These animals can cause big trouble. Why are states unleashing them by the millions?

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Introduced species can wreak havoc on native ecosystems. Many states are flooding their waterways with them.

When animals that aren’t native to an area harm the environment, we usually label them as invasive and consider them bad. State wildlife agencies spend tens of millions of dollars a year trying to eliminate them.

That makes this fact peculiar: Those same agencies also regularly and purposefully release nonnative fish into the environment that, in many cases, damage local ecosystems.

The reason for this apparent contradiction is that anglers everywhere want something nice to catch. Many US streams, ponds, and lakes no longer support healthy native fish populations, or never did. Without flooding them with brown trout, rainbow trout, largemouth bass, and a whole host of other nonnative species, there wouldn’t be much to fish.

A more complex explanation is money: The very revenue streams that fund state conservation come in part from selling fishing licenses. Stocking nonnative fish helps states sell more of them.

But along with those benefits, stocking local waters with nonnative species comes at an under-appreciated environmental cost, several scientists and wildlife advocates told me. The practice is ironic for publicly funded agencies charged with protecting native wildlife and biodiversity, they said.

The research on this is hard to parse. The worst, original impacts of releasing these animals occurred a long time ago in most places, and states — which also want to meet the needs of anglers — are far more careful when they stock streams and lakes today. What it clearly reveals, however, is a deeper problem facing conservation in the US, rooted, at least in part, in a funding model lodged in the past.

The strange history of fish stocking

Many of the most infamous invasive species in the United States arrived or spread accidentally, such as zebra mussels, spotted lanternflies, and Burmese pythons.

Do you have a story tip or feedback on our reporting? Reach out to benji.jones@vox.com.

That has not been the case with fish.

In the late 1800s, as we built dams and polluted waterways, native fish began disappearing from streams and lakes across the country. So the federal government — and later state wildlife agencies — began raising fish in hatcheries and dumping them into waterbodies.

Many of those fish were brought in from other states or even other countries; they were nonnative. In those days, some trains actually had “fish cars” that transported tanks of fry from coast to coast. To deliver them to mountain lakes, pack mules and horses would often carry them in milk cans or barrels.

Florida has become a zoo. A literal zoo.

This wasn’t scandalous. At the time, there wasn’t much awareness around invasive species or that you might not want to unleash foreign animals into a landscape. The late 19th and early 20th centuries were notorious golden years of intentional (and later regretted) species introductions — that’s when European starlings were brought to New York City, for example, and cane toads were released in Australia. Plus, it was far easier to fill a stream with hardy nonnative fish than to fix the underlying environmental problems that endangered the local fishery in the first place.

Fast-forward to today and, of course, we know a lot more about the impacts of invasive species, which are now considered one of the leading drivers of extinction. Stocking looks a lot different, too. Officials no longer use trains and mules to transport fish but specialized trucks, planes, and helicopters.

What hasn’t changed is that states are still stocking streams and lakes with millions and millions of nonnative fish.

What happens when the fish are unleashed

The most obvious impacts of these fish stocking programs are in mountain lakes — many of which never had fish to begin with, until we brought in trout, said Angela Strecker, a freshwater ecologist at Western Washington University.

“In lakes, we know that the consequences are quite dramatic,” Strecker said.

Introduced trout become apex predators in these ecosystems and gobble up native tadpoles, salamanders, and insects. Research shows that in the western US, trout released in alpine lakes has harmed numerous native amphibian species, including frogs and salamanders. And the consequences of these introductions don’t stop at the water’s edge, Strecker added. Birds and other land animals that eat insects can lose a crucial source of food in lakes where fish have been dropped in. As authors of a recent study put it, “predatory fish introductions fundamentally restructure alpine lake food webs.”

In regions that do have native fish, such as streams, stocking with nonnative species can push out those local varieties, said Alex Alexiades, a fisheries biologist at Heritage University. “If you introduce a predator fish, it’s going to displace local fish,” he said. On the East Coast, for example, releasing brown trout and rainbow trout — which are native to other continents and the US West Coast, respectively — can push out the native brook trout, he said. Hatchery-raised fish tend to be more aggressive and better at fighting for limited resources, Alexiades told me.

“The science is pretty clear,” said Helen Neville, a senior scientist at Trout Unlimited, a large nonprofit, founded by anglers, that works to conserve streams where native trout live. “There’s a limited amount of habitat and resources,” she said. “When fish have to compete for those resources and spend a lot of their time on aggression, or they are pushed into more marginal habitats because of the presence of another species, that is seen as generally pretty negative across the board.”

Neville also pointed out the problem of hybridization — when nonnative fish breed with the local, native population. Their offspring are hybrid and have traits from both parents. And over time, that can eliminate the genetic lineage of fish that have evolved over millions of years to their local environment. “For some of these [native fish] species, hybridization is the path to extinction,” said Andrew Rypel, a freshwater ecologist at Auburn University. One of the biggest threats to a Texas fish called the Guadalupe bass, for example, is hybridization from stocked nonnative smallmouth bass, Rypel said.

Other consequences are more complex. There’s some research, for example, that suggests that releasing nonnative fish has helped spread other invasive species — a phenomenon dubbed “invasion meltdown.” Nonnative sunfish that states have stocked in the Western US, for example, prey on dragonfly larvae that would otherwise eat the tadpoles of bullfrogs. Bullfrogs are invasive in that part of the country and eat their way through populations of native species, and stocked sunfish may be helping them spread.

The US has a bullfrog problem

Some environmental activists have highlighted these consequences in an effort to restrict or put an end to nonnative stocking practices. Earlier this year, for example, a small nonprofit in Massachusetts launched a campaign to stop routine stocking of nonnative fish in the state. Massachusetts is spending public resources on a program that endangers local ecosystems for the benefit of only a small population (i.e., anglers), Brittany Ebeling, the group’s executive director, told Vox. (Those resources largely come from people who fish, whether or not they want to catch native fish.)

