Like flyways for birds, we need to map swimways for fish

For almost a century, migratory flyways have been a cornerstone of bird conservation. Knowing where these aerial highways are helps protect habitats and monitor species through carefully mapped routes that connect breeding, feeding and resting grounds.

But birds aren’t the only kinds of animals taking vast treks. Freshwater fish do, too.

Unlike birds, whose flight paths are visible to the naked eye, these fish migrations remain hidden beneath the surfaces of rivers. This invisibility has left them largely overlooked, even as migratory freshwater fish populations worldwide have plummeted by more than 80 percent since 1970. Habitat fragmentation, overfishing and pollution have taken a devastating toll on species essential to aquatic ecosystems and human livelihoods.

Now though, global swimways for migratory fish are emerging as a vital framework for maintaining freshwater ecosystem connectivity.

“Fish are the most heavily impacted of all migratory species globally,” says Twan Stoffers, a fish ecologist at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin. “Because their habitats and migratory paths are underwater, we often don’t know where these routes are, or even if all species can still use them.”

So researchers such as Stoffers are working to create detailed maps of species-specific migration routes in rivers worldwide, identifying pathways that migratory fish depend on for their survival. These maps are designed to guide sustainable infrastructure development, conservation planning and habitat restoration by highlighting crucial corridors and bottlenecks.

“There are still tremendous gaps in knowledge regarding fish migrations, particularly for species in less-studied parts of the world like the tropics and across much of the Southern Hemisphere,” says Michele Thieme, deputy director of freshwater for World Wildlife Fund U.S., who is part of the endeavor.

The greatest obstacle to preserving swimways? Dams. These barriers block breeding and feeding habitats, disrupting migration routes. While dam construction has virtually stopped in the United States and most of Europe, it has surged in Southeast Asia, with hundreds built throughout the Mekong River system over the past two decades. Home to the world’s largest inland fishery, the Mekong sustains tens of millions of people’s livelihoods and over 1,000 fish species, with migratory fish possibly comprising up to 70 percent of the catch.

One highly migratory species is the Mekong giant catfish (Pangasianodon gigas), which has suffered sharp population declines as dams block access to its breeding grounds. Some fear it may be on the brink of extinction. Yet moments of hope remain. In Cambodia, conservationists recently tagged and released 18 giant catfish in a single month, a record high number. Events such as this suggest that, despite the pressures on the Mekong system, migratory fish such as giant catfish still make use of their natural pathways.

“The more comprehensively and more quickly we can identify Mekong swimways for these fish, the better solutions we can find to protect them,” says Zeb Hogan, a fish biologist at the University of Nevada, Reno, who leads the U.S. Agency for International Development–funded research project Wonders of the Mekong.

Globally, only about one-third of the world’s major rivers remain free-flowing, with most located in the Amazon and other remote regions. In Europe, the most dammed continent in terms of rivers being blocked, rivers are so fragmented that identifying historical swimways is almost impossible. Restoration efforts elsewhere, however, have shown what’s possible.

On the Klamath River in the northwestern United States, the removal of four dams, completed in 2024, allowed salmon to return to spawning grounds they hadn’t accessed in over a century. Such successes highlight both the resilience of migratory fish and their river habitats.

The concept of global swimways first gained traction at a 2020 summit organized by Herman Wanningen, a Dutch fish ecologist and founder of the World Fish Migration Foundation. The Global Swimways Initiative, which has been underway since November 2023, is analyzing data for all the roughly 2,400 migratory freshwater fish species — about 13 percent of the 18,000 freshwater fish species that have been described globally so far. Rather than tagging and tracking fish themselves, which is quite costly, the scientists are synthesizing existing data from sources such as Fishbase and the International Union for Conservation of Nature Red List of Threatened Species, along with studies that have already collected relevant information.

“I’m happy with the amount of information we could find,” Stoffers says, “but I’m also shocked at how many species we still have little information for.” He notes that technology for tracking freshwater fish migrations still lags far behind that used to study birds.

Researchers also point out that rivers are often misunderstood as simple, linear systems when their reality is far more complex. They connect laterally to floodplains, have depth and change seasonally, creating dynamic, multidimensional ecosystems.

“From a person’s perspective, a fish moves upstream or downstream. But from a fish’s perspective, it’s moving between habitats, often under specific environmental conditions,” Hogan says. “The swimway concept is trying to show that not all fish migrations are created equal.”

The Amazon Basin exemplifies this complexity, with its immense flooded forests and wetlands that serve as crucial habitats during the rainy season. Many rivers also span multiple countries, making international cooperation essential to keeping migration routes open and accessible. “Fish and their migrations don’t pay any attention to borders,” Thieme says.

Just as migratory flyways revolutionized bird conservation, global swimways could redefine how humanity approaches river management and protection, the researchers argue.

“We need to take our understanding of fish migration to the next level,” Hogan says, emphasizing the importance of addressing fish migration with the same nuance and attention given to birds. “Mapping these underwater highways is essential to ensuring the survival of freshwater fish and the ecosystems and communities that depend on them.”