US scientists have suggested that unexplored deep pools in the Mekong River in an area of Stung Treng could potentially be home to significant populations of giant freshwater stingrays, one of the world’s largest freshwater fish species.

This comes as a fisherman hooked a 180kg stingray in the Mekong last week, and a rescue team documented and released the specimen back into river unharmed, an incident which the scientists said highlights the area’s global significance.

The endangered giant freshwater stingray, nearly 4m in length, was hooked by the fisherman from a small island community in the middle of the Mekong River on May 5 in Siem Bok district’s Koh Preah village and commune.

“The catch, together with other data collected by a recent scientific expedition to the area, suggests that the remote site – which is characterised by pools up to 260 feet [79m] deep and is essential habitat for such iconic animals as Irrawaddy dolphins and giant softshell turtles – could also be home to the world’s largest freshwater fish,” the University of Nevada’s Reno campus said in a press release on May 10.

According to the press release, interviews with fishermen working in the region suggest that the 180kg stingray caught recently is dwarfed by other catches perhaps twice that size in the same remote area over the past 20 years.

The giant freshwater stingray is Southeast Asia’s largest fish and one of its rarest. It is the focus of a recent expedition by well-known underwater explorers as part of the Wonders of the Mekong project funded by the US Agency for International Development (USAID).

The expedition’s base camp was adjacent to one of the Mekong’s deepest pools and to a nature reserve designed to protect spawning fish. The area itself is dotted with islands covered in seasonally flooded forests and occupied by small fishing camps.

“The fishermen here tell stories of catching what would be record-breaking stingrays and other giant fish,” said Zeb Hogan, University of Nevada fish biologist and project lead of the Wonders of the Mekong project. “It’s truly a wonder that a place like this still exists.”

Members of the Wonders of the Mekong team worked with local fishermen to rescue the massive stingray, which was captured accidentally after it swallowed a smaller fish that had eaten a baited hook.

The rescue team was able to unhook the stingray, weigh it and measure it. Then it was released back into the river unharmed. The successful release highlighted the utility of the project’s network of fishermen in studying and conserving the rare and unusual biodiversity of the Mekong River, said the press release.

The Wonders of the Mekong’s study area is located just downstream from a Ramsar wetlands site of international importance and is home to many of the Mekong’s almost 1,000 fish species, including most of the Mekong’s imperilled giants – the giant stingray, Mekong giant catfish and giant barb.

The giant freshwater stingray can grow up to 5m in length counting its tail, 2m or more across and up to 600kg in weight. The Mekong giant catfish and giant barb can grow up to 3m in length and 300-400kg in weight.

Researchers believe the area is critically important as a dry season fish refuge and spawning site and as a last refuge for many endangered giant fish. It is a region that is recognised as special and protected by local communities.

Hogan said the rescued stingray and its habitat highlight the characteristics of the Mekong’s aquatic ecosystems generally: diverse, globally significant, poorly understood – and under threat.

“These are unseen worlds, underappreciated and out of sight,” he said.

Leading up to the field expedition, the Wonders of the Mekong scientists conducted community and market surveys to gather information about aquatic biodiversity and then in collaboration with the Cambodian Fisheries Administration they also established a network of fishermen who agreed to report any catches of giant or endangered fish.

Sin Piseth, a river guard based in Koh Preah village, said the stingray was caught after it swallowed a fish that had swallowed the baited hook at the end of a line cast into the water by a fisherman using an ordinary rod and reel.

Pheng Boeun, head of the Koh Preah fishing community, said this is not the first time that a giant stingray has been caught by local fishermen.

Boeun said that the Mekong River in Siem Bok district has many such stingrays, but most of them weigh between 30-40kg. All of the stingrays that are caught are usually released as there are no buyers for them because they aren’t traditionally eaten by Cambodians.