This giant stingray could be world's biggest freshwater fish

Bungin, IndonesiaWe know that the whale shark is the biggest fish in the ocean, but what is the world’s largest freshwater fish? The answer to that seemingly simple question has remained a mystery as murky as the Southeast Asian rivers in which it’s most likely found.

Officially, the heavyweight title belongs to the Mekong giant catfish, with a specimen caught in northern Thailand in 2005 weighing an astounding 646 pounds. But researchers have long believed there are bigger fish out there, with a mysterious giant freshwater stingray being the top contender.

Now, there is increasing evidence they might be right. A recent survey of rivers in Indonesia suggests that a species known as the giant freshwater stingray may indeed grow much heavier than the giant catfish, with one stingray caught by a fisherman in South Sumatra said to have weighed more than 800 pounds, or twice as much as an adult mountain gorilla. Unverified reports from Borneo suggest catches of rays of similar size or even larger.

Zeb Hogan, a fish biologist at the University of Nevada, Reno, and a National Geographic Explorer, has been searching for the world’s largest freshwater fish species for the past 15 years. He says it’s almost impossible to confirm such accounts, but he’s convinced that giant stingrays can grow to record sizes. In Cambodia and Thailand, where he has focused his search for record rays, there have been reports of stingrays over a thousand pounds. “I’m not surprised by these reports from Sumatra and elsewhere,” Hogan says. “It seems likely that the giant freshwater stingray is the biggest freshwater fish in the world.” (Read more about Hogan’s mission to document freshwater megafish.)

What is clear is that the study of the species has taken on increased urgency as some ray populations are declining, along with many other large-bodied freshwater fish. A recent study showed that global populations of freshwater giants have crashed by almost 90 percent in the past four decades—twice as much as the loss of vertebrate populations on land or in the oceans.

Poisonous but inquisitive

Most people are only familiar with the stingrays that live in the seas, but there are dozens of species of rays found in rivers and lakes around the world. Most of them are poorly understood, including the giant freshwater stingray. First identified in Indonesia in 1852 by Dutch ichthyologist Pieter Bleeker, it was essentially forgotten for more than a century before being described as a new species in 1990. Then, in 2008, scientists concluded that this new species was in fact the same as the one Bleeker had identified, and it came to be named Urogymnus polylepis.

A major reason why scientists know so little about giant freshwater rays is because they hide at the bottom of the river. They’re not considered a good food fish in Southeast Asia, so they’re rarely targeted by fishers, though they occasionally get caught in nets. If hooked, the car-size stingrays put up a ferocious fight, and there are reports of boats dragged around for hours on the river by hooked rays. (Check out five cool facts about rays and skates.)

While it packs a poisonous, serrated stinger up to 15 inches long, it’s typically a non-aggressive, inquisitive creature, says Hogan, who first saw a giant freshwater stingray in the Mekong River in Cambodia more than 15 years ago. Later, he began working with Thai researchers and recreational anglers on the Mae Klong River, near Bangkok. The team there has caught several huge fish over the years, but they’ve never been able to get a verified weight of a record-breaker.

Credible accounts

In late November, Mohammed Iqbal, a biologist at Sumatra’s Sriwijaya University, and I go to check out a report of a possible record-breaker at a small village in South Sumatra, where a fisherman named Kamar accidentally caught a giant freshwater stingray in 2016.

For four hours, we speed over the choppy Musi River in an old, wooden speedboat. Torrential downpours break out several times during a journey that takes us past numerous oil palm plantations and deforested jungle. With huge barges carrying coal upriver, there are signs everywhere of heavy human pressure on the ecosystem.

Giant freshwater stingrays hadn’t even been documented on Sumatra until 2016, which was when Iqbal began compiling a record of newspaper and internet reports of a dozen ray catches in local rivers. Several accounts he found told of huge rays, with the biggest being caught at the mouth of a small river called Bungin. He’d heard it reportedly weighed more than 800 pounds.

By mid-afternoon, we finally arrive at the Bungin fishing settlement, which consists of a few wooden houses raised on stilts to keep it above the tide.

