The population of the Mekong giant catfish has plummeted 90 percent in just two decades and the species are at risk of extinction, says a new WWF report.

According to the report titled River of Giants: Giant Fish of the Mekong, the world's biggest freshwater fish and four out of the top 10 giant freshwater fish species can be found in the Mekong River, which flows through Cambodia, China, Laos, Myanmar, Thailand and Vietnam.

But their existence has been threatened by a combination of infrastructure development, habitat destruction and overharvesting, the WWF said.

“More giant fish live in the Mekong than any other river on Earth,” said Dang Thuy Trang, Mekong River Ecoregion Coordinator for the WWF Greater Mekong Program.

“Currently, the Lower Mekong remains free-flowing, which presents a rare opportunity for the conservation of these species. But the clock is ticking.”

The report said the single most important threat is the hydropower dams in the lower Mekong and large tributaries.

“For many of the fish, the obstruction posed by these developments block migration routes to spawning grounds; if the mega fish stop migrating, they will stop spawning,” it said.

There are plans in various stages of development for 11 dams on the lower Mekong mainstream, including the Sayabouly hydropower dam in Laos, that will place further pressure on remaining populations of giant fish, WWF said.

It added that the impacts of lower Mekong River mainstream dams are not restricted to these Mekong giants and they would also exacerbate the impacts of climate change on the Mekong River Delta.

WWF said it supports a delay in the approval of the mainstream dams, including the Sayabouly Dam. It advises sustainable hydropower projects on tributaries of the Mekong River.