On May 5, scientists working along a stretch of the Mekong River in Cambodia released a giant freshwater stingray that had been caught on a fisherman's line. The animal pancake was 13 feet long and 400 pounds.

It was shaking, and I told her to calm down, we would release you soon.

The world's largest freshwater stingray, Urogymnus polylepis, is also known as a whipray. The animals have dusky-brown tops and creamy white bottoms. They can grow to epic proportions, but over-harvesting for the meat, accidental deaths in fishing nets, and degradation from dams, pollution, and other human activities have made the animals vulnerable.

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 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.

Ms. Chea and her colleagues took samples that would be used to study the species. They helped guide the colossus back to the depths.

Ms. Chea said that she swam away calmly, but appeared again and made them feel happy.

The stingray had to be released delicately both to keep the animal safe and to protect humans from its venomous barb. Video by University of Nevada, Reno.

The experts said that it was extraordinary that a stingray of this size could still be in the water.

Sudeep Chandra is a limnologist at the University of Nevada, Reno and co-scientist on the Wonders of the Mekong Project.

Ms. Chea said that it does not always play out like this. The people who live along the river rely on the bounty for food and income. There are many stories about larger rays being chopped into small pieces for sale in the local market. Ms. Chea said that another giant stingray was caught in April. It was already dead when they found it.

The stretch of river needs to be preserved for the sake of giant freshwater stingrays. There are giant softshell turtles, giant catfish, and giant barb in the area. Scientists are working with the Wonders of the Mekong partnership to better understand the habitat.

The Mississippi River and rivers in Europe are where most of what is known about big rivers comes from. Dr. Chandra said that all of these are in the tropics. The Mekong is prone to huge, seasonal deluges. He said that the ecology of the Mekong is mostly un studied.

Dr. Chandra and his team were surprised to discover that there were hidden pools more than 250 feet deep beneath the surface of the Mekong. The Statue of Liberty's torch would remain above water if you dipped it into one of these chasms.

The life cycle of the river's giants is likely to be affected by such pools. Scientists working with the Wonders of the Mekong Project hope to learn more about these habitats and protect them from environmental threats by using underwater submersibles, environmental DNA sampling and sensors that can provide information about the river's changes in real time.

Ms. Chea has been working with the people who share the river with these species since 2005. The work seems to be paying off. When someone accidentally hauls in a giant creature, they may reach for a phone instead of a filet knife.

Ms. Chea was told that the leader had never seen a giant freshwater stingray. She watched as he spoke with two young boys.

She said she heard him tell them to protect the animal because their kids will know about it.