How a giant fish could help save its home river

Researchers may have solved a big mystery surrounding a verybig fish. Around the world, freshwater fish are in trouble. That’s especially true of large species. But one recent episode surprised scientists: A massive stingray was pulled out of the Mekong River by Cambodian fishermen last year.

The fish, a female, weighed 661 pounds, or about 300 kilograms, and set a record for the heaviest freshwater fish ever caught. The discovery was surprising because the species, known as the giant freshwater stingray, like many of the other big fish of the Mekong, is listed as endangered.

Yet, here was evidence that huge ones, somehow, still exist. “Imagine an era where whale populations are in broad decline — numbers are declining dramatically, whales are getting smaller and are seldom seen — and then, all of a sudden, Moby Dick appears,” said Zeb Hogan, an aquatic ecologist at the University of Nevada, Reno.

“It’s a shock and also opens the door to so many questions.” Nearly a third of freshwater fish worldwide are threatened with extinction. Since 1970, 94 percent of bigger species, those that weigh more than 66 pounds, have declined, researchers have found. In the Mekong, all of the other big fish are on the brink of extinction. “So how does the world’s largest freshwater fish persist?” Dr. Hogan said.

“And what can we learn from them about saving the Mekong system as a whole?” Cambodian scientists named the record-breaking stingray Boramy, which is Khmer for “full moon,” inspired by her round shape and the lunar phase that evening.

Before releasing her, in June last year, American researchers implanted an acoustic telemetry tag near her tail. Giant stingrays are not aggressive, but the team had to work with caution. That tail has a venomous barb that can reach almost a foot long and can penetrate bone.

The team has been tracking Boramy’s movements ever since as part of the Wonders of the Mekong project, which aims to maintain the economic, ecological and cultural assets of the Lower Mekong, a stretch of the river that is central to the livelihoods of some 50 million people. It turns out, one of the keys to Boramy’s sturdy constitution might be the fact that she tends to stay close to home.

According to findings published in May in the journal Water, her territory is surprisingly small for a fish of such size, encompassing just a few miles in a stretch of river known for its deep pools, its high species count and its population of endangered Irrawaddy dolphins.

The area is under consideration for designation as a UNESCO World Heritage site, which would lead to protection from the Cambodian government. But several major hydropower projects, which would require huge dams, have also been proposed.

Overall, the Mekong is increasingly threatened by dams, and also by overfishing, sand extraction, pollution and climate change. Boramy’s short-range tendencies stand in sharp contrast to other large species in the river, like the Mekong giant catfish, which can migrate 600 miles or more to spawn and feed.

And, Boramy’s preference for a small territory probably applies to giant freshwater stingrays in general, according to another study by Dr. Hogan and colleagues that was published in June.