The Mekong -- among the world's most biodiverse rivers, surpassed only by the Amazon and the Congo -- is home to some 1,148 recognised species of fish, with millions of people relying on its waters for their incomes.
But it faces a multitude of threats, according to environmentalists, including dam-building, sand mining, poorly managed fisheries, habitat loss and the introduction of non-native species.
The report said 19 percent of fish species in the river are seriously threatened by the changes, highlighting how depleted fish populations will affect millions whose livelihoods rely on the river.
"The alarming decline in fish populations in the Mekong is an urgent wake-up call for action," said Lan Mercado, WWF Asia-Pacific Regional Director.
"We must act now to reverse this disastrous trend because the communities and countries of the Mekong cannot afford to lose them."
The report from 25 regional and international groups examined the impacts on different parts of the 4,900-kilometre (3,040 miles) river -- the longest in Southeast Asia -- including Cambodia's Tonle Sap lake, where they said fish populations collapsed by 88 percent between 2003 and 2019.
The authors said 74 fish species were assessed as "at risk of extinction" on the International Union for Conservation of Nature Red List of Threatened Species.
The internationally recognised indicator of biodiversity health lists 18 species that are critically endangered.
"Officially this means that an estimated 19 percent of known Mekong fish species are threatened," the report said.
However, the report said publicly available data was minimal because many river species were under-researched and "it's safe to say that the true number of globally threatened fish species in the Mekong is much higher than 74".
The report also said disappearing fish species could exacerbate regional deforestation as millions who previously relied on the river are forced to farm. The Mekong accounts for some 15 percent of global inland catch.
"It's clear that we are risking a new biodiversity crisis for the Mekong River basin. But it's not too late," said Herman Wanningen, Managing Director of the World Fish Migration Foundation, which was part of the report.
In its recommendations, the report urges Mekong nations to commit to the Freshwater Challenge and protect and restore river ecosystems.
Increasing the river's natural flow, improving water quality, protecting critical habitat and species, and removing obsolete river barriers were among six pillars it recommended to help mend the Mekong.
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