The world's largest recorded freshwater fish, a giant stingray, has been caught in the Mekong River in Cambodia, according to scientists from the Southeast Asian nation and the United States.

The stingray, captured on June 13, measured almost four metres from snout to tail and weighed slightly under 300 kilograms, according to a statement on Monday by Wonders of the Mekong, a joint Cambodian-U.S. research project.

The previous record for a freshwater fish was a 293-kilogram Mekong giant catfish, discovered in Thailand in 2005, the group said.

The stingray was snagged by a local fisherman south of Stung Treng, in northeastern Cambodia. The fisherman alerted a nearby team of scientists from the Wonders of the Mekong project, which has publicized its conservation work in communities along the river.

'It is hard to comprehend'

The scientists arrived within hours of getting a post-midnight call with the news and were amazed at what they saw.

"Yeah, when you see a fish this size, especially in freshwater, it is hard to comprehend, so I think all of our team was stunned," Wonders of the Mekong leader Zeb Hogan said in an online interview from the University of Nevada, Reno. The university is partnering with the Cambodian Fisheries Administration and USAID, the U.S. government's international development agency.

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, such as the huge beluga sturgeon.

The stingray's catch was not just about setting a new record, Hogan said.

"The fact that the fish can still get this big is a hopeful sign for the Mekong River, " he said, noting that the waterway faces many 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. In particular, scientists fear a major program of dam building in recent years may be seriously disrupting spawning grounds.

Tagged by scientists

"Big fish globally are endangered. They're high-value species. They take a long time to mature. So if they're fished before they mature, they don't have a chance to reproduce," Hogan said. "A lot of these big fish are migratory, so they need large areas to survive. They're impacted by things like habitat fragmentation from dams, obviously impacted by overfishing. So about 70 per cent of giant freshwater fish globally are threatened with extinction, and all of the Mekong species."

The team that rushed to the site inserted a tagging device near the tail of the mighty fish that will send tracking information for the next year, providing unprecedented data on giant stingray behaviour in Cambodia.

"The giant stingray is a very poorly understood fish. Its name, even its scientific name, has changed several times in the last 20 years," Hogan said. "It's found throughout Southeast Asia, but we have almost no information about it. We don't know about its life history. We don't know about its ecology, about its migration patterns."

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 hot spot for the species.

Local residents nicknamed the stingray "Boramy," or "full moon," because of its round shape and because the moon was on the horizon when it was freed on June 14. In addition to the honour of having caught the record-breaker, the lucky fisherman was compensated at market rate, meaning he received a payment of about $600.