Fishing is a way of life for people who live on the floating villages of Cambodia’s Tonle Sap Lake.

These Southeast Asians depend on fish that migrate in and out of the lake. And how many are there depend on the time of year.

The wet season runs from May to October. Rains rush off land in the Mekong River basin to the Tonle Sap River and lake. This raises levels in much of the lake — the largest in Southeast Asia. Starting around one meter (3 feet) deep, it can swiftly rise up to 10 meters (30 feet) deep. That’s like going from a shallow pool to enough water to cover a two-story building.

With all that water come vast numbers of fish. Once in the lake, many of these fish spawn. Near-shore places with lots of plants will shelter the young hatchlings from predators.

Later, when the dry season comes, the river reverses its flow. Lake levels now drop. Levels of oxygen in the water dip, too. This sends many of the fish heading back up river.

It’s a cycle that repeats, year after year.

Millions of Cambodians rely on the wet season. “They depend on the fish to eat,” says Mathieu Chevalier. He studies aquatic ecosystems at Ifremer. It’s a national institute for ocean science and technology in Brest, France.

But those fish are in trouble.

Overall, their numbers in Tonle Sap Lake have been falling dramatically. Compared to how many were there in 2003, just 12 percent as many could be found in 2019. Chevalier was part of a team that shared those data, two years ago, in Science of the Total Environment.

“If the trend continues,” he says, “there will be nothing [for people there] to eat any more.”

Such changes are not just happening in Southeast Asia. Across the planet, freshwater fish are facing multiple threats to their existence.

Dams are a big problem, Chevalier says. These structures break up river systems. They also reduce flooding, like the type that brings fish to Tonle Sap Lake each year. As people keep fishing, the share of fish left there could dwindle even more.

Pollution threatens freshwater ecosystems, too. People have also been changing the land in ways that degrade the habitat in lakes and streams. With growing human populations, overfishing becomes more of a threat. And then there’s a growing influx of invasive species: Many now disrupt ecosystems that once nourished native fish. Lastly, global warming has been taking its toll by stressing both fish and their prey.

Today, some one in four freshwater fish species are at risk of going extinct. That’s the finding of a December 2023 report by the International Union for the Conservation of Nature.

“We’re losing a lot of species,” says David Strayer. “And,” he adds, “we’re in danger of losing a whole bunch more.” A freshwater ecologist, Strayer is based in Ann Arbor, Mich. He’s long been a researcher for the Cary Institute of Ecosystem Studies (based in Millbrook, N.Y.). Strayer summed up the threats in his 2024 book: Beyond the Sea — The Hidden Life in Lakes, Streams, and Wetlands.

Still, the situation isn’t hopeless. People pose many of today’s threats to freshwater fish. So people also can act to address them. Researchers are working to identify the most effective ways to do that. Here are some of their new findings.

Get smarter about dams

People build dams to store water for later use. These systems also can harness the flow of water to generate electricity.

The Mekong River basin around Tonle Sap Lake has hundreds of dams. Plans to add hundreds more would bring the total dam count to more than 1,000. An international group shared these data in March 2024. Hundreds of hydroelectric dams are also planned for South America, Africa and elsewhere.

But those dams can pose lots of problems for animals, a December 2023 report found. Dams make it hard for fish to migrate. They can slow water flow. That may mess with cues fish look for when it’s time to spawn. And it can make it harder for young fish to survive: The eggs may sink — or hatch in the wrong place.

But it’s possible to limit these risks, Strayer says. One way: Remove outdated or ill-placed dams. The largest such U.S. project took down four hydropower dams from the lower Klamath River. It runs through Oregon and California. Those dam removals opened up 676 kilometers (420 miles) of river habitat for the first time in more than a century. Less than two weeks after work ended in late 2024, thousands of salmon were swimming upstream through this newly opened stretch.

Elsewhere, dam operators can reduce harm by changing river flows. Limiting flow too much can cause downstream areas to dry up. “You don’t need a PhD to know a dry riverbed is bad for fish,” Strayer says. Instead, “we can make sure … to at least keep the stream bed wet enough for fish to live in.”

