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National Geographic

How 600-lb. megafish became a collector's item in Thailand

The huge fish are dwindling in the wild, but thriving in private homes. Scientists see an opportunity for their recovery. In Thailand, some threatened megafish, like this giant barb photographed at Gilham's Fishing Resort in Khao Thong, can be found in government hatcheries, temples, farms, and backyard ponds. Bangkok, Thailand — When a 200-pound Mekong giant catfish turned up at a flooded train station in the Thai city of Chiang Mai last year, it stopped people in their tracks. Seeing a six-to-seven-foot long fish trapped outside a ticket booth was a surreal sight—and it raised an obvious question: Where did it come from? It certainly did not come from its natural habitat—the Mekong River, which runs through several Southeast Asian countries. The critically endangered species is one of the world's largest freshwater fish and has become vanishingly rare in the wild in Thailand. Instead, the train station fish was surely raised in captivity—possibly escaping from a private pond, temple pool, or stocked reservoir after floodwaters had breached containment. The incident provided a rare glimpse of a hidden world: Across Thailand, megafish like the Mekong giant catfish (Pangasianodon gigas) and the similarly threatened giant barb (Catlocarpio siamensis) are being bred in large numbers in captivity. Government hatcheries, commercial farms, and private owners raise them for fishing ponds, religious purposes, or ornamental displays. Altogether, these captive fish form a vast and largely undocumented population of endangered fish that dwarfs what's likely left in the wild. Now, scientists are beginning to ask a pivotal question: Could these captive stocks play a role in reversing the decline of megafish populations in the wild? 'Captive stocks could be more than just a fallback,' says Zeb Hogan, a biologist at the University of Nevada, Reno, who has studied Mekong megafish for decades. 'With the right science and conservation efforts, they might help bolster wild populations and keep these iconic fish from disappearing altogether.' (Meet the world's largest freshwater fish.) Fish farming is common in the Mekong Delta, and catfish (like these pictured in 2008) are popular. Their larger brethren, Mekong giant catfish, also seem to do well in captivity. Photograph By Justin Mott/Redux Megafish have a sacred status Mekong giant catfish can grow over 600 pounds, and they were once plentiful, caught for food and revered for their size. Thai fishers recorded annual catches in the hundreds in the early 1900s and later dozens. 'I would go to take a bath in the river and catch a fish for dinner at the same time,' recalls 79-year-old Boonrian Jinarat, who spent decades fishing in the town of Chiang Khong. Where guests are guardians But those days are gone. While no exact wild population numbers exist, Thailand hasn't seen wild catches in years. Today, a massive golden catfish statue stands downstream from Chiang Khong where the giant fish once swam—a monument to what's been lost in the wild. Recognizing the threats to the giant catfish, the Thai government launched a breeding program aimed at preservation in the early 1980s. The species, it turned out, adapted well to hatchery techniques, and populations steadily grew in government facilities. Over time, these giants found their way into private hands, with collectors, temple caretakers, and fish farmers raising giant catfish alongside other iconic species such as the giant barb, the world's largest carp, and the striking but rare seven-striped barb. 'Their large size gives them a sacred status that is often linked to religious beliefs,' says Chaiwut Grudpan, a fish biologist at Ubon Ratchathani University. The fish also appeal to the nostalgia of some collectors. Sittitam Ruengcharungpong grew up in Bangkok surrounded by fish his father raised as a hobby. Today, he breeds dragon fish and arapaima commercially but keeps several Thai megafish species in his two home ponds. 'I want to grow them to full size, like the ones I saw when I was young, and be able to show my children how big they can get,' he says. This gold catfish statue at a Buddhist temple, known as Wat Pla Buek, in the northern Thai town of Chiang Khong commemorates the Mekong giant. Photograph By amnat, Almay For Jirawat 'Organ' Sangphoo, Thailand's fascination with giant fish has become a thriving business. Outside Bangkok, he rents a former shrimp farm the size of a football field, where he keeps hundreds of fish ranging from exotic imports to native megafish. His customers include private collectors, aquariums, and fishing parks, and business is steadily growing. The most prized fish, he says, is the giant barb—also known as the Siamese carp—a slow-growing species native to the Mekong and Chao Phraya river basins that is widely considered a symbol of prosperity and good luck. 'Organ' once sold a single specimen weighing 266 pounds (121 kilograms) to a fishing park for 1.7 million baht, or about $52,000. Given their value and the short time he handles them, he says, 'I take care of the fish as if they're my pets.' Avoiding a genetic bottleneck Researchers first need to know how many captive fish there are, where they're being kept, and how genetically diverse they are. Tools like environmental DNA could help detect the presence of megafish in backyard ponds and fish farms. Hogan and his colleagues hope to track the origins of captive megafish and sample the gene pool beginning with a study of Mekong giant catfish. (Read more about scientists' efforts to save the world's largest freshwater fish.) An exact count of Thailand's captive population may not be possible, but Hogan, who leads the Wonders of the Mekong research project, speculates there are more than a million Mekong giant catfish in captivity throughout the country, at least a thousand times more than remain in the wild. That staggering number exists largely because the Mekong giant catfish has proven so resilient in captivity. 