Three adult giant catfish were caught on a single day on Tuesday in Mekong River weighing from 95 kg to 131 kg, two of them longer than two metres.
All three were measured and tagged for tracking purposes before being released back into the river, with DNA samples of the fish collected.
The Wonders of the Mekong project, a U.S.-funded conservation initiative, described the catch of so many giant catfish in just a few days as "a remarkable and unprecedented event".
"I've never heard of this before," said its project lead, Zeb Hogan, a research biologist at the University of Nevada Reno.
"By tagging these fish, we gain critical information about their ecology, their migrations, their habitat...to try to help these fish survive in the future."
Three other giant catfish were caught, tagged and released in the days prior to that, one in the Mekong and two on the Tonle Sap River near the capital Phnom Penh.
Some giant catfish can weigh as much as 300 kg and span three metres.
The catch indicates spawning activity in the Mekong in Cambodia and follows 25 years of conservation work by the group in collaboration with local fisheries organisations and communities.
Wonders of the Mekong said the catch boosts hopes for survival of a species that has become increasingly rare in many of its habitats.
Reuters