The waters of the mighty Mekong have sustained generations of families but nowadays its fishermen often find their nets empty and fear hydropower mega-dams will destroy their livelihoods.

Pat Chaiwong has fished the Mekong in northern Thailand for three decades but good days are increasingly rare.

"Some days I can catch fish. Some days I don't catch any," lamented the 67-year-old, one of about two dozen fishermen in the community of Wiang Kaen in the northern province of Chiang Rai.

For many in the community, the reason lies upriver in the Chinese province of Yunnan where dams on the upper reaches of the river disturb the delicate cycle of nature.

"Usually the water would rise (and fall) with the seasons," said fisherman Decha Chaiwong, 48.

Now it ebbs and flows depending on whether the dams are open or closed, he said.

"That's why the fish have decreased."

Today a new threat looms downstream in neighbouring Laos.

The Xayaburi hydropower project is one of 11 planned on the lower Mekong, raising worries for the future of the 60 million people in the region estimated to depend on the river in some way.

The 4,800 kilometre (3,000 mile) long waterway, the longest in Southeast Asia, is home to hundreds of species of freshwater fish including the endangered giant Mekong catfish, according to conservation group WWF.

Environmentalists warn that damming the lower Mekong would trap vital nutrients, increase algae growth and prevent dozens of species of migratory fish -- including the giant catfish -- swimming upstream to spawning grounds.

"If there is the Xayaburi dam, fish cannot lay eggs and the numbers of fish and their breeds decrease. There will definitely be a big impact," said Niwat Roykaew, president of the Natural Resources and Culture Conservation Network in Chiang Rai province.

Many of the roughly 200 species in the lower Mekong swim upriver to spawn -- one of the most important mass river migrations in the world, according to the Mekong River Commission (MRC).

The hydroelectric project at Xayaburi, led by Thai group CH Karnchang, has sharply divided four Mekong nations -- Laos, Vietnam, Cambodia and Thailand -- who formed the MRC, an intergovernmental group.

Communist Laos, one the world's most under-developed nations, believes the planned 1,285 megawatt dam -- which will cost $3.5 billion according to state media -- will help it become "the battery of Southeast Asia".

Thailand has agreed to buy most of the electricity generated by the project, but Cambodia and Vietnam fear the dam could seriously affect fish migration and sediment flows, hitting their farming and fishing industries.

Despite the concerns, construction on the main part of the dam began in November with Laos predicting completion by the end of 2019.

Niwat described the decision to go ahead without listening to people's concerns as "a coup d'etat against the Mekong River".

"We're the children of the Mekong River. We were born and grew up on the Mekong. It has taken care of us and provided for us. Then one day the dams came," he added.

His association has filed a lawsuit against the state-run Electricity Generating Authority of Thailand (EGAT) and the Thai government in an attempt to block the project.

Neither EGAT nor developer CH Karnchang responded to request for comment.

Laos has modified the design to try to minimise the impact, said Hans Guttman, chief executive officer of the Mekong River Commission Secretariat.

The changes include a system to flush sediment through the dams and allow fish to migrate through fish passages, he said.

"It is still obviously up for question whether this sediment flushing will work as envisaged because it has never been tested, and there is also concern about whether the fish passages will work on a structure this big," he added.

In a study published in 2011, the MRC warned that the construction of 11 dams on the river in Laos and Cambodia as well as dozens more on tributaries, could cause fish catch to drop by at least 25 percent by 2030.

Thai villagers are fighting "on behalf of the Mekong" and also for people in Laos, Vietnam and Cambodia who have less freedom of speech, said Pianporn Deetes with campaign group International Rivers.

"There are many festivals and traditions connected to the Mekong river," she said, such as the mythical Naga snake that protects the Mekong.

"But if the dams are blocking the river, this means the Naga cannot move upstream," she added.

The same goes for the Mekong giant cat fish, one of the world's biggest freshwater fish which can reach three metres (10 feet) in length and 300 kilos (660 pounds) in weight.

Already threatened by overfishing, only about 200 are estimated to remain, according to a recent study by WWF, which fears that dam construction will drive the iconic creature to extinction.

It has been years since one ended up in fisherman Pat's net.

"In the past, I caught a lot of them but now not at all," he said. "I don't know where they all went."