Migratory Fish: 81% Decline Since 1970 Reveals a Silent Crisis in the World's Rivers
While the world debates climate change and deforestation, an ecological catastrophe of colossal proportions advances silently beneath the surface of rivers: freshwater migratory fish populations have plummeted 81% since 1970, according to a report by the United Nations Convention on Migratory Species (CMS) released in 2026. The number is more brutal than the decline of terrestrial and marine animals over the same period. Of the 58 migratory fish species listed by the CMS, 97% are threatened with extinction — and 325 additional species have been identified as urgent candidates for conservation. Led by researcher Zeb Hogan, the study exposes an alarming reality: 93% of the planet's rivers are fragmented by dams, transforming ancestral migratory routes into dead ends.
What Happened
The Report That Exposed the Invisible Crisis
The Convention on Migratory Species (CMS), a United Nations environmental treaty, published in 2026 a comprehensive report on the state of freshwater migratory fish populations worldwide. The data compiled by an international team of researchers, led by biologist Zeb Hogan — one of the world's foremost experts on freshwater fish — revealed a devastating scenario.
Since 1970, freshwater migratory fish populations have suffered an average decline of approximately 81%. This number represents a catastrophic loss of biodiversity that surpasses even the decline observed in terrestrial and marine animals over the same period. Freshwater fish, which depend on connected rivers to complete their life cycles, are disappearing faster than any other monitored group of vertebrates.
Shocking Numbers
The report presented statistics that put the scientific and conservation community on high alert:
- 81% decline in freshwater migratory fish populations since 1970
- 97% of the 58 species listed by the CMS are threatened with extinction
- 325 species were identified as urgent candidates for conservation measures
- 93% of rivers worldwide are fragmented by dams and other human structures
These numbers are not statistical abstractions — they represent the real collapse of fish populations that sustain entire ecosystems and feed hundreds of millions of people around the world.
Identified Causes
The CMS report identified four main causes for the catastrophic decline of freshwater migratory fish:
Dams and river fragmentation — With 93% of the world's rivers now fragmented, the migratory routes that fish have used for millions of years to reproduce and feed have been blocked. Hydroelectric dams, irrigation reservoirs, and flood control levees have transformed free-flowing rivers into sequences of stagnant reservoirs, preventing fish passage between essential habitats.
Overfishing — Both commercial and artisanal overfishing has reduced migratory fish populations to unsustainable levels in many river basins. Large-bodied species that take years to reach reproductive maturity are particularly vulnerable.
Habitat degradation — Deforestation of riverbanks, urbanization of floodplains, and conversion of wetlands to agricultural land destroy the habitats fish depend on for spawning, feeding, and refuge.
Water quality pressures — Agricultural pollution (pesticides and fertilizers), industrial effluents, and domestic sewage degrade river water quality, making them inhospitable for sensitive species.
Context and Background
The Ecological Importance of Migratory Fish
Freshwater migratory fish are species that regularly move between different habitats along rivers and river systems to complete their life cycles. Some migrate hundreds or even thousands of miles between spawning, feeding, and growth areas. These migrations are not nature's whims — they are evolutionary strategies refined over millions of years that allow fish to exploit the best available habitats at each stage of their lives.
Among the best-known examples are salmon, which are born in rivers, migrate to the ocean, and return to their rivers of origin to spawn. But hundreds of fish species make migrations entirely within freshwater systems, moving between main rivers and tributaries, between rivers and floodplains, or between stretches at different elevations.
The Case of the Hucho Taimen: A Giant in Decline
An emblematic example of the decline documented by the report is the Hucho taimen, the world's largest salmonid, which can exceed 5 feet in length and weigh more than 110 pounds. Historically distributed across vast areas of Russia, Mongolia, and China, the taimen has seen its populations plummet in all these regions.
In Russia, overfishing and degradation of Siberian rivers have drastically reduced populations. In Mongolia, where the taimen is considered sacred by some nomadic communities, pressure from mining and uncontrolled sport fishing threatens the species' last refuges. In China, dam construction and industrial pollution have virtually eliminated the taimen from rivers where it was abundant just a few decades ago.
The taimen's decline is not an isolated case — it is representative of what is happening to hundreds of migratory fish species on every continent.
The Era of Great Dams
The fragmentation of the world's rivers is a relatively recent phenomenon on the scale of human history. The vast majority of existing dams were built in the 20th century, especially between the 1950s and 1980s, during the boom in hydroelectric construction and large-scale irrigation projects.
Today, there are more than 60,000 large dams in the world (over 50 feet tall), plus millions of smaller dams. Together, these structures have radically transformed the planet's river systems. Rivers that once flowed freely for thousands of miles are now sequences of reservoirs, with flows controlled by human operators instead of natural cycles of rain and drought.
The finding that 93% of the world's rivers are fragmented is particularly alarming because it means only 7% of the planet's river systems still offer conditions for migratory fish to complete their life cycles without artificial obstacles.
Faster Decline Than Terrestrial and Marine Animals
One of the most concerning aspects of the CMS report is the finding that freshwater migratory fish are declining faster than terrestrial and marine animals. While the WWF's Living Planet Index documents an average 69% decline in wild vertebrate populations since 1970, freshwater migratory fish have lost 81% — a significantly higher rate.
This disparity is explained by the particular vulnerability of freshwater ecosystems. Rivers and lakes represent only 0.01% of the planet's water but harbor a disproportionate share of global biodiversity. At the same time, they are the ecosystems most directly impacted by human activities — receiving pollution from cities and farms, being dammed for energy and irrigation, and having their banks deforested and urbanized.
