Why we still haven’t caught the world’s largest fish

Two decades ago, Zeb Hogan was working on Southeast Asia’s Mekong River when he got the idea for the Megafishes Project, a quest to find, study and protect the world’s largest freshwater fishes. At the heart of the project was the question: Which species is the largest? “I thought there would be a simple answer,” says Hogan, a fish biologist at the University of Nevada, Reno. “I was wrong.”

For years, he scoured Earth’s waterways, often as the host of Nat Geo Wild’s “Monster Fish” show. Although he came close on many occasions, he could not find a fish with a verified weight greater than a 646-pound Mekong giant catfish caught in Thailand in 2005, a catch that had sparked his search. Instead, Hogan found that river giants around the world were often poorly studied and in most cases in serious decline, with some species about to go extinct. (Read more about how scientists are saving megafishes.)

Eventually he and I began writing a book, Chasing Giants: In Search of the World’s Largest Freshwater Fish, published this month by University of Nevada Press. But even after finishing the manuscript last year, he still did not have an answer to the central question.

Then, miraculously, Hogan’s research team in Cambodia got a call from fishers who said they had caught a giant freshwater stingray in the Mekong River, much bigger than ones previously caught. The ray, as it turned out, measured 13 feet long and weighed 661 pounds, which Guinness World Records recognized as the largest-ever recorded freshwater fish in 2022.

The discovery energized conservation efforts to protect these behemoths. Unlike the catfish that was killed and sold for meat in 2005, the stingray caught last year was released alive after being fitted with an acoustic receiver. This has allowed researchers to track its movements to learn more about a species they knew next to nothing about.

“We’ve been losing these fish without truly knowing them,” Hogan says. “Now we have the tools to study and protect these amazing creatures, so we need to take that newfound knowledge and turn it into meaningful protection. It’s not too late, the big fish are still out there.”

And Hogan says he’s still on the hunt—for a fish even heavier than the giant stingray.

Contenders for biggest fish

For his project, which was supported by the National Geographic Society, Hogan identified more than two dozen species under the term “megafishes,” freshwater fish that could grow at least six feet long or reach weights of 200 pounds. These fish are a diverse assemblage of creatures of different shapes and life histories, from colossal carps and catfish to electric eels and gargantuan gars.

What they have in common, besides their huge size, is that many of their populations have been decimated by overfishing, dam building, pollution, invasive species, and climate change. The Chinese paddlefish, which could grow over 20 feet long, was declared extinct during the early stages of Hogan’s search. “The realization that these fish, some of which have been on Earth for hundreds of millions of years, were disappearing in our lifetime was incredibly disturbing,” he says.

Giant fish are indicators of river health, so their decline is a worrying sign for the world’s freshwater ecosystems, which studies show are often more degraded than their marine and terrestrial counterparts.

Having focused on the Mekong giant catfish early in his career, Hogan came to suspect that the species, along with the world’s largest carp, the giant barb, had more or less disappeared in the Mekong River.

Other possible contenders for biggest fish included the air-breathing arapaima, which dwells in South America’s Amazon Basin, and the wels catfish, Europe’s largest freshwater fish and a species with a voracious appetite.

Emerging as the top contender for the title, however, was the giant freshwater stingray that is found in rivers in Southeast Asia, where most of its populations are endangered but may still be doing better than many other giant fish species. For several years, Hogan focused his search on the Mae Klong River in central Thailand, not far from Bangkok, where sport fishers were catching and releasing enormous rays. During a “Monster Fish” shoot, Hogan’s team caught a stingray that he believed was of record-breaking size, but the team did not have a scale large enough to weigh the massive animal.

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Hogan suspected, however, that the Mekong River in Cambodia also held record rays. The USAID research project he’s been leading since 2017, Wonders of the Mekong, has increasingly focused on a biologically rich stretch of the river in northern Cambodia, where deep pools are believed to be a refuge for many large fish. (Read about a 13-foot-wide stingray found in a deep hole in the Mekong.)

Fishers told Hogan they regularly caught stingrays weighing as much as a thousand pounds, but those accounts were difficult to verify. 

As it happened, a team of American scientists were in the area at the time to set up a first-ever telemetry study in northern Cambodia to learn more about migration patterns and behavior of the fish in the Mekong River system. The standout female became the first fish to be implanted with an acoustic tag for the study. 

Hydrophone records showed the tagged stingray stayed almost exclusively in the specific area of the river where she was discovered. This means a place-based management approach, such as setting up a no-fishing reserve in the area, would help protect the stingrays. Such reserves are common in Southeast Asia, though they require strong buy-in from local communities. 

“We plan to extend our work more in creating public awareness and local participation to protect these species, including the stingray,” says Heng Kong, director of the Inland Fisheries Research and Development Institute of the Cambodian Fisheries Administration.

The hunt goes on

For Hogan, the search for the largest freshwater fish is not over. “This ray is likely not the biggest individual stingray that exists in the river,” he says. “Fishers tell me all the time they’ve caught bigger rays than this.”

There are some experts who suggest that other species, in particular the arapaima, may reach similar weights as the stingray. Scientists studying growth ring deposition on the scales of arapaimas living in Guyana’s Essequibo River have shown those animals may grow much heavier than those from central Brazil. (Read about a new arapaima species identified in 2016.)

For instance, the length-mass relationship developed by the scientists suggests that an arapaima caught a couple of years ago in Brazil, which measured about 10 feet and weighed 540 pounds, would have weighed over 700 pounds in Guyana had it been of similar length.

“I predict the ultimate world record arapaima will come from Guyana,” Donald Stewart, a fisheries professor at State University of New York, who led the research, said by email.

Hogan says his search was always about more than just finding the biggest fish.

“It was always about learning more about these animals,” he says. “Finding this record ray is proof and evidence that these fish are still out there, and that’s a positive sign, but we’ve just scratched the surface when it comes to our knowledge about them.”