A recent analysis looked at some Chinese dams along the upper Mekong River. Dams can block sediment. Less silt has been flowing to the Mekong’s mouth, shrinking the delta — where millions of people live. To reverse that trend, a team of scientists recommends letting more water and sediment through Mekong dams. As a side benefit, Tonle Sap might see more water flow, too, the team noted in the May 2024 Science Advances. 

Smarter dam placement would also help. Alex Flecker is a freshwater ecologist at Cornell University in New York. Many new hydropower dams are planned in South America’s Amazon River basin. He was part of a team that recommended sites for them that might minimize their harm.

Siting plans often consider such things as a dam’s cost, how much electricity it’ll produce and whether that power can get to where it’s needed. Flecker’s team wanted to consider environmental factors, too — such as how best to protect aquatic species. Other goals: to keep parts of the river connected and sediment moving through.

Using a computer to model different plans really helps here, his team found. These models can identify dam sites posing the worst environmental threats. And, depending on the environmental goals, the models point to which sites should work best.

Modeling works best when considering large areas, such as a whole river basin (not just small segments of a river), Flecker says.

Protect and restore habitats

How people use the land can greatly affect what happens to water flowing through its streams and lakes. Roads, buildings and other land changes can alter shorelines. And then there’s the risk of pollution, fertilizer and sediment running off of farms and city streets. All of these risk harming fish.

John Gray is a fish biologist at the University of British Columbia in Vancouver, Canada. He studies a small fish called the Nooksack dace. This rare minnow lives in just a few parts of British Columbia and Washington State. Growing to about 15 centimeters (6 inches) in length, it likes to hang out in riffles. These are parts of streams with rocky or uneven riverbeds.

Over time, people have turned the land around many streams into farms. They’ve made other changes, too. Rains now wash sand and sediment into the water. This has filled in nooks and crannies, taking away many fish hangouts.

Gray and others recently tested whether restoring stream habitat might aid the Nooksack dace. They added larger river rocks — cobble — to parts of two creeks. Other sites got smaller gravel. Still other places got a mix of the two — or no changes.

A year later, more Nooksack dace showed up in study areas treated with cobble than in untreated areas. Spaces between the cobble provided more nooks for little fish to hide. Fish populations dropped, however, where the team had added more gravel and sand.

The group shared its findings last December in Facets.

These data show that restoring habitat can help — if done right. But this work costs money. And it’s hard to restore entire streams, Gray says. So, it’s better to avoid excess runoff in the first place. He argues that “the cheapest habitat is the one you leave alone.”

Keep out alien invaders!

Sometimes, habitat isn’t the problem. It’s strangers that have moved into the neighborhood.

In South Africa, ecologist Mohammed Kajee studies where fish species live across the country. Invasive species are species that never naturally lived at some site. As newcomers, these invaders may compete with native species for food. They’ll definitely compete for habitat — such as good spots for laying eggs or hiding from predators. Often, the alien invaders have few or no natural predators in their new neighborhood.

Kajee works at the Freshwater Research Centre and is a graduate student at the University of Cape Town. He’s also been part of a team that mapping biodiversity in freshwater ecosystems across South Africa. It’s identifying where different fish species live.

“Wherever you find invasive species, you also see a decrease in the population size of the native species,” Kajee now reports.

That’s especially worrying for species already at risk of extinction. More than half such species could be found in places with non-native fish. Those invaders can make matters worse for already-at-risk species.

“If we do not deal with the invasive species, then many of South Africa’s native species are at risk of going extinct,” Kajee says.

In the United States, invasive carp show how non-native fish can cause long-lasting problems. People deliberately brought bighead, black, grass and silver carp into some southern states in the 1960s and 1970s. By the 1990s, these species had spread through the Mississippi River basin.

“They are competing with many of the freshwater fishes,” says Mamie Parker. This fisheries biologist is a consulting scientist in Washington, D.C. She used to work with the U.S. Fish and Wildlife Service. The alien carp gobble up much of the food that native fish need. And the adult carp have few predators.