As another thunderstorm sweeps over the village, the fisherman, Kamar, who goes by only one name, recounts how the catch happened. One day, three years ago, he laid his net in the open estuary where several rivers empty into the South China Sea. He left it overnight, hoping to catch fish to sell and to eat. In the morning he was shocked to find a massive stingray in it. The fish was far too heavy to pull into the boat, Kamar says, so had to drag the net with the fish in it behind his boat back to the village.

It took 15 men to lift the ray out of the water and onto land. The men say they’d never seen such a huge ray before. They borrowed a large scale from a local fish storage facility to weigh it. Kamar tells us it weighed “almost 400 kilos,” or nearly 880 pounds. Was he aware that meant he had caught the largest freshwater fish in the world? He shakes his head, no.

The legitimacy of the account may rest on Kamar’s word. But what lends his account extra credence is the fact that not only was the fish weighed, but Kamar also saved the ray’s skin after it was cleaned out and sold for meat. The skin has since shriveled, but it suggests a ray with a width of about eight feet, possibly big enough to beat the record (and bigger than a giant stingray found in 2009).

Despite the fact that the Bungin ray was caught in the brackish or saltwater of the river mouth, Iqbal is certain it is the freshwater Urogymnus polylepis, based on its shape and markings. The other rays Iqbal identified were caught upstream in rivers, sometimes more than 60 miles from the ocean. “It seems like the species lives in freshwater but that it is also able to tolerate saltwater,” he says.

Earlier this year, Iqbal conducted a similar survey of newspaper and online reports of catches of giant freshwater stingrays around Borneo and found several more instances of fish that may have been over 646 pounds, the weight of the record-holding Mekong giant catfish.

In a fishing community on Melintang Lake in Indonesian Borneo, several fishermen last month said they had caught rays recently. One fisherman, El-Hadji Janadi, says he snared a nearly 500-pound ray earlier this year using a 328-foot-wide trawling net dragged at the bottom of the lake. “I didn’t want to catch the stingray,” he tells me. “The meat does not fetch much money.”

Dwindling populations

Researchers fear the giant freshwater stingray could disappear before we’ve ascertained how big it can get and what other secrets it holds. While fishing pressure and habitat loss threaten it throughout its Southeast Asian range, another major concern is pollution, says Mabel Manjaji-Matsumoto, a marine biologist at the University of Malaysia, Sabah, who’s been studying the species since the mid-1990s. She says runoff from fertilizers used on oil palm plantations have turned parts of rivers in eastern Malaysia “all the colors of the rainbow,” endangering many of the river’s wildlife, including the giant freshwater stingrays.

On the Mae Klong River in Thailand, waste discharged from an ethanol plant three years ago killed at least 70 giant stingrays, with the carcasses of rays rising to the surface and being carried down the river like crashed UFOs, a shocking sight to local residents. A subsequent investigation showed that significant amounts of cyanide had been released into the river.

Last year, when Thai researchers from Chulalongkorn University in Bangkok conducted the first comprehensive tagging study of the ray population in the Mae Klong River to learn more about the movements of the rays, they found far fewer individuals than they’d counted before the spill and almost no large ones.

Veterinarian Nantarika Chansue, who led the National Geographic Society-funded tagging project, says there are likely no stingrays larger than 650 pounds left in the Mae Klong. “This was quite a healthy population, but with uncontrollable pollution, the risk to lose them is very high,” she says. The rays have also been targeted for the burgeoning aquarium trade, she adds. (Read how river sanctuaries are helping some giant fish recover.)

For Hogan, the quest to learn more about the stingrays—and all the threatened megafish—continues. He points out there’s an unstudied but relatively healthy population in the biodiverse upper Cambodian Mekong, which overlaps the distribution of other rare animals like the Irrawaddy dolphin and Cantor’s giant softshell turtle.

“It’s not just about finding the biggest fish,” he says. “Maybe we’ve found it with the giant freshwater stingray. But what’s clear is that we must do more to protect these fish before they disappear forever.”