Cambodian fisherman Sles Hiet lives at the mercy of the Mekong: a massive river that feeds tens of millions but is under threat from the Chinese dams cementing Beijing's physical — and diplomatic — control over its Southeast Asian neighbours.

The 32-year-old, whose ethnic Cham Muslim community live on rickety house boats that bob along a river bend in Kandal province, says the size of his daily catch has been shrinking by the year.

“We don't know why there are less fish now,” he told AFP of a mystery that has mired many deeper into poverty.

It is a lament heard from villages along a river that snakes from the Tibetan plateau through Myanmar, Laos, Thailand, Cambodia and Vietnam before emptying into the South China Sea.

Nearly 4,800 km (2,982 miles) long, the Mekong is the world's largest inland fishery and second only to the Amazon for its bio-diversity.

It helps feed around 60 million people across its river basin.

Yet control over its taps rests to the north with China, whose premier Li Keqiang will land in Phnom Penh on Wednesday to lead a new regional summit that could shape the river's future.

Beijing has already studded the Mekong's upper reaches with six dams and is investing in more than half of the 11 dams planned further south, according to International Rivers.

Environmental groups warn the blockages pose a grave threat to fish habitats by disrupting migrations and the flow of key nutrients and sediment — not to mention displacing tens of thousands of people with flooding.

Communities in the lower Mekong countries have reported depleted fish stocks in recent years and are blaming the dams.

Experts say it is too early to draw full conclusions given a lack of baseline data and the complex nature of the river's ecosystem.

But what they do agree on is that China has the upper hand over a resource that serves as the economic lifeblood of its poor southern backyard.

The lower Mekong countries are “not able to stand up to China geo-politically,” said Thitinan Pongsudhirak, a foreign policy expert at Bangkok's Chulalongkron University. That allows Beijing to keep “undermining habitats and millions of livelihoods downstream.”

China rewrites the rules

With control over the headwaters of the river — known there as the Lancang — Beijing can dam its section of the river while the impacts are felt downstream.

It can also modulate water levels, a powerful bargaining chip displayed in 2016 when China opened dam gates on its soil to help Vietnam mitigate a severe drought.

The regional superpower is now asserting its authority through the nascent Lancang-Mekong Cooperation forum, while appeasing its Southeast Asian neighbours with investment and soft loans.

Leaders from all six Mekong countries will attend the LMC this week in Cambodia.

China's foreign ministry bills the forum, which also covers security and trade issues, as a way to foster “economic prosperity, social progress and a beautiful environment”.

But environmentalists say the LMC aims to replace the long-standing Mekong River Commission — a regional body that has tried to manage development along the river — albeit without China.

“There is major concern that China's leading role and relative influence will see it prioritising its own interests over meaningful co-operation,” warned Maureen Harris, Southeast Asia programme director at International Rivers.

Chinese companies are investing billions of dollars in many of the dams but have so far failed to carry out full environmental and social impact assessments.

Firms and state agencies from Thailand, Vietnam and Laos also stand to gain from their investments in the hydropower projects.

“Much of the benefit will be reaped by the financial and business interests involved, with impacts to hit hardest local communities along the river,” Harris said.

Calls to protect the river have largely gone unheeded in Southeast Asia, where governments are eager to meet energy needs and unwilling to stand up to China or resist its cash.

That makes the Mekong's dependents, such as fisherman Sles Hiet, an afterthought. “We depend on the Mekong river,” he said. “Even though there are less fish we are still trying because we don't have any other jobs and we have no land to farm. “