Study co-author Hogan heads the National Geographic Society's Megafishes Project, a three-year program launched this year that will document the world's largest freshwater fishes.
(National Geographic News is part of the National Geographic Society.)
Many catfish species are believed to be migratory, Hogan noted, but they live in murky rivers, making documentation of their movements difficult.
"We did suspect that Pangasius krempfi was anadromous, because fishermen reported the species in the South China Sea and far up the Mekong," Hogan said.
"The challenge was to connect the dots—to show that the fish from the South China Sea and the Mekong were part of the same population."
The scientists examined chemical markers called isotopes in bone and muscle tissue of catfish from the Khone Falls region of the Mekong.
The team found evidence that the freshwater-dwelling fish had migrated recently from a marine habitat.
John Lundberg is curator of fish in the department of ichthyology at the Academy of Natural Sciences in Philadelphia, Pennsylvania.
"As far as I know … this is the first record of a riverine catfish migration between marine and freshwater habitats based on radioisotope evidence," Lundberg said via email.
"This is a highly regarded method, so I would say the finding is sound. … [O]ne hopes now that catch data will confirm the presence of Pangasius krempfi in seawater."
National University of Singapore's Ng added: "The fact that these large catfishes have been fished for hundreds of years, and yet it is only recently that we know that these are anadromous, shows how little we know about the biology of many of these catfish species."
Famous Soup
P. krempfi is closely related to more than ten other species of Asian catfish.
Its relatives include the Mekong giant catfish and the Chao Phraya giant catfish—often called the "dog-eating" catfish because of its taste for canine carcasses.
A Mekong giant catfish caught in 2005 measured about 9 feet (2.7 meters) long and weighed 646 pounds (293 kilograms).
P. krempfi grows up to 4 feet (1.2 meters) long, and the species is a popular food. In Vietnam—one of six countries the Mekong passes through—the fish is the main ingredient in a famous soup.
But news that the species is migratory could have serious implications for its conservation, experts warn.
Scientists point out that migratory fish are more vulnerable to extinction, because they fish rely on a variety of habitats to complete their life cycles.
Fishers can also easily target such species during spawning runs.
The new study suggests that the fish's ability to spawn might be compromised by the plans to build a dam at Khone Falls.
More than 98 percent of the P. krempfi caught each year in the Khone Falls region are taken between mid-May and June, the research shows.
This leads scientists to believe that the catfish swim through the area only once a year, probably during spawning.
Study co-author Hogan said that "plans for a dam on the mainstream Mekong threaten migratory fish, which in turn threatens the livelihoods of the millions of people in the Mekong that depend on fish for food."
Ian Baird, a study co-author at the University of British Columbia in Vancouver, said that the study highlights the need for all six countries that contain the Mekong to cooperate on protecting the river and its wildlife.
"Fish species do not recognize socially constructed national borders," he said.
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