroelectric dams in Laos and China block the natural migration of fish and disrupt the seasonal ebb and flow that is vital to mating and reproductive cycles. But development in the Tonle Sap region is a big factor in the lake's woes. Illegal logging has made many hills bare, causing silt to run off into the lake. Pesticides from neighboring farms seep into the lake, poisoning the fish and other wildlife. Logging and agriculture are already drying up surrounding wetlands, removing spawning ground for the lake's 1,300 species of fish and threatening rice crops. In short, too many people are clamoring for a piece of the lake. Cambodia's population doubled in the past 20 years and is expected to double again by 2020.

Drastic action is needed to save the lake, environmentalists say. Although the Cambodian government has worked with the Mekong River Commission to address some of the problems upstream, aid organizations say it has failed to stop the current plunder. "That would ...