Impact on the Population
Direct Consequences for Billions of People
The decline of migratory fish is not just an environmental issue — it is a crisis of food security, economy, and public health that directly affects hundreds of millions of people worldwide.
| Dimension | Current Situation | Consequence | Affected Populations |
|---|---|---|---|
| Food security | Freshwater fishing in global decline | Loss of essential protein for riverside communities | 200+ million people depend on freshwater fishing |
| Economy | Collapse of commercial and artisanal fisheries | Loss of income and jobs in fishing communities | Millions of artisanal fishers in Asia, Africa, and South America |
| Ecosystems | 93% of rivers fragmented by dams | Collapse of aquatic food chains | Global freshwater biodiversity |
| Biodiversity | 97% of CMS species threatened | Mass extinction of migratory fish | 325 species candidates for urgent conservation |
| Culture | Culturally significant species in decline | Loss of ancestral fishing traditions | Indigenous and riverside communities on every continent |
Food Security at Risk
Freshwater fishing is a critical source of protein for more than 200 million people, especially in developing countries. In the Mekong Basin in Southeast Asia, freshwater fishing provides up to 80% of the animal protein consumed by riverside communities. In the Congo Basin in Africa and the Amazon Basin in South America, similar patterns repeat.
The decline of migratory fish directly threatens this food source. Migratory species tend to be the largest and most productive in rivers, and their disappearance cannot be easily compensated by smaller, sedentary species. For communities that depend on fishing as their primary protein source, the loss of these species can mean malnutrition and food insecurity.
Cascading Effects on Ecosystems
Migratory fish play ecological roles that go far beyond serving as food for humans. They transport nutrients between different parts of aquatic ecosystems, control invertebrate and algae populations, and serve as prey for birds, mammals, and reptiles. The disappearance of these species can trigger cascading effects that affect the entire food chain, from microorganisms to apex predators.
What the Stakeholders Say
Zeb Hogan and the Research Team
Zeb Hogan, the researcher who led the study, is an internationally recognized freshwater fish biologist known for his work with endangered migratory species in rivers like the Mekong, Amazon, and Congo. The researchers involved in the report emphasized that the 81% decline is not a future projection — it is an already-accomplished reality. Without immediate and coordinated action, many species are heading toward extinction in the coming decades.
The United Nations' Position
The CMS, as a UN environmental treaty, has the authority to recommend conservation measures to signatory countries. The 2026 report includes specific recommendations for the protection of 325 species identified as urgent candidates for conservation, in addition to the 58 already listed.
UN representatives emphasized that the migratory fish crisis is a symptom of a broader problem: the systematic degradation of the planet's freshwater ecosystems. Healthy rivers are essential not only for fish but for drinking water, agriculture, energy, and the health of billions of people.
Reactions from Conservation Organizations
Organizations such as the Nature Conservancy, the Smithsonian, and WWF responded to the report with urgent calls for action. The Nature Conservancy highlighted that removing obsolete dams and installing fish passages on existing dams are immediate measures that can make a significant difference.
The Smithsonian published a detailed analysis of the report, emphasizing that the decline of migratory fish is faster and more severe than that of any other monitored vertebrate group, and that the window of opportunity to reverse this trend is closing rapidly.
Next Steps
Urgent Conservation Measures
The CMS report identified several priority actions that can help reverse the decline of migratory fish:
Removal of obsolete dams — Thousands of dams around the world no longer serve their original function (many were built for mills or small power plants that no longer operate). Their removal can restore river connectivity and allow migratory fish to resume their ancestral routes. In the United States and Europe, dam removal programs have already demonstrated positive results, with fish populations recovering rapidly after free flow restoration.
Fish passages — On dams that cannot be removed, installing fish passages (fish ladders, elevators, or lateral channels) can allow migratory species to bypass obstacles. However, the effectiveness of these structures varies enormously depending on design and target species.
Fishing regulation — Establishing science-based fishing quotas, creating protected areas in critical river stretches, and effective enforcement against illegal fishing are essential measures to allow population recovery.
Habitat restoration — Recovering riparian forests, restoring floodplains, and improving water quality are fundamental to creating favorable conditions for the survival and reproduction of migratory fish.
Inclusion of 325 New Species on the CMS List
One of the most significant developments from the report is the identification of 325 species as urgent candidates for inclusion on the CMS protection list. This inclusion would require signatory countries to implement specific conservation measures for these species, including protection of critical habitats and fishing regulation.
International Cooperation
Since migratory fish frequently cross national borders along their migratory routes, effective conservation requires international cooperation. The CMS report emphasizes the need for bilateral and multilateral agreements for shared management of transboundary river basins.
Closing
The 81% decline in freshwater migratory fish populations since 1970 is one of the most severe and least discussed environmental crises of our time. While debates about climate change and deforestation dominate headlines, the silent destruction of freshwater ecosystems advances with devastating consequences for biodiversity, food security, and the economy of hundreds of millions of people.
The 2026 CMS report, led by Zeb Hogan, leaves no room for ambiguity: 97% of listed species are threatened, 93% of rivers are fragmented, and 325 species need urgent protection. The window of opportunity to reverse this trend still exists — dams can be removed, habitats can be restored, and fishing can be regulated sustainably. But time is running out, and each year of inaction means more species pushed to the brink of extinction.
The world's rivers are the planet's arteries, and migratory fish are the pulse that indicates their health. When that pulse weakens, the entire organism suffers.
Sources and References
- The Guardian — Migratory freshwater fish populations decline 81% — Journalistic coverage of the CMS report
- Smithsonian Magazine — The crisis facing migratory fish — Detailed analysis of the decline and its causes
- UN Convention on Migratory Species (CMS) — Official Report — Official report with complete data
- Nature.org — Freshwater fish conservation — Conservation measures proposed by the Nature Conservancy
- Earth.com — Migratory fish decline — Report on the report's data
- EurekAlert — Scientific press release — Scientific press release about the research