Migratory freshwater fish are in trouble: Will we act in time to save them?

SALOBRA, Brazil — Life revolves around water in this quiet fishing village in Brazil’s southern Pantanal, the world’s largest wetland. Here, a clear tributary meets the broader Miranda River, part of a vast floodplain where multiple streams spill across the landscape and wildlife thrives. Jaguars prowl riverbanks, giant otters patrol the channels, parrots fill the skies, and beneath the rippling surface, fish of every size and shape swim through a shifting watery mosaic, tying a vast aquatic ecosystem together.

For 72-year-old retired fisherman Alberto Oriozola, the Miranda River was once both livelihood and lifegiving. He recalls, as a young man, looking down from a hilltop and seeing the river bottom appear to move with swimming pintado — the spotted surubim catfish (Pseudoplatystoma corruscans), their jaguar-like markings visible in the clear water.

“You could choose the size you wanted to catch,” he remembers. Fish 3 meters (10 feet) long were part of the catch. Now, the largest surubim are maybe half that length, and far less common.

These days, Oriozola’s grandson-in-law, Maycon Lopes da Silva, 26, works the same waters but in a different way. He guides sport fishers who come in search of surubim (in Portuguese, or sorubim in Spanish), along with other large species, and he shares the river with his more than 18,000 followers on Instagram. Bare-chested and quick to talk, da Silva learned the river from older anglers as he navigated a more precarious changing economy where fishing is as much about tourism as subsistence. “My feeling is that nowadays it’s much harder,” he says.

But as da Silva’s Instagram account shows, the Pantanal still draws people for its fish.

The pintado, a large catfish, remains one of the biome’s iconic species, while the golden dorado (Salminus brasiliensis), known as the “river tiger,” is prized for its speed and strength. This stretch of the Miranda River remains a hotspot for fishers, even though it is now straitjacketed by catch-and-release rules and other measures that reflect a growing need to manage and conserve what’s left.

One thing that hasn’t changed: These fish move! The pintado and dorado that define the Pantanal’s aquatic life are long-distance migratory fish, species that seamlessly meld into a vast freshwater landscape, traveling great distances along with other South American migratory fish, linking headwaters to floodplains and coastal estuaries.

Some of those journeys are truly extraordinary: The dourada (Brachyplatystoma rousseauxii) undertakes the longest-known freshwater migration on Earth, making roundtrip treks of up to 11,600 kilometers (7,000 miles) between the Andes mountains and Amazon estuary.

Epic migrations at risk

These continent-sprawling movements sustain river ecosystems and the people who depend on them worldwide. Yet it is people and their activities, especially dam building, that threatens these life-giving waters. For migrating fish, a river-straddling dam is much the same as a barbed wire fence to a herd of pronghorn antelope — posing a potentially fatal denial of habitat crucial to their life cycles.

That’s true not just across South America but in other great river basins, such as the Mekong and Congo. According to a new report, migratory freshwater fish have declined by an estimated 81% since 1970, making them among the most threatened vertebrates on Earth.

Despite these staggering losses, migratory freshwater fish remain severely underrepresented within the United Nations Convention on Migratory Species (CMS), a treaty aimed at protecting wildlife that crosses national borders.

Only two dozen migratory freshwater fish are listed under the current CMS agreement, despite hundreds more at high risk in South America, Asia, Africa and elsewhere; a clear reflection of how these species and their Herculean journeys have been neglected.

But that may be changing. At the CMS COP15 meeting in Campo Grande, Brazil in March, countries adopted a new action plan for Amazonian migratory fish and added the Pantanal’s spotted surubim catfish to the treaty’s Appendix II list of threatened species requiring coordinated management across their range.

A released at the meeting identified 325 migratory freshwater fish species worldwide in need of urgent conservation attention; without those species, the world would be a far poorer place economically, ecologically and culturally.

Long-distance swimmers vital to food webs

Migratory fish are the backbone of inland fisheries worldwide. In tropical regions especially, these species sustain livelihoods, local and regional economies and daily diets.

“We often think of rivers mainly as water supplies, but for hundreds of millions of people, healthy rivers are actually food systems, producing protein from migratory fish on a scale that rivals agriculture,” says Zeb Hogan, the CMS-appointed councilor for freshwater fish and lead author of the global assessment.

Many people associate fish migration with species that move between oceans and rivers. Species like Atlantic salmon and European eel depend on these journeys to complete their life cycles. Most sturgeon species, which are largely anadromous, meaning they migrate from the sea to freshwater to spawn, are listed under CMS.

