How the World’s Largest Freshwater Fish Grew to 660 Pounds

  • A local fisherman in northeastern Cambodia caught a 660-pound stingray in the Mekong River and alerted scientists with the Wonders of the Mekong project.
  • Researchers say the massive growth of this fish showcases the relative health of a stretch of the Mekong River.
  • To grow huge, it’s essential to have enough space and access to food.

    If you’re a fan of giant fish, you already know about the Mekong River. Now, familiarize yourself with the new record-holder in the largest freshwater fish category: a giant stingray that measures about 13 feet long and weighs 660 pounds. Earlier this month, a local fisherman caught the beast in the Mekong River, located in northeastern Cambodia.

    The stingray—which researchers with the Wonders of the Mekong conservation project saved—beat out the previous freshwater record-holder, a 646-pound catfish found in Thailand in 2005, also in the Mekong River. But how did these fish become so huge in the first place?

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    Zeb Hogan, Ph.D., a research biologist at the University of Nevada, Reno and director of the Wonders of Mekong project, tells Popular Mechanics that finding a fish of this size offers “an indication that this stretch of the Mekong River, the longest remaining free-flowing section, is still relatively healthy.”

    Hogan says giant stingray populations have declined in other regions of southeast Asia more heavily impacted by pollution, dams, and overfishing.

    The Wonders of the Mekong project, funded by the US Agency for International Development, has partnered with the Cambodian Fishers Administration and worked with local communities to improve conservation efforts along the river. When the fisherman—who was financially compensated at market rate for his catch, even though the fish went back into the river—saw what he’d pulled in from the river, he contacted scientists with the project, who documented the fish and tagged it with a tracking device before it was released back into the waterway in order to study it.

    But getting so large was more a product of space and food access than anything else.

    “The Mekong is one of the largest and most productive rivers in the world,” Hogan explains. “Over two million tons of fish are harvested from the Mekong River each year. So, historically, the Mekong has been a great place to be a big fish. In fact, the Mekong River is home to many of the world’s largest freshwater fish: giant freshwater stingray, giant barb, giant pangasius, seven-striped bard, giant wallago catfish, and goonch catfish.” The area is also home to freshwater dolphins and giant soft-shell turtles. A handful of giant stingrays have already been seen in the area this year, leading researchers to believe the spot could be a popular spawning location.

    “Unfortunately, populations of most of these species have plummeted, and some are now on the brink of extinction,” Hogan says. “It’s an issue throughout Asia, where populations of giant freshwater fish have declined by over 95 percent since 1970.”

    Ronald Oldfield, Ph.D, a senior instructor at Case Western Reserve University, tells Popular Mechanics that capturing such a large fish in the Mekong River shows it “might not be too late and the river might still be capable of supporting such large fishes if it is managed wisely.”

    The Mekong River, already with a storied history of mega-sized freshwater fish, runs through six countries—China, Myanmar, Laos, Thailand, Cambodia, and Vietnam—but new dams can dampen fish’s ability to naturally grow. New dams could fragment habitats, lead to overfishing, and threaten the giant Mekong species, Hogan says.

    Huge catfish hauled from Mekong River being admired by locals.

    Robert NickelsbergGetty Images

    John Janssen, Ph.D., a professor at the University of Wisconsin-Milwaukee’s School of Freshwater Sciences, tells Popular Mechanics the Mekong is an old, large river, which allows enough time for evolution to produce an extra-large fish. He says the adequate space allows a fish to grow big and not get stranded while having the opportunity to get enough food.

    And Janssen doesn’t think the recent catch is an outlier, either for its species or for other large, old-river species. “The trouble is that these large rivers tend to get damaged, which messes up reproduction, so the really big species go extinct or become rare,” he says. Simply put, they need: “space and food.”

    In fact, there may not be a limit on how big a fish can grow, Oldfield says, as the more than 35,000 diverse species on Earth often exhibit indeterminate growth, allowing them to continue growing throughout their entire lives. “Many fishes start reproducing and then continue growing and reproducing for years and years to come,” he says. “However, many fishes do indeed stop growing at some point.”

    Of course, being able to evade humans is a necessity.

    Oldfield likens the Mekong to the Amazon River, both large rivers that have seen fish evolve into huge-growing species. “Looking at both rivers, we see this parallel evolution of completely different stingray species and completely different catfish species growing to huge sizes,” he says, “and in both rivers, it has happened in species whose ancestors came from freshwater, catfishes, and from saltwater, stingrays.”

    That need for human-free space is where the conservation efforts from the Wonders of Mekong project come in, along with the tracking effort. “We would like to learn about the ecology and migration patterns of the species,” Hogan says of tracking the recently-found record-holder for largest freshwater fish, a species that little is known about. “Why is it found in the area? What is its home range size? Is it sedentary or a highly migratory species? This information can be used to develop a conservation plan.”

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