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The predatory fish can grow up to 4 feet in length, and has a conspicuous knob at the tip of its lower jaw. A striking patch of yellow surrounds its large eyes.

With the last confirmed sighting in 2005, “this species of fish seems to have disappeared from the Mekong region for decades,” said Chheana, who is a co-author of a in the journal Biological Conservation that documents the recent sightings.

Since 2017, biologists tracking migratory fish species in Cambodia have developed relationships with local fishing communities, asking them to alert any unusual sightings.

That's how the three giant salmon carp found in the Mekong River and a tributary in Cambodia between 2020 and 2023 came to the attention of researchers.

“I was really surprised and excited to see the real fish for the first time,” said Bunyeth Chan, a study co-author and researcher at Svay Rieng University in Cambodia.

Researchers say the sightings give them new hope for the fate of the species. One nickname for the species is “ghost fish.”

“This rediscovery is very exciting, positive news,” said Zeb Hogan, a fish biologist at the University of Nevada, Reno, who was part of the team.

But the plight of the fish also spotlights the perils facing all migratory species in the Mekong, which faces industrial pollution and overfishing.

More than 700 dams are built along the river and its tributaries and there are very few functional “fish passages” to help species navigate obstructions, said Brian Eyler, director of the Southeast Asia Program at the Stimson Center in Washington, who was not involved in the research.

The biologists said they hope that working with local communities in Thailand and Laos will enable them to confirm if the fish still swims in other stretches of the Mekong River.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group. The AP is solely responsible for all content.