But many of the most important migrating fish species never touch the sea and occur entirely within freshwater. In South America’s Amazon, Southeast Asia’s Mekong and along river systems worldwide, freshwater fish move within floodplains, upstream to spawn and downstream to feed, sustaining food webs and ecosystem balance.

Via their travels, they connect headwater, floodplain and estuarine habitats, allowing populations to replenish. They also support predators and population stability along the way. “In tropical rivers, fish don’t just occupy the food web, they are the food web,” says Hogan, a fish biologist at the University of Nevada, Reno.

Amazonia’s migratory fish and dams

The Amazon offers a vibrant example of freshwater biodiversity; it holds more fish species than any other river system and its researchers estimate that around 80% of the daily catch consists of migratory fish.

But the Amazon River, its tributaries and other river systems worldwide are under mounting pressure. Dams, water extraction and habitat loss disrupt migration routes, with cascading effects across aquatic ecosystems.

“Infrastructure is the biggest concern in the Amazon,” says Guillermo Estupiñán, a freshwater ecologist with the Wildlife Conservation Society in Brazil.

Unlike free-ranging marine and terrestrial species, freshwater fish are confined to a linear habitat. When a new dam blocks the path that a migratory fish species has traversed for millennia, their ecosystem and life cycles are abruptly cut in two.

Fish cannot go around a dam. And fish ladders, while touted by hydropower companies, often don’t allow much passage. Cut off from spawning and feeding grounds, migratory fish populations dwindle.

Take the Paraná River Basin for example, extending through Brazil, Paraguay and Argentina for some 4,880 km (3,030 mi). Its large dams, such as Itaipú (completed in 1982) and Yacyretá (1994), now block the ancient migration routes for species like the spotted surubim, shrinking their range.

“The Paraná is so full of dams that fish can no longer move,” says Carla Polaz, a Brazilian freshwater fish specialist with the International Union for Conservation of Nature (IUCN).

Dams not only impact long-distance swimmers. They also affect shorter-distance migrators, and scientists are now rethinking what migration means for freshwater fish. Migration, once mainly associated with long-distance, cross-border journeys, is currently understood to include a range of lesser journeys up and down river systems.

Species once thought sedentary are increasingly recognized as highly mobile, including arapaima (Arapaima gigas), South America’s largest freshwater fish and a key food source across Amazonia. These fish can travel up to 160 km (100 mi), often moving laterally into floodplains as waters rise and retreat annually, helping disperse plant seeds and shape periodically flooding forest ecosystems.

“Migration happens at different scales,” Estupiñán says.

The Mekong’s ‘billions and billions of fish’

More than half of the 325 at-risk migratory fish recently identified in the global assessment occur in Asia, with a major concentration in the Mekong Basin. Rising in the Tibetan Plateau and flowing through six countries to the South China Sea, the Mekong River supports the world’s largest inland fishery in the world and provides livelihoods and essential protein for tens of millions of people, especially in Cambodia and Vietnam.

Nowhere is the ebb and flow of this great river system more visible than along Cambodia’s Tonle Sap, which links the Mekong to Tonle Sap Lake, the ecological heart of the basin. Each year, the river reverses direction with the seasonal flood pulse, expanding the lake many times over before draining back toward the Mekong.

Just outside Phnom Penh, Cambodia, rows of large stationary bag nets, known as the dai fishery, are set across the river to intercept fish following this seasonal flow. During peak months, the nets here are lifted from the water every few hours, each haul bringing up around a ton of fish, or more, as waves of migrants move downriver.

“Right in this river below us is one of the largest animal migrations on Earth,” Hogan says during a recent visit to the dai nets. “Billions and billions of fish seasonally move down the Tonle Sap River from the lake and out into the Mekong.”

That movement underpins the entire basin’s fishery. An estimated 40-70% of the Mekong’s catch depends on migratory species, which link distant parts of the basin into a single, functioning system.

The Mekong is also home to some of the largest freshwater fish on Earth. Species such as the Mekong giant catfish (Pangasianodon gigas), which historically has reached weights of 300 kilograms (660 pounds), and the giant barb (Catlocarpio siamensis) depend on long-distance migrations to survive. But once widespread across the basin, many of these fish have disappeared in Thailand and Laos.

Cambodia, where stretches of the river remain free-flowing, now persists as a sort of last refuge. “When fish can no longer live in certain sections of the Mekong, they have to move elsewhere,” says Peng Bun Ngor, dean of faculty of fisheries and aquaculture at Cambodia’s Royal University of Agriculture.

