Hidden beneath the surface of the world’s rivers, some of Earth’s great animal movements unfold – , in sheer biomass, the famous mass movements of zebra and wildebeest across the Serengeti.
For centuries, fish migrations were as predictable as the seasons. Salmon, sturgeon, giant catfish and many other species moved through rivers rising water, flood pulses and evolved biological cues.
These species are extraordinarily diverse, ranging from – massive fish that can live for more than a century and produce the world’s most prized caviar – to , , gold-flecked shad and goliath catfish, all of which travel to survive, in some cases over hundreds or even thousands of miles.
Their journeys can span continents. But the .
For most migratory fish, movement is not optional; it is how they survive. When dams block routes, when fishing intensifies at migratory bottlenecks and when floodplains and spawning grounds are cut off or degraded, most migratory fish do not simply go somewhere else. They cannot. First the migration thins, then it falters. In some rivers, especially those blocked by dams, .
A new I led for the March 2026 international meeting of parties to the Convention on the Conservation of Migratory Species of Wild Animals provides the clearest picture yet of this decline – and what’s needed to stop it.
My co-authors and I reviewed more than 15,000 species of freshwater fish, identified which of them migrate, and . We then focused on migratory species with declining populations and identified those where countries will have to work together to help them recover and thrive.
The results are sobering.
We identified 325 migratory freshwater fish species as candidates for coordinated international conservation actions under the . Many of the largest species, the giants that make the longest and most dramatic journeys, are in the most trouble. Among migratory fish already listed under the Convention on Migratory Species, . In Asia, populations of by over 95% since 1970.
The disappearing giants of the Mekong
For the past 25 years, I have studied the world’s largest freshwater fish as at the University of Nevada, Reno; host of Nat Geo Wild’s series; and the Convention on Migratory Species councilor for freshwater fish.
One of these extraordinary animals, the , grows to more than 650 pounds. It once migrated hundreds of miles along the Mekong River, supporting fisheries and cultural traditions across the region. Today it is critically endangered because its route to spawning grounds and overfishing at migration bottlenecks is killing the large adults that the population depends on.
In Cambodia, small migratory fish known as trey riel are so significant that they gave their name to the national currency. In South Asia, one , is so culturally important that it is sometimes given as a wedding gift, wrapped in ornate cloth and adorned with flowers.
Migrations of these fish, like migrations of buffalo on the American plains once did, shape ecosystems, livelihoods and culture. In the Mekong Basin alone, fisheries produce each year, helping to feed tens of millions of people. When these fish disappear, people suffer.
Long migrations under threat
Declines are unfolding in other great river systems as well.
In the Amazon, some of the largest catfish on Earth migrate across much of the continent. The dorado, or gilded catfish, can reach six and a half feet (2 meters) in length and complete a between Andean headwaters and coastal nurseries, the longest freshwater fish migration ever recorded.
At Teotônio Rapids between Bolivia and Brazil, fishers once hung from wooden scaffolding above turbulent waters to spear dorado as they surged upstream – until the rapids were flooded by new dams. Altered river flows, barriers and overfishing are increasingly disrupting these journeys, and in upstream Bolivia have plummeted.
Across the Northern Hemisphere, migratory fish such as salmon, sturgeon and shad have suffered major losses because rivers have been dammed and polluted, while many populations were heavily overfished.
In the Columbia River basin, dam construction transformed an immense river system into a and blocked fish from large parts of their historical range.
In South Asia, fish such as , goonch catfish and are also declining under pressure from dams, overharvesting, sand mining, pollution and habitat loss, even as they remain central to fisheries and river cultures across the Ganges, Brahmaputra and Indus basins.
Why migratory fish are struggling
Migratory freshwater fish depend on long, connected river corridors, often across multiple countries. , are breaking those connections. Once routes are cut, .
This is increasingly an international problem. More than 250 rivers and lakes worldwide cross national borders, and about . Yet freshwater fish are still too often managed at a local or national scale, as if rivers and fish movements stop at political boundaries.
That is why international agreements matter. is the only global treaty specifically designed to encourage countries to work together to conserve migratory animals.
For freshwater fish, cooperation can begin with something as simple as countries sharing data and can extend to coordinated actions to reduce overharvesting, protect floodplains and spawning grounds, and keep rivers connected. The most fundamental solution is to manage rivers as connected ecological systems rather than as isolated national waterways.
Of the , many could be considered for listing under the convention. Listing does not automatically save a fish, but it provides a mechanism to enable countries to coordinate monitoring, management and conservation across borders. That matters because freshwater fish remain , despite the scale of their decline.
We found that the river basins where international cooperation is now include the Amazon and La Plata-Paraná in South America, the Danube in Europe, the Mekong in Asia, the Nile in Africa and the Ganges-Brahmaputra in South Asia.
How to bring back migratory fish
Restoring migratory fish populations means , reconnecting rivers fragmented by dams and channelization, improving fisheries management, protecting floodplains and wetlands, and restoring habitats that have been drained, cleared or isolated by development.
There are examples of success. In Washington state, dam removals on the Elwha and White Salmon rivers reopened habitat that had been inaccessible for migrating fish for about a century, .
The world’s great fish migrations have not disappeared everywhere, but they are fading. This new assessment offers a clearer picture of where international cooperation is most urgently needed. It is up to humanity to protect these extraordinary aquatic animals, which support enrich their lives, and make the world a more wondrous place.
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