Yin Vuth, one of hundreds of Cambodians who protested against the Don Sahong dam over the weekend, said that if construction on the project in Laos goes ahead, the fish will disappear, and once the fish disappear, the dolphins will be next.

“All we will have left will be water contaminated by the dam,” said Vuth, 53, who runs a dolphin-spotting tourism business about 2 kilometres from the construction site in Laos’s Champasak province, just over the border from Cambodia.

About 400 protesters traversed the Mekong River on 50 or so longboats through Kratie province’s Sambor district and Stung Treng’s Thala Barivat district near the two countries’ border in the northeast.

The demonstrators wore white shirts and unfurled banners as they negotiated the waterway, calling for the preservation of the river system and the cancellation of the multimillion-dollar dam.

They called on the governments of Laos, Cambodia, Thailand and Vietnam to axe the development at a meeting of the Mekong River Commission later this week, which will be attended by Prime Minister Hun Sen.

“This dam was established against international law and the Mekong Agreement of 1995, which was signed by Laos, and it was not discussed with the people in other countries either,” Vong Kosal, legal aid officer at the NGO Forum, said.

“The Mekong basin has been chopped up into stairs for building the hydro-electric dams, and it brings more disadvantages,” Kosal said.

Sean Kin, Kratie provincial director of fisheries, said that the river was his constituents’ spiritual home, a source of essential nutrition and tourism income.

“Fresh water dolphins are our living heritage, which brings in more tourists,” he said. “The dam will slow down the water currents, and the environment will be changed for both animals and people.”

The 260-megawatt Don Sahong dam is one of 12 large-scale hydropower projects planned for the lower Mekong river basin. It has drawn strong criticism from rights groups and local communities.

In a recent report on the dam, the World Wildlife Fund said that judging by the low standards of an environmental impact assessment for the project, the negative impact on millions of people could be severe.

“If it depends on this weak evaluation, Mekong river fishery resources and the 60 million people who are living in the sub-Mekong river basin will face high risks,” the report said.

The WWF estimates there are only about 85 Mekong River dolphins left in the wild.

“If Laos really constructs the dam, we will lose our rare resources, especially the dolphins first,” said Sam Sovann, an executive officer of local NGO Northeastern Rural Development. “Dolphins bring in money, but do not need money to be fed, like pigs or chickens.”

But if they die, the resources will not be able to be replaced by spending money, he added.

According to Sovann, the communities gathered more than 3,000 signatures to submit to Hun Sen ahead of the MRC summit on Saturday.