PHNOM PENH (AFP) - Cambodian fishermen on the Mekong River got a shock when they inadvertently hooked an endangered giant freshwater stingray 4m-long and weighing 180kg, scientists said on Wednesday (May 11).

The female leviathan, one of Southeast Asia's largest and rarest species of fish, was caught by accident last week in Stung Treng province when it swallowed a smaller fish that had taken a baited hook.

An international team of experts on the US-funded Wonders of the Mekong project worked with the fishermen to unhook the ray before weighing and measuring it and returning it unharmed to the river.

The giant Mekong is a crucial habitat for a vast array of species large and small, but project leader Zeb Hogan, a fish biologist from the University of Nevada, said the river's underwater ecosystem was poorly understood.

"They are unseen worlds, underappreciated and out of sight," he said in a statement issued by his university.

More than 1,000 fish species call the Mekong home and the stingray is not the only giant lurking in the muddy waters - the giant catfish and giant barb also reach up to 3m-long and 270kg in weight.

The study group said in the statement that the remote location where the ray was caught has pools up to 80m deep and could harbour even bigger specimens.

But they also warned that underwater video footage showed plastic waste even in the deepest stretches of the Mekong, along with "ghost nets" - abandoned by fishers but still able to snare fish.

Environmentalists have long voiced concerns about dam building along the Mekong River that will destroy fish stocks. The famous waterway starts in China and twists south through parts of Thailand, Laos, Myanmar, Cambodia and Vietnam, feeding 60 million people through its basin and tributaries.