The megafish of the Mekong River are shrinking, a new study has found.

In the most comprehensive analysis of species size in Southeast Asia’s Lower Mekong Basin, researchers have tracked a generational shrinkage among the river’s iconic gargantuan fish, which are among the largest freshwater fish in the world.

The size decline in the Mekong is a troubling trend for the ecosystem on which more than 65 million people across six countries depend. It also mirrors shrinkages of other megafauna in river basins around the world.

“At its core, the analysis shows that the Mekong River’s biggest, slowest-to-mature fish species, and especially those at highest risk of extinction, are the ones shrinking fastest,” Zeb Hogan, a co-author of the study and biologist at the University of Nevada, Reno, said in an interview with Mongabay. “Declining fish size isn’t just a symptom of overfishing — it’s a warning sign of deeper population instability.”

The study results are based on more than seven years of catch-monitoring data that tracked 257 species across 23 sites in Laos, Cambodia and Vietnam. More than 397,000 samples were collected from mid-2007 to mid-2014.

These samples were compared to historical data from conservation information hub FishBase and the Mekong River Commission’s fisheries monitoring database.

Changes in fish body size are a key indicator of fish stock health that informs fisheries management and conservation planning.

Small and medium-sized fish species didn’t show significant change, the study found. But fish exceeding 60 centimeters (2 feet) in length — which were often classified as endangered or critically endangered on the IUCN Red List — displayed the most shrinkage.

Hogan, who is also a co-author of the book Chasing Giants: In Search of the World’s Largest Freshwater Fish, said the issue is best characterized by some of the river’s most legendary species: the Mekong giant catfish (Pangasianodon gigas) and the Mekong giant barb (Catlocarpio siamensis). The average observed size of these critically endangered giants, which can measure up to 3 meters (10 ft) long and weigh up to 300 kilograms (660 pounds) on average, respectively, have declined by more than half.

Shrinking sizes are an indication of fish being consistently caught before they reach maturity. When that happens, Hogan said, they produce fewer offspring, which “can trigger a downward spiral in population size, ultimately leading to collapse or extinction.”

The study also found that large species whose conservation status hasn’t been assessed for the IUCN Red List were similarly shrinking, like the shovelnose sea catfish (Hemiarius verrucosus), spoon-snouted catfish (Cryptarius truncatus), eeltail catfish (Plotosus canius) and spotted sea catfish (Arius maculatus).

The findings indicated that these fish “require greater attention” since “they may face an elevated risk of extinction due to a lack of data.”

“Knowing these giants are shrinking, both in size and number, is like losing a small piece of Cambodia itself,” Sophorn Uy, the study’s lead author and a researcher at the Royal University of Agriculture in Phnom Penh, said in a release about the findings. “Their existence carries deep cultural and ecological significance.”

Megafish are etched into the temple walls within the famous Angkor Wat complex in Cambodia. Species like the Mekong giant catfish are known in the Khmer language as trey reach, which translates to “king of fish.”

“The loss of large fish isn’t just about fewer fish — it’s about a breakdown in population dynamics and ecological function,” Sovan Lek, a co-author of the study, said in a release about the research. “These animals shape food webs, migrate across countries, and support one of the most important fisheries in the world.”

Nathaniel Ng, who was not affiliated with the study and is the Southeast Asia program coordinator for the freshwater conservation organization SHOAL, said the findings “did not come as a surprise” at all. Given the many “layered problems facing the Mekong” that affect the spawning and migration of megafish, the situation, he said, “has become very dire for some of these species.”

“You would lose a lot of the flavor of what makes the Mekong the Mekong” if these fish went extinct, Ng said in an interview. “This ecosystem is very much defined by the species that live and survive in it. When you remove these really iconic, huge species, the ecosystems are just irreparably changed,” he said.

“Not all hope is lost but we are definitely on a stopwatch and action needs to happen quickly and at a large enough scale for this ecosystem to actually recover,” Ng added.

The Mekong findings reflect a broader global trend, according to the researchers who are seeing freshwater giants in other river basins similarly shrink.

In the Amazon Basin, populations of arapaima (Arapaima gigas), which rivals the Mekong giant catfish for the claim of the world’s largest freshwater fish, have declined, and the survivors are shrinking — a blow for fisherfolk in the basin.

In the Nile Basin, the Nile perch (Lates niloticus) has diminished in size because of overexploitation and habitat degradation, on top of other pressures bearing down on Africa’s freshwater species.

“This research fits the classic pattern we see around the world,” Hogan said. “Large-bodied fishes are intrinsically more vulnerable under heavy exploitation: they’re more catchable, grow slowly, and mature late.”

Overfishing and indiscriminate harvesting are the main causes behind the shrinking species in the Mekong. But the study stressed that, like in other river basins, a perfect storm of additional threats, ranging from dam construction and deforestation to sand mining, pollution, climate change and habitat loss, is brewing.

These “multiple and potentially synergistic stressors” alter habitat and migration, which, the study said, may tip the balance toward population collapse.

“There is no silver bullet in this game,” Marc Goichet, the freshwater lead in the Asia-Pacific for WWF, told Mongabay. “We really need to put everything we have to save those fishes because the threats are real and increasing and the time to respond is closing.”

But funding cuts — perhaps most significantly for the Mekong, the dismantling of USAID, which funded this most recent study before its shutdown by the Trump administration earlier this year — may force conservation and research to slow.

“We do really need all the funds we can get,” said Goichet, who was not affiliated with the study. “Cutting funds now is not only an issue because international aid has been an important part of this important work, but also because of how suddenly it is affecting us. We do not have much time to find alternative resources.”

Read more:How private funding helped one NGO survive the USAID cuts

While the shrinking megafish and slashes to funding are worrisome, conservationists point to recent megafish discoveries as an indication that not all hope is lost.

The 2022 catch and release in Cambodia of the largest freshwater fish ever recorded — an absolutely gargantuan giant freshwater stingray (Urogymnus polylepis) that weighed 300 kg — proved to researchers that life, however big, continues to find a way.

Chea Seila, a project manager of the Wonders of the Mekong program, who was part of the record-breaking tagging team, told Mongabay that “even when the biggest fish in the Mekong are shrinking and vanishing, discovering that the world’s largest freshwater fish is surviving in Cambodia was an amazing surprise and a call to action.”

“That 300-kilogram stingray showed us that it’s not too late,” Hogan said. “New records are still being set. But they will mean little unless we act now to protect these species and the river system they call home. The future of the Mekong’s giants, its biodiversity, food security, and cultural heritage hangs in the balance.”

Banner image: A 1.72-meter (5.6-foot), 102-kilogram (225-pound) Mekong giant barb caught in Tonle Sap Lake in December, 2002. Image courtesy of Zeb Hogan with Wonders of the Mekong.

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Citation:

Uy, S., Hogan, Z. S., Grenouillet, G., Lek, S., Chandra, S., & Ngor, P. B. (2025). Declining fish sizes across the Lower Mekong Basin highlights urgent conservation needs. Biological Conservation, 310, 111384. doi:10.1016/j.biocon.2025.111384