The stingray, which measured almost four metres (13ft) from snout to tail and weighed slightly under 300kg (661lb), was captured on 13 June, according to a statement by Wonders of the Mekong, a joint Cambodian-US research project.
It was caught by a local fisherman south of Stung Treng in the northeast of the country and nicknamed “Boramy”, or “full moon”, by locals because of its round shape.
The fisherman alerted a nearby team of scientists from Wonders of the Mekong.
The previous record for a freshwater fish was a 293kg (646lb) Mekong giant catfish, discovered in Thailand in 2005, the group said.
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Mr Hogan added that the fact fish can still get this big is a “hopeful sign for the Mekong River”, despite environmental challenges.
The Mekong River runs through China, Myanmar, Laos, Thailand, Cambodia and Vietnam.
It is home to several species of giant freshwater fish but environmental pressures are rising with particular fear from scientists around a major programme of dam building in recent years that may be seriously disrupting spawning grounds.
Freshwater fish are defined as those that spend their entire lives in freshwater, as opposed to giant marine species such as bluefin tuna and marlin, or fish that migrate between fresh and saltwater like the huge beluga sturgeon.
Researchers say it’s the fourth giant stingray reported in the same area in the past two months, all of them females.
They think this may be a spawning hotspot for the species.