CAMBODIA (CBS) — Fishermen on the Mekong River in Cambodia got quite a shock when they reeled in a giant stingray by mistake.

The 13-foot endangered stingray weighed nearly 400 pounds. The massive ray had swallowed a smaller fish that took the bait.

As luck would have it, marine biologists from the University of Nevada were on an expedition nearby.

When the fisherman asked for help, they answered the call.

“We were able to get the stingray into a nice spot of the river, get the hook out, we placed it on a tarp, and then pushed it out into the river,” says Dr. Zeb Hogan, a fish biologist and professor at the University of Nevada.

Dr. Hogan says colossal catches like the stingray are almost never reported, so it’s a rare and remarkable discovery.

“This catch was significant because it confirms the existence of these big fish in this stretch of river.

This is a very remote stretch of river, it’s not well studied, it’s incredibly important for fisheries and biodiversity,” he says.

Giant freshwater stingrays are believed to be the world’s largest stingray species.

And this one seemed happy to be set free, ready to join other unknown giants lurking below the surface.

The American marine biologists are part of the “Wonders of the Mekong” project which is funded by the U.S. government.

The famous river starts in China and twists through Asia, feeding an estimated 60 million people.