- As national representatives gather at the UN COP15 Convention on Migratory Species (CMS) meeting this week in Brazil, a new global report has been released profiling a dangerously neglected category of migratory animal: the world’s freshwater fish.
- Migratory freshwater fish populations have fallen by 81% since 1970, says the report, with 325 species worldwide urgently needing coordinated international conservation action. However, only 23 migratory freshwater fish species are currently listed under CMS.
- More than half of the 325 at-risk freshwater migratory fish species documented by the report are in Asia, with the Mekong River of major concern. While international conservation cooperation is urgently needed, China and other Mekong basin nations are non-parties to CMS, as are the U.S. and Russia.
- What is needed now, conservationists say, are transnational migratory freshwater fish species conservation action plans that cover entire river systems, with those plans managed cooperatively by multiple nations within each river basin.
CAMPO GRANDE, Brazil — In the wide, sandy stretches of Brazil’s Araguaia River, the piraíba, South America’s largest catfish, is a cornerstone of local fisheries. Fishers often recognize individual fish by markings and scars, catching the same fish in the same places over time, leading many to believe the species did not move very far. But new work by researcher Lisiane Hahn reveals something surprising: The piraíba travels hundreds, if not thousands, of kilometers to spawn.
Using radio telemetry, Hahn and her colleagues implanted transmitters in nearly 100 catfish, some nearly 2 meters (6.5 feet) long, and tracked their movements via a network of receivers. The data provide the clearest evidence yet that piraíba (Brachyplatystoma filamentosum), which can reach 200 kilograms (more than 440 pounds), undertake extensive, long-distance migrations.
“As a researcher, it’s emotional to discover this in such an iconic species,” said Hahn, who leads Projeto Peixara at the Federal University of Mato Grosso in Brazil. “You imagine the largest catfish in South America moving across entire river systems.”
The finding underscores a broader reality: Some of the most important wildlife migrations in South America — and around the world — have remained largely out of view. In the Amazon and other rivers in South America, giant catfish move underwater between nations along routes that connect Andean headwaters to floodplains and coastal nurseries, sustaining fisheries that feed millions and support regional economies. Yet these aquatic movements are still poorly understood and severely under protected.
Freshwater fish remain underrepresented within the UN’s Convention on Migratory Species (CMS), even as a new global assessment released this week at the CMS COP15 meeting here in Brazil paints a stark picture: Migratory freshwater fish populations have dropped by an estimated 81% since 1970, with 325 species globally in urgent need of coordinated international conservation action, says the new report.
“This is the first global assessment of migratory freshwater fish, and it shows just how much has gone unnoticed,” said Zeb Hogan, the report’s lead author and the COP-appointed CMS scientific councilor for freshwater fish. “We’re only now beginning to understand the scale of these migrations, and how much they are at risk.”
Migratory freshwater fish the world over depend on connected river systems that span vast distances and often cross national borders. Many fish move between floodplains, main channels and estuaries at different life stages. But dams, overfishing and habitat loss are breaking those pathways, causing populations to collapse.
“Rivers don’t recognize borders, and neither do the fish that depend on them,” said Michele Thieme, lead freshwater scientist for WWF. “The crisis unfolding beneath our waterways is far more severe than most people realize.”
Under CMS, a species qualifies for listing if it regularly crosses national borders and would benefit from international cooperation. CMS Appendix I includes species at risk of extinction, while Appendix II focuses on those requiring coordinated management across their range. Only 23 migratory freshwater fish species are currently listed under CMS, 19 of them being sturgeon. By comparison, 1,189 migratory animal species are listed overall, with 962 of them being birds.
The new report identifies 325 fish species that spend most of their time in freshwater and meet conservation criteria, meaning they are migratory, transboundary and in decline. More than half are in Asia, with the Mekong River in Southeast Asia a major concern. Yet none of the countries along the Mekong are parties to CMS, including China. The U.S. and Russia are also non-parties, leaving large portions of key river basins (including the vast Missouri-Mississippi river system and many Arctic rivers) outside the framework.
“It highlights the need to bring more countries, especially in Southeast Asia, into CMS if species are going to be protected,” said Zach Bess, a fish ecology graduate student at the University of Nevada, Reno in the U.S., and a co-author of the report.
Experts say the new assessment points to a fundamental mismatch between how rivers function and how they are managed. “What stands out for me is that … we are still managing rivers in fragmented ways,” said Twan Stoffers, a fish ecologist at Wageningen University in the Netherlands.
But there are signs of progress. Dam removals in parts of Europe and North America have helped restore migration routes, and at CMS COP15, countries in the Amazon basin presented a new multi-species action plan for migratory catfish, one of the first coordinated efforts of its kind.
In Cambodia, cooperation with fishing communities has contributed to increases in some critically endangered species, including the Mekong giant catfish (Pangasianodon gigas), offering a model for recovery. But the Mekong River, despite hosting 80 of the 325 identified at-risk species, still lacks coordinated basin-wide management. “There is a big gap in international cooperation in the Mekong for migratory fish,” said Hogan, who is a fish biologist at the University of Nevada, Reno, and has studied Mekong fish species for decades.
Back on the Araguaia, Hahn and her team are expanding their study area to capture data from a larger stretch of the river to gain a better understanding of how far these fish move. So far, only two South American migratory freshwater fishes — the dorado (or gilded catfish, Brachyplatystoma rousseauxii), which makes the longest known freshwater migration in the world, and the piramutaba (Brachyplatystoma vaillantii) — are listed under CMS, even though other large species move across vast distances.
Hahn has also tracked other South American fishes, including Jaú catfish (Zungaro zungaro) and pirapitinga (Piaractus brachypomus), a large fruit-eating species known for seasonal movements. In one case, a fisher in Peru reported a tagged pirapitinga nearly 800 kilometers (497 miles) from where it was released. “We’re starting to find the pieces of the puzzle,” Hahn said. “And we’re only just beginning to understand what these fish are doing.”
Banner image: Researcher Lisiane Hahn and colleagues have tagged and released piraíba fitted with tracking devices, generating new data that reveal previously undocumented migrations of this South American species. Image courtesy of Lisiane Hahn.
Citation:
Hogen, Z., Bess, Z., Thieme, M., & Stoffers, T. (2025). Global Assessment of Migratory Freshwater Fishes. CMS Secretariat, Bonn, Germany. Retrieved from: https://www.cms.int/sites/default/files/document/2025-11/cms_cop15_doc.25.6.1_annex1_freshwater-fish_e.pdf
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