Cambodian fishermen on the Mekong River got a shock when they hooked a giant stingray four metres long and weighing 180 kilos, scientists said on Wednesday.

The female stingray, 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 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 many species large and small, but a fish biologist from the University of Nevada said the river's underwater ecosystem was poorly understood.

More than 1,000 fish species call the Mekong home and the stingray is not the only giant living in the muddy waters -- the giant catfish and giant barb also reach up to three metres long and 270 kilos in weight.

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.