Rapid Decline in Tonle Sap Raises Alarms For Key Southeast Asian Fishery

Sharp reduction in sediment buildup blamed on hydroelectric dams and riverbed dredging.

Tonle Sap and the Mekong region.

Cambodia’s Tonle Sap, a lake known as the country’s “beating heart,” faces a more rapid decline than previously estimated, according to a new study.

Experts say that urgent countermeasures are needed to save Southeast Asia’s largest freshwater lake, which has served Cambodia for millennia.

Millions of Cambodians depend on the Mekong River and the Tonle Sap for fish catches that provide them with their main source of protein.

Cambodian fishermen pull more than 400,000 tons of fish out of the Tonle Sap each year, making the lake the world’s largest and most productive inland fishery.

The new study focusing on the sediment flow in Southeast Asia’s longest river was produced by the Stockholm Environment Institute (SEI), an independent nonprofit research organization.

The damage done to fish migration to and from the Tonle Sap by upstream dams in China and Laos has already been widely reported.

The dams and their reservoirs block adult fish trying to migrate upstream and  larvae and juveniles trying to migrate downstream.

But the damage to the Tonle Sap caused by a decline in sediment flows which have been disrupted by the dams has been less well studied.

Robust sediment flows to the Tonle Sap lakebed help to nourish the plants and smaller organisms on which the fish feed. Without this sediment, the lake’s food chain is likely to collapse.

According the SEI report, factors leading to “a drastic reduction in sediment loads” in the Mekong include hydroelectric dams, riverbed mining for sand, land-use changes, and climate change.

Among these factors, the report says, the most important are the dams’ reservoirs and riverbed mining.

Climate change, which has brought extreme weather changes, including increasingly intense floods and drought, exacerbates the situation, it says.

The sand dredged from the Mekong riverbed goes mostly into concrete. The concrete has been used in the dams and in other Chinese projects being built along the riverbanks and for the Chinese-backed construction of a China-Laos railroad and its tunnels.

The Stakes

The stakes are huge. According to the World Wildlife Fund, the Mekong counts for up to 25 percent of the global freshwater catch and provides livelihoods for at least 60 million people.

The Food and Agriculture Organization of the United Nations (FAO) says that the Mekong is the second most biodiverse river in the world after the Amazon.

The river supports the largest inland fishery in the world. But dams have been disrupting the migration of more than 100 species of fish.

In Cambodia, researchers have warned in recent years that a loss of fish resources caused by dam building could devastate the health of rural children.

And experts have documented signs of chronic malnutrition in a number of villages.

In 2010, The Mekong River Commission (MRC) recommended that all damming be postponed through 2020.

The MRC was formed in 1995 through an agreement among the governments of Cambodia, Laos, Thailand, and Vietnam to ensure the “reasonable and equitable use” of the Mekong River system.

But the commission’s findings are nonbinding, and China is not a member of the MRC.

The importance of sediment

Sediment might sound like an arcane subject studied by no one but agricultural experts. But for farmers and fishermen living downstream from China, it can mean life or death.

According to Thanapon Piman, a researcher for the SEI and lead author of its report, sediment is “critical for the formation and stabilization of deltas and the ecosystems they support.”

Sediment, or silt, enrichs the entire Lower Mekong Basin and supports farms, irrigation, fisheries, water quality, and water-based plants.

Dr. Thanaphon says that if all of the dams proposed for the Lower Mekong Basin are developed, including dams that are planned or ongoing, this could prevent more than 90 percent of Mekong’s sediment load from being transported downstream.

The SEI report isn’t the first of its kind to warn of looming, mostly man-made disasters on the Mekong.

A 2012 study by the Washington, D.C.-based National Academy of Sciences identified Cambodia’s Lower Sesan 2 tributary dam as the most destructive to fish biodiversity of the country’s dams.

The dam was expected to endanger 56 fish species. That impact will be felt across Cambodia as well as in the neighboring countries of Laos, Thailand, and Vietnam.

In June 2014, The New York Times reported that a growing population of fishermen at the Tonle Sap were engaging in overfishing. The cutting of mangrove swamps that shelter fish added to the damage.

During the dry season, the Tonle Sap feeds the Mekong River, releasing water that irrigates and nourishes crops to the southeast

But during the rainy season, the Mekong backs up into the Tonle Sap.

In brief, the waters of the Tonle Sap ebb and flow with the annual cycle of the river that feeds it. The Tonle Sap River connects the lake to the Mekong.

The ebb and flow of its waters gives Southeast Asia’s largest freshwater lake the nickname “Cambodia’s beating heart.”