Today, the Mekong poses an iconic example of the fragility of the world’s freshwater migratory systems and their transboundary regulatory challenges. Many Mekong species cross national borders during their life cycles, making them especially vulnerable to new dams and other barriers. But despite this, none of the countries in the Mekong Basin are signatories to the Convention on Migratory Species.

“You have one of the most productive rivers on Earth, with some of the largest migrations, but very limited international coordination to protect them,” Hogan says.

Africa’s altered refuges

In parts of the world, especially Africa, even the most basic data on migratory fish are missing. Scientists say migratory species there are likely to be as important to rivers and people as they are in the Amazon or Mekong, supporting fisheries and shaping ecosystems, but their movements remain poorly documented.

In many cases, even fundamental questions — where fish travel, when they move and how far — are still not well understood. And yet, millions of human lives depend on the survival of those fish for food security.

The Congo Basin, the world’s second-largest river system, offers a striking example. Among the few researchers working on these unanswered questions, Tobit Liyandja of the University of Toronto and University of Kinshasa has documented migrations in large, fast-swimming Labeo — a genus of river carps in the family Cyprinidae — that move with seasonal flows and support local fisheries. Liyandja’s genetic work suggests distinct populations of L. sorex — a carp species that travels along river stretches that mark international borders — which could face growing conservation risks.

But these river carp findings only hint at much larger, mostly unseen migratory systems. “The list of migratory species within the Congo Basin is certainly longer,” Liyandja says, “but the data needed to rigorously document their movements are currently lacking.”

On the Zambezi River in southern Africa, seasonal migrations involve species such as Labeo carps and mormyrids, or elephantfish. “When the water drains from the floodplains, they start moving upstream,” says Clinton Hay, a fisheries scientist at the University of Namibia, who has studied fish there for 35 years.

Some Zambezi species migrate by day, others at night, even tracking lunar cycles, underscoring how tightly these movements depend on shifting stream conditions.

In Ethiopia, pressure on rivers is exposing the vulnerability of migratory fish. In the Lake Tana Basin, 17 closely related species of Labeobarbus (another genus in the Cyprinidae family) migrate from the lake into tributaries to spawn, sustaining local fisheries. These movements concentrate fish at predictable times and places, making them highly susceptible to overharvest. “It’s very easy to catch a lot of fish,” says Michael Cooperman, a fisheries ecologist with PlusFish Philanthropy who studies the river system.

But as irrigation development expands across the Lake Tana Basin, all six rivers used by these fish have been reshaped by dams, diversions and water withdrawals. Migration routes are cut off, spawning grounds are no longer accessible, and fish are funneled into smaller stretches of river where they are heavily harvested. The result has been a sharp decline in catches.

Likewise, a newly built reservoir in the basin has become an unintended experiment in whether altered African rivers can sustain migratory fish. The Ribb Reservoir has trapped part of the stream’s migratory fish population upstream, with early surveys finding at least a dozen of the 17 species present there.

“Can the newly created reservoir function as a refugia for the species to persist?” Cooperman asks. The answer could have implications far beyond Ethiopia, testing whether fragmented systems can replace connected rivers ecologically.

Uncertain riverine futures

Even as new research fills critical gaps, the full scale and importance of migratory freshwater fish globally remains only partly grasped — with a race underway to understand, mitigate harm and help these fish species persist and thrive.

“Our knowledge of migratory fish gets better every year, but there is still a lot we don’t know,” Hogan says.

Yet there are hopeful signs. At the recent CMS meeting in Brazil, long overlooked freshwater fish species drew far greater attention than at previous gatherings. Brazil, scientists say, is emerging as a leader in pushing freshwater fish migrations onto the global conservation agenda.

Back in Salobra, those global discussions feel distant though deeply relevant. But as researchers rush to fill data gaps, the rate of change on the world’s rivers is escalating in the face of rapidly expanding infrastructure and onrushing climate change.

But fishers, those who know rivers most intimately, say the fish that once defined these waters are harder to find, even as these great migrators remain central to daily life.

“Fishing used to be easier,” says da Silva, the fishing guide.

Banner image:The Golden dorado (Salminus brasiliensis), a powerful migratory freshwater fish native to the Pantanal and Paraná River Basin, shown here being held by researcher Zeb Hogan. Highly prized by recreational anglers and important to commercial fisheries, the species has declined in parts of its range due to dam construction. Image courtesy of Zeb Hogan.

Citation:

Hogen, Z., Bess, Z., Thieme, M., Stoffers, T., 2025. Global Assessment of Migratory Freshwater Fishes. CMS Secretariat, Bonn, Germany.

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