One big goal has been to keep the carp from moving into the Great Lakes. A large canal in Chicago links the Mississippi River system to the Great Lakes. Electric barriers near the canal try to keep carp from passing through. Sounds, bubbles and lights also aim to deter the invaders. But holding them in check is an ongoing challenge.

A success story

The fight against one invasive fish — the sea lamprey — highlights a major success.

During the early 20th century, these bloodsuckers moved through a canal from Lake Ontario into other Great Lakes. By mid-century, populations of many native fish were crashing. A decades-long battle against the lampreys used traps, odor repellents and barriers. People also released chemicals that kept young lampreys (but no other fish) from becoming adults that could reproduce.

In most areas, the numbers of sea lampreys now is just one-tenth of what it had been at its height. That’s according to the Great Lakes Fishery Commission. And new research confirms the controls are working — but also that they must keep going.

Benjamin Marcy-Quay likens this to “more like keeping the lawn mowed, rather than cutting down a tree.” This fish biologist in Millersburg, Mich., works for the U.S. Geological Survey’s Great Lakes Science Center.

During the Covid-19 pandemic, travel bans in 2020 and 2021 limited sea-lamprey-control work in Lake Ontario. Afterward, “we saw sea-lamprey populations bounce back like a coiled spring just waiting to be released,” Marcy-Quay says. Since then, renewed control work has been lowering numbers again.

He and others shared the news on March 25 in Fisheries. 

But controlling invaders is only part of the work.

Other efforts have been helping native species recover. Fisheries workers collect the eggs of lake trout, for instance, to raise in hatcheries. When they’re old enough, these fish will be released into the lakes.

In November 2024, the Great Lakes Fishery Commission said lake trout numbers are finally high enough that people no longer need to restock them in Lake Superior. It’s the largest of the five Great Lakes. And, Parker notes, populations in the other four are improving.

Ongoing challenges

Populations of freshwater fish usually face more than one threat. Overfishing, pollution and dams, for instance, threaten those in Tonle Sap Lake. Both invasive species and overfishing put South Africa’s native fishes at risk. And climate change is affecting waters everywhere.

“We have to think of threats as acting [together],” says Gray in British Columbia. That makes it tough to find fixes. No one-size-fits-all solution will fix most problems. But multiple approaches may help.

While upstream dams remain a problem for Tonle Sap Lake, dealing with other threats may help its fishes. Fishing limits now apply to a few areas. This could protect at least some of the fish that that stay there during the dry season. Other projects try to help struggling families earn money from work other than fishing.

In 2022, scientists tried restocking some fish species at Tonle Sap Lake. However, nearly half the fish were were caught less than three months later. Researchers reported the finding in Water, later that year. Ultimately, they wrote, people need to actively manage fishing in the lake — and work to preserve its flow and connection to the Mekong River.

With each success or failure, scientists learn how better to deal with these big problems.

“The good news is that we can control all of those kinds of impacts,” Strayer says. The challenge is whether people and governments will invest time, money and other resources to do that.

But there may not be much time to reverse or even slow losses of freshwater species. Strayer and a colleague gave that warning, last September, in Biological Reviews.

Teens and tweens can get involved, Strayer notes. Many conservation groups welcome help from people of any age. He urges kids to contact public officials. Popular support may move governments to protect freshwater fishes, he says.

“The easiest way to control a species invasion is to make sure it never happens in the first place,” Marcy-Quay adds. “You probably can’t end an invasion yourself,” he says. “But you can definitely prevent one.”

For instance: If you fish, don’t release bait that doesn’t already live in an area pond or stream. Clean your boots and boats before traveling so you don’t accidentally spread a species from one area to another. Also, “spread the word about what you’ve learned,” he says. That way “others can learn too.”

And take time to appreciate the freshwater fish that may live near you. Many face threats. But stay optimistic, Gray adds: “Sometimes it’s important to come back to your backyard — to your little creek around your home — and remind yourself that there’s hope.”