Cambodia & Nature: The Silent Giant of the Mekong, the Giant Mekong Catfish on the Brink

At the bottom of the Mekong's brown waters, an immense shadow is becoming increasingly rare. Pangasianodon gigas, the giant Mekong catfish, is a large migratory species endemic to the Mekong River basin. Due to its decline and shrinking distribution, it is now listed on the IUCN Red List as "critically endangered."

A recent study published in the journal Global Ecology and Conservation has just provided a detailed and alarming assessment of its health status.

A Colossus with Clay Fins

Pangasianodon gigas is one of the world's largest freshwater fish, reaching nearly three meters in length and weighing up to 300 kilograms. In 2005, it set the Guinness World Record for the largest freshwater fish ever observed. Yet this scaleless giant, whose presence is attested by 3,000-year-old rock paintings near Ubon Ratchathani in Thailand, is today on the brink of extinction.

The global population is estimated to have declined by about 90% over the past twenty years. Poaching, habitat degradation, and hydroelectric dams have turned this living symbol of river biodiversity into a phantom species.

Bycatch, the Invisible Threat

Between October 1999 and January 2025, 132 specimens of P. gigas were documented as bycatch in the Cambodian Mekong river system. The post-release survival rate exceeds 83%, with seasonal capture peaks occurring between October and January during migration. However, the data reveal a worrying trend: the average weight of captured individuals is decreasing over time, indicating changes in the population's demographic structure.

This size reduction is not unique to this species. A separate study published in 2025 in Biological Conservation, based on analysis of over 397,000 samples from 257 species in Laos, Cambodia, and Vietnam, shows that the average size of the giant Mekong catfish and the giant Mekong barb has dropped by more than half compared to historical data. The size reduction is a sign that fish are being regularly caught before reaching maturity.

Tonlé Sap, the Vital Lung Under Pressure

Migratory data confirm the irreplaceable role of the Tonlé Sap. Spatial and temporal capture patterns show a wide distribution between the lake and the Tonlé Sap River and the Mekong's main channel, confirming the continued use of these historic migration routes at different life stages, from juveniles to breeding adults.

Yet this ecosystem is under pressure itself. In 2019, the combination of climate change, El Niño, and Mekong dams caused the Tonlé Sap River's flow reversal to last only a few weeks instead of several months, leaving the lake warm, shallow, and oxygen-poor. That year, Tonlé Sap fisheries collapsed by an estimated 80 to 90%.

Dams: A Growing Threat

Large-scale hydroelectric dams on the main stem of the Lancang-Mekong pose a risk of major environmental impacts, including reduced sediment flows and declining fish catches. In the Sesan and Srepok basins, two major Mekong tributaries where dam construction has intensified, fish species declined from 60 to 42 species and from 29 to 25 species, respectively, between 2007 and 2014.

China has built twelve dams on its section of the river, and lower Mekong countries have followed suit. In March 2020, the Cambodian government announced a ten-year moratorium on hydroelectric dam construction along Mekong stretches within its territory. Late 2023, the Cambodian Prime Minister reaffirmed this commitment. A decision welcomed by scientists, but insufficient on its own to halt the decline.

Community Fishing, the Last Watchpost

Faced with scarce data in such a vast and hard-to-access environment, researchers are increasingly relying on local fishing communities. Joint efforts between scientists and fishers have become essential for monitoring the region's rarest species.

Tagging, release, and reporting of catches: these collaborative practices now form the main monitoring net for a species whose stealth defies conventional methods.

Environmental DNA techniques, which detect the presence of the giant catfish from simple water samples, have also been developed, with successful in situ validation in reservoirs where the species was known to be present.

Race Against Time

The species enjoys international protection: it is listed in CITES Appendix I and the Convention on Migratory Species (CMS). Commercial fishing bans have been in place since 1947 in Thailand, 2003 in Cambodia, and 2009 in Laos. But a quarter-century of accumulated scientific data paints an increasingly grim picture.

If the dam-building dynamic on the Mekong continues, a large part of the river's fish production and the economic, nutritional, and social benefits it generates could be lost in the coming decades, researchers warn in a specialized journal. For the giant Mekong catfish, time is running out.

Sources: Global Ecology and Conservation (2026); Biological Conservation, Uy et al. (2025); WWF; IUCN; Mekong River Commission; Mongabay; International Crisis Group (2024); Stimson Center, Mekong Dam Monitor (2024).