Scientists working in Thailand’s Mae Klong River made a big find last week: an enormous stingray that they think is a contender for the largest freshwater fish ever documented by researchers.

The ray was caught and released in about 65 feet (20 meters) of water in the Amphawa District, about an hour outside Bangkok.

Nantarika Chansue, a veterinarian and professor at Chulalongkorn University in Bangkok, helped catch and measure what she calls the “big one.” The ray (Himantura polylepis or H. chaophraya) was 7.9 feet (2.4 meters) across and 14 feet (4.3 meters) long and weighed an estimated 700 to 800 pounds (318 to 363 kilograms), she said via e-mail.

The team was unable to get an exact weight because “it's really hard to weigh these things without hurting them, because they are such big, awkward animals,” says Zeb Hogan, a National Geographic fellow and a professor of biology at the University of Nevada, Reno.

“Certainly [this] was a huge fish, even compared to other giant freshwater stingrays, and definitely ranks among the largest freshwater fish in the world,” he says.

Hogan has a connection to this particular ray: The same animal was caught and tagged in 2009 under a program he runs with Nantarika. (Learn about a colossal, 280-pound catfish.)

Nantarika performed a portable ultrasound on the ray while it was held in a cage in the river, revealing that the animal was pregnant with two fetal rays. Records show that she was also pregnant when caught in 2009.

“That indicates it was found in an area that is likely a nursery ground,” Hogan says.

In 2009, the ray was 6.5 feet (2 meters) across and 15 feet (4.58 meters) long. “Her tail might have been shortened by some accident,” says Nantarika. The ray also had bite marks that may have come from a male ray.

By knowing how much time had elapsed since the ray was last studied, scientists now have a better idea about how fast rays can grow. Like most fish, they keep growing as long as they live and can find enough food. Giant stingrays are bottom feeders, preying on fish, prawns, mussels, clams, and whatever else they can find. Scientists don’t know how long the rays can live, but Nantarika estimates this one is between 35 and 40 years old, based on its size.

Record Catch?

Hogan keeps track of freshwater fish size records through his Megafish Project, which studies the world’s biggest freshwater fish, and says the previous size record for a freshwater fish was an estimated 693 pounds (314 kilograms), for a Mekong giant catfish. The largest freshwater ray he has measured was about 400 pounds (180 kilograms). Unconfirmed reports have suggested sizes for the rays as big as 1,100 to 1,300 pounds (500 to 600 kilograms).

The Guinness Book of World Records lists the Mekong giant catfish, which also lives in Thailand, as the “world’s largest freshwater fish,” weighing up to 660 pounds (300 kilograms). The 2015 edition of the book breaks out the largest saltwater ray as a separate category from bony fish, even though rays are technically a type of fish. Anthony Yodice, a spokesperson for Guinness World Records North America, said the organization would not comment on whether this catch constitutes a new record until a formal application had been submitted and reviewed.

The ray was the seventh that has been recaught by Hogan and Nantarika over the course of their ten-year program.

The animals are hard to find, so the scientists work with partners such as Fishsiam Ltd, a company that offers guided catch-and-release fishing trips to tourists and that caught last week's huge ray.

Fishsiam's guides measure rays that are caught and attach tags and microchips, which Nantarika and Hogan use to track growth rates and movements. The company uses a special net and cage to keep the animals in the water and to reduce stress while they are being studied.

The catch was filmed for an upcoming episode of the ABC TV show Ocean Mysteries, hosted by Jeff Corwin. “For the most part they are a gentle giant,” Corwin says of the rays.

The giant freshwater stingray has the longest spine of any species of ray, up to 15 inches, and carries powerful venom. The animal uses its spine for defense and it can be lethal, but injuries to people are quite rare.

Watch this archive video of Zeb Hogan examining another freshwater stingray.

A Threatened Species

Giant freshwater stingrays are listed as endangered by the International Union for the Conservation of Nature, in part due to Hogan's and Nantarika's work. “These rays are decreasing rapidly since there is no national law to protect them,” Nantarika says.

Other large fish in the region, including the Mekong giant catfish, are in worse shape because they are easier for fishermen to catch. Freshwater rays are so large and strong that they break most fishing gear that isn’t specially designed to catch them. And since they aren’t a popular food item, there isn’t much commercial fishing pressure.

The rays are threatened by pollution, oil spills, and dams that have fragmented their habitat, however. (See more photos of megafish.)

The fact that the big ray found this week was pregnant again is good news for the species, Hogan says, and proves that the animals can survive the catch-and-release process.

Correction: An earlier version of this story incorrectly suggested that the largest ray previously measured was 693 pounds; that was a Mekong giant catfish. The largest freshwater ray measured was about 400 pounds.

Hogan will speak at National Geographic headquarters in Washington on March 26, following the opening of the new exhibit "Monster Fish: In Search of the Last River Giants." Nat Geo Wild is showing a Monster Fish marathon on March 22 from 12 p.m. to 7 p.m. EST.