- In February, an international team of researchers conducted a two-week survey of fish species sold in markets in the Mekong River towns of Stung Treng and Kratie in Cambodia.
- The survey builds on a benchmark set by a 1994 survey in Stung Treng, allowing scientists to detect patterns in the size and diversity of fish being pulled from the river.
- The team identified 130 species, compared with 113 in the 1994 survey; 46 species were newly documented, many of them linked to aquaculture, while 29 species documented in 1994 were not found.
- Survey members say the tally shows the resilience of the Mekong, especially in places like Stung Treng where it remains undammed, but also points to worrying trends such as smaller fish dominating catches.
STUNG TRENG, Cambodia — Sunrise is still a long way off when the first fishing boats slip into the landing site at this provincial town along the Mekong River in northern Cambodia. The night’s catch is hauled ashore and moved in tubs and woven baskets to a nearby side street off the town’s main boulevard.
By daylight, vendors have arranged the fish across tarps and reed mats laid directly on the street. Snakeheads, catfish, barbs and loaches lie in dense, gleaming rows as the market swells into a blur of motion and sound. Motorcycles crowd the edges while buyers weave through narrow passageways. Vendors weigh, sort and pack fish for kitchens, restaurants and traders heading off to Phnom Penh.
On this morning in early February, a team of Cambodian and international researchers also converged on the fish markets here and in Kratie, a town about 140 kilometers (87 miles) downstream, to begin a two-week survey documenting the aquatic wealth of the world’s most productive river system. More than 2 million tons of fish are harvested from the Mekong each year.
The survey builds on a rare historical benchmark. In 1994, the late ichthyologist Tyson Roberts conducted a detailed inventory of fish species appearing in the main Stung Treng market. Three decades later, researchers are replicating that work, returning in the same seasons, to enable a direct comparison across generations. A second survey is planned for the rainy season later this year.
By the end of the first morning, researchers had identified 115 species across Stung Treng’s main market and from two smaller markets in town, surpassing in one day the 113 species Roberts documented in Stung Treng’s one main market.
Over the following two weeks, the combined total of species found in Stung Treng and Kratie markets reached 130.
The findings suggest that this stretch of the Mekong remains highly productive. Unlike many upstream sections in Laos and China, the river here continues to flow freely, a condition long linked to the Mekong’s extraordinary fisheries. “It’s a sign that when the river remains relatively free flowing, you still see the abundance and diversity that the Mekong is famous for,” says Zeb Hogan, a fish biologist at the University of Nevada, Reno, in the U.S. who has studied Mekong fish since the 1990s and leads the survey. (Stefan Lovgren, who wrote this story, and Hogan are co-authors of a 2023 book on megafish.)
Still, the results carry important caveats. Several of the river’s largest and most threatened species were absent, including the seven-striped barb (Probarbus jullieni) and freshwater eel (Anguilla marmorata). Iconic giants such as the critically endangered Mekong giant catfish (Pangasianodon gigas) and giant barb (Catlocarpio siamensis) were nowhere to be seen.
While 29 species recorded in 1994 were not observed, many of the 46 newly documented species are linked to aquaculture.
“More species does not necessarily mean healthier fisheries,” says Peng Bun Ngor, dean of faculty of fisheries and aquaculture at Cambodia’s Royal University of Agriculture and a survey member. “Most species are still present, but fish sizes are declining and smaller individuals now dominate the catch.”
Long-term ecological change in a vast, transboundary river system like the Mekong is notoriously difficult to measure directly. The river spans six countries, flows through deep channels and floodplains and remains turbid much of the year. Conducting comprehensive fish surveys across such a system is expensive, technically challenging and often limited in scope.
Which is why fish markets offer a practical alternative by bringing together catches from multiple fishing grounds into a single place, effectively pooling information from across a wide area of the river. By documenting what appears in trade, and what does not, scientists can detect patterns in diversity, size, rarity and seasonality that would be difficult to observe underwater. “Fish markets are windows into what’s happening in the river,” Hogan says.
Deep pools
The Mekong River faces mounting pressures across its basin, from dam construction and overfishing to pollution and sand mining. While the river’s main stem remains undammed in Cambodia, major tributaries have been heavily altered. Not far from Stung Treng, the Lower Sesan 2 Dam spans the Sesan River, and studies have linked it to sharp declines in migratory fish.
Yet the stretch of the Mekong from the Laos border south to Kratie, about 200 km (125 mi), remains one of the river’s most biologically intact and productive reaches. Here, the Mekong is joined by three major tributaries — the Sesan, Srepok and Sekong — a hydrological crossroads that helps drive its extraordinary fisheries. Deep pools provide refuge for aquatic life, including some of the world’s largest freshwater fish. In 2022, fishers landed a 300-kilogram (661-pound) giant freshwater stingray (Urogymnus polylepis), later confirmed as the largest freshwater fish ever recorded.
Scientists estimate that vast numbers of migratory fish move through and spawn in these waters each year. “One reason we’re seeing a lot of fish here is because they’ve been chased out of other areas,” Ngor says.
The majority of species recorded in 1994 were again documented, pointing to a persistent backbone of commonly traded fish that continue to underpin local diets and commerce.
