Researchers may have solved a great mystery surrounding a very large fish.

Around the world, freshwater fish are in trouble. This is especially true for large species. But one recent episode surprised scientists: a massive stingray was pulled from 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 Mekong’s other large fish, is listed as endangered. However, here was evidence that huge ones, somehow, still exist.

“Imagine an era where whale populations are in broad decline — numbers are dropping dramatically, whales are getting smaller and rarely 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.”

Almost a third of freshwater fish worldwide are threatened with extinction. Since 1970, 94 percent of larger species that weigh more than 66 pounds have declined, researchers found.

In the Mekong, all other large fish are on the verge of extinction. “So how does the world’s largest freshwater fish last?” Dr. Hogan said. “And what can we learn from them about saving the Mekong system as a whole?”

Cambodian scientists named the record stingray Boramy, which is Khmer for “full moon,” inspired by her round shape and the moon phase that evening. Before releasing her, in June of last year, US researchers implanted an acoustic telemetry tag near her tail. Giant stingrays are not aggressive, but the team had to work carefully. That tail has a poisonous barb that can reach nearly a foot in length and can penetrate bone.

The team has been tracking Boramy’s movements since then as part of the Wonders of the Mekong project, which aims to preserve 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, hers territory is surprisingly small for a fish of this size, spanning 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 being considered for nomination as a UNESCO World Heritage Site, which would lead to protection by the Cambodian government. But several major hydropower projects have also been proposed, which would require huge dams.

In general, the Mekong is increasingly threatened by damsand also through overfishing, sand extraction, pollution and climate change.

Boramy’s short-distance tendencies stand in sharp contrast to other large species in the river, such as 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, which was published in June.

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

“We were quite surprised by this because we thought they would migrate around,” said Dr. Chayanis Daochai, an aquatic veterinarian at Chulalongkorn University in Bangkok and 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 typically 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 endangered 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 live in areas where the water quality is still good and local communities are committed to conservation.

In July, Dr. Hogan and his colleagues published further evidence indicating that giant stingrays are still around regularly caught in the Mekong and other Asian rivers. In addition, anglers throughout the species’ range have reported 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, as 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 way forward to save the stingray seems less insurmountable,” said Dr Hogan. “And efforts to protect it could also benefit these other species that face extinction.”

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