'You put this huge fish in a little pond and it does just fine,' says Hogan, who is also a National Geographic Explorer. By comparison, other freshwater giants have struggled to survive outside their natural habitats. The Chinese paddlefish—a native of China's Yangtze River and once one of the world's largest freshwater species—couldn't survive in captivity and is now believed to have gone extinct in the wild in the early 2000s. With enough food and space, Thailand's other megafish might fare better, but getting them to reproduce in captivity could be another challenge. Mekong giant catfish are among the largest freshwater fish in the world. In the early 1900s, fishermen would catch hundreds of fish annually, but today, wild catches are extremely rare. Photograph By Fargriv, Shutterstock Adapting to captivity alone isn't the only hurdle; a central concern is genetics. Decades of artificial selection and inbreeding in closed systems can lead to genetic drift and bottlenecks, weakening traits critical for survival in the wild. Fish like the Mekong giant catfish produce thousands of offspring per spawn, but in hatcheries, a single pair can generate tens of thousands of fish. Without careful tracking, this narrows the gene pool, heightening inbreeding risks and undermining long-term survival. 'Releasing these fish could dilute the genetic diversity of wild populations, making them less resilient to diseases, environmental changes, or new threats,' says Apinun Suvarnaraksha, a fisheries lecturer at Maejo University in Chiang Mai. Compounding the issue, captive fish don't face the same challenges as their wild counterparts. In controlled environments, they often develop traits—like tameness or dependency on hand-feeding—that reduce their ability to survive in the wild. 'To rebuild wild populations, it's not enough to have fish,' says Hogan. 'We need to ensure they're genetically strong enough to survive.' In Cambodia, fishers still sometimes see Mekong giant catfish in the wild. Here Cambodian officials release one back into the wild. Part of a last-ditch effort The role of private collectors in conservation is contentious. Critics say it risks normalizing wildlife ownership, while others see it as a last resort for species with nowhere else to go. For some collectors, being part of the conservation effort is part of the appeal. Nantarika Chansue, a veterinarian at Chulalongkorn University in Bangkok and a leading Thai expert in aquatic animal health, keeps around 20 Mekong giant catfish in a lily-covered pond at her home outside the city. She also cares for two dozen giant freshwater stingrays, many of them rescued from fishermen. 'It makes me proud to care for species that are so rare in the world,' she says. In Vientiane, the capital of neighboring Laos, one individual has taken matters into his own hands. A businessman—who requested anonymity out of concern for provoking the government—has built a custom tank to house a handful of critically endangered wolf barbs (Luciocyprinus striolatus), also known as monkey-eating fish. He rescued them from a Mekong tributary that may be their last refuge but is now threatened by a dam under construction. 'When the dam is finished, the wolf barb will disappear from the wild,' he says. Watching the fish—sleek and fast as they dart through the water—he calls his effort 'the last option.' 'There is no manual for this,' he adds. 'I'm just trying to save the species.' (Rivers and lakes are the most degraded ecosystems in the world. Can we save them?) A river to return to An even bigger challenge awaits beyond the ponds. Reintroducing animals that thrive in captivity back into the wild is notoriously difficult, especially for large freshwater fish. Mekong's megafish face relentless pressures from dam construction, overfishing, and increasingly climate change. Many, like the giant catfish, are long-distance migrators that depend on connected, healthy river systems. While Thailand has released hatchery-raised fingerlings into reservoirs, efforts to reintroduce them into the Mekong itself have struggled. 'The focus must shift towards a more holistic approach, which includes not only breeding and release but also habitat protection and restoration,' says Suvarnaraksha. 'That means controlling illegal fishing, addressing dam impacts, and working closely with local communities.' Thailand may not have seen wild catches of the Mekong giant catfish in years, but it can look to Cambodia, where the river system is healthiest, as a sign of what's still possible. Late last year, Cambodian fishers caught and released 17 Mekong giant catfish in the Tonle Sap and Mekong rivers, the highest number recorded in more than two decades. Several exceeded 200 pounds, and the fish spanned multiple age classes, indicating natural reproduction is still occurring. Whatever role megafish in private collections might end up playing, Thailand's rivers would need to both support fish survival and reproduction. 'It's not the solution, but it's not the problem either,' says Hogan. 'It only becomes a problem when people think having the fish in captivity is enough. It's not. Real conservation means giving them a river to return to.'