At the market, the sellers are all women. Toak Kunthea has been selling fish here for a decade. Like most sellers, she doesn’t specialize in any species but trades in some of the most common fish found across the market, where vendors display river catfishes, including the long whiskers Mystus gulio; several carp species, among them trey riel (Henicorhynchus entmema and H. siamensis), the small Siamese mud carps used to make Cambodia’s fermented fish paste, prahok; small-scale mud carp (Cirrhinus microlepis); and freshwater croakers.
“Business is good,” Kunthea says. While she says the overall volume of fish she sells has declined, prices have risen in the last two or three years. Traders increasingly purchase fish for restaurants in Phnom Penh, she explains, and sometimes for export.
Some species can fetch extraordinary sums. Among them is the striking two-faced fish (Incilisabeo behri), an endemic species easily recognized by the pronounced bulbous bump on its head and its vivid orange-yellow coloration. Prized less for its taste than for its status, the fish has become one of the market’s most valuable offerings. On this morning, Sles Rany, 32, who goes by Mary, recounts selling one for nearly $700. “I think it may be going abroad,” Mary says.
Such eye-catching prices worry scientists. High market values can drive fishing pressure, creating incentives to target species that may already be under strain. The two-faced fish is currently listed as vulnerable, yet fishers recall a time when it was more common in the river.
History offers cautionary parallels. Several of the Mekong’s most iconic giant fish, including the Mekong giant catfish and giant barb, were once abundant before heavy fishing pressure and habitat disruption drove their populations into steep decline. As these large, slow-growing species became rarer, their market value often rose, reinforcing the cycle. “I worry that the two-faced fish might be going the way of the Mekong giant catfish,” Hogan says. “Once a species becomes valuable enough, it can quickly become a target.”
Shifting trade
Vendors at Stung Treng’s main market appeared aware of restrictions on selling endangered fish, perhaps reflecting stronger enforcement by fisheries authorities. At a smaller market on the outskirts of town, however, researchers documented an endangered giant freshwater stingray offered discreetly for sale.
Researchers suspect that portions of the trade in threatened species have shifted online, making it far harder to monitor. “This is something that needs more investigation,” says Chea Seila, program manager for the Wonders of the Mekong research project, who participated in the survey.
Alongside warning signs, the surveys made encouraging discoveries. Researchers encountered species that may represent new records for Cambodia, including what appears to be an undocumented type of spiny eel in the family Mastacembelidae, which are not true eels but eel-like fishes found across Southeast Asia.
The survey also shed light on one of the Mekong’s rarest fishes, the giant salmon carp (Aaptosyax grypus), sometimes called the “Mekong Ghost.” Long feared extinct before being rediscovered in recent years, a handful of individuals have since appeared at the Stung Treng market.
Until now, however, scientists did not know where the fish were being caught. During the survey, researchers identified a vendor who was able to pinpoint its source, a crucial step for conservation. “If you want to protect a species, you need to know where the fish is being caught. In this case, the fish market helped solve the mystery,” Hogan says.
The survey also hinted at overlooked biodiversity beyond fish. Researchers from the National University of Singapore identified what may be a freshwater crab genus not previously recorded in Cambodia, a finding still being verified.
The discovery underscores how much remains undocumented in the Mekong system. Market surveys offer “useful insight,” says Tan Heok Hui, a taxonomist at the National University of Singapore who took part in the study, even if they capture only the species entering trade and represent just “a slice of information” from a short window in time.
Identification improvements
The surveys also point to the growing role of aquaculture in Cambodia’s fish trade, a pattern largely absent when Roberts conducted his 1994 study. Alongside wild river species, markets now feature increasing numbers of farmed fish, including non-native tilapia and native species raised in ponds and cages. Black-eared catfish (Pangasius larnaudii) and other river catfishes, once only wild-caught, are now commonly produced through aquaculture.
Researchers say this shift is reshaping the composition of fish entering trade, reflecting broader changes in how fish are supplied, consumed and valued across the Mekong Basin.
They also note that the ability to conduct this kind of survey has improved dramatically from three decades ago. Advances in taxonomy and digital tools now allow far more precise identifications. The Mekong Fish App, a mobile field guide with species images and diagnostic features, helps scientists quickly distinguish species that might previously have been recorded only at the genus level.
Improved scientific capacity within Cambodia has also played a key role. “We can now document biodiversity in ways that simply weren’t possible before,” Ngor says, pointing to both better identification tools and growing local expertise.
By revisiting markets surveyed decades ago, researchers hope to document the shifting baselines in an ecosystem undergoing rapid change, which can help inform fisheries management and conservation. Hogan says he wasn’t sure what the survey would reveal but remains hopeful. “What we see here is a sign of the remarkable resilience of the river,” he says.
Stefan Lovgren is a journalist based in Las Vegas, NV. He is the founder of Freshwater Frontlines, an independent journalism initiative to report on freshwater issues globally.
In the Mekong’s murky depths, giants abound, new expedition finds
Citations:
Roberts, T. R. & T. J. Warren (1994). Observations on fishes and fisheries in southern Laos and northeastern Cambodia, October 1993-February 1994. Natural History Bulletin of the Siam Society, 42.
Sor, R., Ngor, P. B., Lek, S., Chann, K., Khoeun, R., Chandra, S., … Null, S. E. (2023). Fish biodiversity declines with dam development in the Lower Mekong Basin. Scientific Reports, 13(1). doi:10.1038/s41598-023-35665-9
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