Scientists are trying to understand why 70 giant freshwater stingrays, some as big as a car, have been found dead in Thailand over the past few weeks. The die-off has been taking place in the Mae Klong River.

The giant freshwater stingray is the largest freshwater fish in the world.Zeb Hogan

Thai officials have found the river to be slightly more acidic than normal, but aren't sure if that could be the cause. Some speculate that the rays may have been poisoned by cyanide or succumbed to a recent spill from an ethanol plant.

"One thing is clear: a reduction of pollution from surrounding factories is needed to improve the health of the river and save the stingrays in the long term," Zeb Hogan, host of the Monster Fish series on Nat Geo Wild, said.

The WWF says that these rays are being increasingly isolated into separate groups due to construction of large hydropower dams, reducing genetic diversity. They are vulnerable to siltation as they spend much of their time along the river's sandy bottom. And they can be caught up in fishing activities such as longlines and gill nets, and may be killed as bycatch.

Freshwater rays are prized as pets by home aquarium hobbyists. A search for "freshwater stingrays for sale" on Google will retrieve more than 200,000 results.

Listed as Endangered by the International Union for the Conservation of Nature (IUCN), giant freshwater stingrays inhabit rivers in Southeast Asia and northern Australia.

Hogan said that one ray recently found alive weighed between 700 and 800 pounds. It measured 14 feet in length and 7.9 feet across. They can grow to more than 1,000 pounds.

Much mystery surrounds the giant freshwater stingray. Cousins of the more numerous and widespread ocean stingrays, they were only identified by scientists in 1990. While the freshwater rays have been seen in brackish waters, it's not known if they ever venture into the ocean. No one knows how many exist in the wild, but they appear to be in decline.

In September, a new conservation effort was launched to help ocean rays and sharks. Actor and environmental activist Leonardo DiCaprio, at the Our Ocean Conference, announced the Global Partnership for Sharks & Rays, a collaborative effort to halt the alarming decline and overexploitation of shark and ray populations due to market demand for shark fin, liver oil, cartilage, leather, meat and ray gill plates.

"Sharks and rays are among the most threatened vertebrates on the planet," Cristina Mormorunni, Global Partnership for Sharks & Rays acting director, said. "For many of these incredible animals, the future is uncertain. Unfortunately, the scale of current conservation efforts and investments don't match the level of urgency sharks and rays face."