The good news is that many internationally recognized scientists have been monitoring and reporting on the Mekong and Tonle Sap.

A team involving some of these scientists has been studying the impact of upstream dams in China and Laos on the Tonle Sap since 2012 and has built a computer model to track developments there.

International organizations such as the Asian Development Bank (ADB) and the European Union have been supporting studies on environmental flows and food security in the region.

The U.S. State Department runs a Lower Mekong Initiative in support of such activities.

The SEI report was published with the support of the United Nations Educational, Scientific and Cultural Organization (UNESCO).

Last year, Radio Free Asia sent a cameraman to document changes occurring along the Mekong. When he visited the Tonle Sap a representative for five communes depending on the lake spoke of its importance to Cambodia.

“If the Tonle Sap is alive,” he said, “then the Mekong is alive.”

But over the past five to six years, the water flow has been unpredictable and the fish catch declining.

As RFA’s cameraman reported in a series of blogs titled “A River in Peril,” fishermen working on the Tonle Sap on a daily basis report that they sometimes find no fish at all.

Some fishermen have given up. Young men are leaving to find work elsewhere. Some head to neighboring Thailand or even as far away as South Korea.

Writing for The Phnom Penh Post on Aug. 1 of this year, Alessandro Marazzi Sassoon summed up the view of many experts that “Cambodia’s most important ecosystem is in crisis.”

‘The ecosystem of the gigantic lake is set to spectacularly collapse,” he wrote.

“At least…,” he says, “if drastic measures aren’t taken—and soon.”

“Such was the prevailing sentiment” he says, at an International Symposium on “Flood Pulse Ecosystems” held in Siem Reap, Cambodia, where researchers convened in late July this year.

Sassoon said that the tone at the conference “alternated between frustrated and funereal.”

What can be done?

Any solution to saving the Tonle Sap and other fishing grounds in or near the Mekong River would have to begin with a curb on dam building, much of it carried out by Chinese state-owned companies.

In all, more than 15 countries’ companies are engaged in dam-building.

In China, six dams have been completed with 13 more under construction or planned.

Laos has plans for 140 dams with roughly 30 percent of them completed. Most of them are dams located on tributaries.

According to Brian Eyler,  an expert on the issues, tributary dams can disrupt “environmental flows,”  such as fish migration and sediment distribution, just as seriously as mainstream dams do.

Eyler directs the Southeast Asia program at the Stimson Center, a nonpartisan research center based in Washington, D.C.

Two mainstream dams in Laos, which are still under construction, have designs intended to mitigate damage to fish built into them, such as fish ladders.

Laos’s Xayaburi dam has extensive fish modifications, including a fish elevator never tried before and accommodations for fish passage in the navigation lock system, Eyler says.

Xayaburi also has installed expensive sediment flushing gates.

The Don Sahong dam uses a modification of natural passages in channels adjacent of the dam site to permit fish passage. The developer deepened and widened these passages to permit fish passage in the dry season.

But it remains to be seen, Eyler says, whether these innovations will produce predicted results.

“The simple truth is that we still don’t know enough about the dams’ impacts on fish migration to determine before and after impacts of these two projects,” says Eyler.

According to Eyler, this is why The Mekong River Commission recommended in 2010 that all damming on the Mekong mainstream be postponed through 2020 until more data is collected.

“These dams use untested fish-mitigation technology, so even modeling results from the developer should be questioned and scrutinized,” says Eyler.

Cambodia itself has plans for 60 dams, with about 10 percent of them completed. But only one of the completed dams, the much-criticized Sesan 2 dam, is located in the Mekong Basin and relatively near the Tonle Sap.

At a workshop held in Ho Chi Minh City on Nov. 30, experts argued that a growing awareness of the inefficiencies and high costs of hydropower should induce Laos and other countries to take another look at renewable energy.

Eyler, who chaired the workshop, has long argued that the current hydropower system in Laos favors the needs of investors, which are short-term and driven by the bottom line.

In Eyler’s view, the energy infrastructure in Laos, while designed to make Laos “the battery of Southeast Asia,” is “highly inefficient.”

Nikky Nkiruka Avila from the Energy and Resources Group at the University of California, Berkeley, said that hydropower dams are becoming more costly.

She advocated the increased use of other renewable energy sources, such as wind and solar powered energy, noting that the prices for the equipment needed for these sources have plummeted.

Laos and Cambodia offer ideal conditions for solar and wind power generation.

These renewable energy sources could potentially substitute for dams now in the planning stage. If built, those dams could become the most disruptive for environmental flows yet.

Dan Southerland is RFA's founding executive editor.