Researchers may have solved a big mystery surrounding a very big 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.

Using acoustic telemetry, the researchers tracked 22 giant freshwater stingrays in a section of the Mekong in Thailand and found that many of the animals also confined themselves to relatively small areas, on the scale of a few miles.

“We were quite surprised by this, because we thought they’d migrate around,” said Dr. Chayanis Daochai, an aquatic veterinarian at Chulalongkorn University in Bangkok and a co-author of the study published in June.

The researchers in Thailand also observed that male and female giant stingrays of all ages tended to live together, another discovery that stands in contrast to other Mekong megafish, which usually spend parts of their lives in separate sections of river.

Taken together, these findings may help explain why giant freshwater stingrays are not yet as imperiled as other large Mekong species, Dr. Hogan said. Because they do not have to migrate long distances as part of their life cycle, they can eke out an existence in places where the water quality is still good and local communities are committed to conservation.

In July, Dr. Hogan and his colleagues published additional evidence indicating that giant stingrays are still regularly caught in the Mekong and other Asian rivers. What’s more, fishermen across the species’ range have been reporting encounters with stingrays weighing several hundred pounds.

The findings suggest that protecting key sections of the river could go a long way toward protecting giant freshwater stingrays, so long as the river as a whole does not become heavily polluted or dammed.

“Given the pervasive and systemic threats to megafish as a group, the path forward for saving the stingray seems less insurmountable,” Dr. Hogan said. “And efforts to protect it could also benefit these other species that are facing extinction.”


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