Towering waves crash onto the rocks while fishermen, submerged in water, tie together long poles of wood and weigh them down with heavy stones. These are the remaining fishermen in Laos’s Four Thousand Islands area of the Mekong River, the only place in the world where fishing traps known as li are used. The practice, which has been passed from generation to generation for hundreds of years, is about to disappear.

The Lao government has given Mega First Corporation Bhd, a Malaysian company, permission to build Don Sahong dam, a hydroelectric project, just one kilometre north of the border with Cambodia. Environmentalists say the dam will stop the flow of fish from the Tonle Sap in Cambodia up to China, and robbing thousands of families of their livelihoods. The Li practice is likely to go with them.

On Friday, the Thai photographer Piyavith Thongsa-Ard exhibited a selection of photographs and documentary footage showing fisherman at work in the Four Thousand Islands area at Opera Café in Phnom Penh.

He first visited the area back in 2000 when he was working as a UNICEF photographer, and was captivated by what he saw. He took photographs, but they were damaged through flooding in Chiang Mai a few years later. When he heard that the tradition was under threat, he returned to Four Thousand Islands last year, staying with the fishermen for seven months.

He is working on a book about the li tradition with the writer Adele Clusen.

He said: “I wanted to document the tradition before it stopped. It’s too late to campaign against the construction now, because it’s definitely going to happen. The government forced the fishermen to sign [a contract] saying they’d stop the fishing.”

With their livelihoods ruined, what is going to happen to the fishermen and their families? Most of them will go to Thailand as illegal immigrants, said Piyavith. In fact, many have gone already.

He added: “Protesting didn’t even cross their minds, because in Laos nobody wants to fight the government. If the government orders them to do something, they just accept it. If they have no work, they have to move to Thailand to become illegal labourers. It’s too expensive for them to buy a passport.”