Watch a Giant Stingray’s Safe Return to Its River Home

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Just after dawn on May 5, scientists working along a stretch of the Mekong River in Cambodia released a giant, endangered freshwater stingray that had been caught on a fisherman’s line. At 13 feet long and 400 pounds, the gigantic animal pancake was larger than a hibachi table.

“It was shaking, and I told her, ‘Calm down, we will release you soon,’” said Chea Seila, a coordinator for the Wonders of the Mekong Project.

The giant freshwater stingray, Urogymnus polylepis, is the world’s largest stingray species, known also as a whipray. With dusky-brown tops and creamy white bottoms, the animals slide across riverbeds in search of fish and invertebrates. Though they can grow to epic proportions, over-harvesting for the stingray’s meat, accidental deaths in fishing nets and habitat fragmentation and degradation from dams, pollution and other human activities have made the animals endangered.

After receiving a call from the fisherman who caught the stingray, Ms. Chea and her team drove eight hours through the night to assist with its release. They arrived at 3 a.m., and waited with the fish until the sun came up. More people were needed to delicately move the animal, which was armed with a venomous barb that could be more than a foot long and is capable of piercing bone.

Before freeing the stingray, Ms. Chea and her colleagues took noninvasive samples that would help with future study of the species. Then, they helped guide the colossus back to the Mekong’s depths.

“She swam away calmly, but then appeared again, which made us feel so, so happy,” said Ms. Chea.

That a stingray of this size could still be found in these waters was extraordinary, the experts said.

“It shows you nature is so beautiful, but also resilient,” said Sudeep Chandra, a limnologist at the University of Nevada, Reno and co-scientist on the Wonders of the Mekong Project. “Even with the major environmental problems in the Lower Mekong, like dams, forest change and overfishing, these large, charismatic species are still there, wanting to persist.”

Of course, it does not always play out like this, Ms. Chea said. People who live along the Mekong rely on the river’s bounty for food and income. Stories abound in those communities about much larger rays that have been chopped into small pieces for sale in the local market, she said. In fact, Ms. Chea said, another giant stingray was caught in April. However, it was already dead by the time they found it.

Giant freshwater stingrays are not the only enormous and endangered creatures that need to be preserved along that stretch of river. It is also home to giant softshell turtles, the Mekong giant catfish and the giant barb, a type of fish. The Wonders of the Mekong partnership is working with scientists to better understand the habitat.

Much of what is known about big rivers as ecosystems comes from the Mississippi River and rivers in Europe. But all of these are in temperate regions, Dr. Chandra said. In contrast, the Mekong is tropical and prone to huge, seasonal deluges. This gives the Mekong a dynamic and mostly unstudied ecology, he said.

For instance, Dr. Chandra and his team were surprised to discover recently that beneath the Mekong’s surface, there were hidden pools more than 250 feet deep. If you could somehow dip the Statue of Liberty and its pedestal into one of these chasms, only the torch would remain above water.

It is probable that such pools play an important role in the life cycle of the river’s giants. With underwater submersibles, environmental DNA sampling and sensors that can provide information about the river’s changes in real time, the scientists working with the Wonders of the Mekong Project hope to learn more about these habitats and protect them from environmental threats.

Ms. Chea has been working in these communities since 2005, developing trust and building partnerships between the project and the people who share the river with these species. And that work seems to be paying off. Now, when someone accidentally hauls in a giant creature, they may reach for a phone instead of a filet knife.

Ms. Chea said a local leader told her that he had never seen a giant freshwater stingray. And during the release, she watched as he spoke with two young boys.

She said she heard him identify the animal to them and say, “you should protect it so your kids in the future will also know that we have a giant stingray in our village.”