But even more mainstream environmental groups that count anglers as their members have expressed concerns about stocking nonnative fish. Fly Fishers International, a nonprofit that advocates for fly fishing, “acknowledges the recreational value many introduced populations provide to the angling public, but also emphasizes the damage caused to native fish communities by the introductions,” the group says. “FFI is concerned about the continued erosion of the genetic integrity of existing native fish populations.”

Trout Unlimited doesn’t have a formal position on stocking nonnative fish, said Mark Taylor, a spokesperson, though it’s currently developing one. Taylor says the group’s “default position is native populations.”

States have all kinds of environmental problems to contend with, from drought and other impacts of climate change to wildlife conflict with ranchers. Why, then, are they continuing to flood their waters with introduced fish, potentially fueling another one?

I reached out to wildlife agencies in all 50 states to ask about this, and more than half of them responded. Nearly every state I heard from stocks nonnative species for recreational fishing — mostly trout and bass — revealing how incredibly common this practice still is.

Several states emphasized that the nonnative fish they stock are not, in fact, invasive species. That distinction is important. By most common definitions, a nonnative species becomes invasive if it does damage to humans or local ecosystems — but the line is often blurry. Honeybees are not native to the US and harm native bees, a highly threatened animal group, but we usually don’t consider them invasive species. Like stocked nonnative fish, honeybees are valuable to human industries and the damage they cause is largely invisible to the public. This is to say: Whether or not we consider animals invasive often comes down to what we value.

What we get wrong about saving the bees

Another key point that states make is that, in many cases, the nonnative species they’re releasing have already been living in local streams and lakes for decades — they’re “naturalized,” as some states put it. In the 1960s, for example, Michigan stocked chinook salmon in Lake Michigan, partly as a way to control alewives, another invasive fish. Now most of the chinook, native to the Northwest and Alaska, are breeding on their own, said Ed Eisch, assistant chief of fisheries at the state’s wildlife agency. States commonly also stock imperiled native fish in an effort to help restore ecosystems.

In other cases, the waterbodies that states stock are humanmade, such as ponds, or have been dramatically altered by things like dams, pollution, or climate change. That means few native animals can survive in them anyway. In other words, the damage to the environment has already been done. What’s a bucket of foreign fish gonna do, especially if anglers will remove most of them anyway?

“Connecticut’s watersheds and fish communities are irreversibly altered by hundreds of years of anthropogenic impacts,” the state’s Department of Energy and Environmental Protection, told me. The environment has changed so much, the agency said, that “it can no longer support fisheries for some native fishes, notably Atlantic Salmon.”

North Dakota Game and Fish, meanwhile, told me that most of the state’s fishing areas didn’t even exist 75 years ago. “The damming of rivers and streams created large reservoirs that are foundational to the state’s recreational fishery,” said Greg Power, the fisheries division chief. Along with an increase in rain, he said, that’s multiplied the number of fishable waters from 23 in 1951 to 450 today.

The problem with the “it’s already broken” thinking is that it doesn’t address the underlying environmental problems that drove native fish out in the first place — some of which are getting worse. Rising global temperatures, for example, are making it hard for certain native trout, which love cold water, to survive in their home ranges.

If nothing else, it seems clear that in most cases stocking nonnative fish is not helping native wildlife populations recover. So while the decline of native aquatic life may be rooted in the past — in the so-called sins of our fathers — continuing to actively stock local waters may be contributing to a lower baseline level of biodiversity.

The other, deeper driver behind the widespread practice of stocking nonnative fish has more to do with how state wildlife agencies are funded.

Recreational fishing is an enormous industry in the US, contributing some $230 billion to the economy each year, according to the American Sportfishing Association, an industry group. Because waters across the country are so degraded, stocking nonnative fish keeps this industry afloat.

That’s obviously good for anglers — especially if they don’t care whether or not their catch is native. But it keeps state wildlife agencies afloat, too. This is key: On average, more than half of the revenue of these agencies comes from selling hunting and fishing licenses, along with federal funds that are distributed based partly on the number of licensed anglers in the state.

This funding model creates an “incentive for agencies to maximize fishing participation,” said Mandy Culbertson, a spokesperson for Wildlife for All, an environmental advocacy group. The more people who fish, the more revenue state wildlife agencies reel in. And this makes sense — activities like hunting and fishing that rely on the environment should support the agencies charged with conserving it.

But as Wildlife for All sees it, that incentive distorts the priorities of those public organizations. For too long, the group says, state wildlife agencies have prioritized maintaining game animals, such as nonnative fish that people catch, over conserving the full diversity of wildlife within their borders, much of which is now threatened. Culbertson also says that some of the money raised by fishing goes towards activities that “are not connected to conservation at all,” including increasing boating access and safety.

In statements to Vox, some states acknowledged challenges with how funding for state conservation works. “Unfortunately, the funding model that we rely on to manage native and non-native fish alike is based on the recreation of sportsmen and women that direct their time and money in the pursuit of relatively few species of fish,” said Jason Henegar, the fisheries division chief at the Tennessee Wildlife Resources Agency.

But Henegar and other state officials told me that this funding is also what allows their agencies to conserve native species, such as by restoring plants along a stream that help limit erosion and keep water cooler. The Tennessee Wildlife Resources Agency, for example, has a division dedicated to conserving non-game wildlife including rare, important, or sensitive fishes, Henegar said. “Tennessee anglers — many of whom fish for stocked non-native species — support this important biodiversity work through their purchase of the Agency’s licenses and permits,” he told me.

Several states, including California and Utah, also repeatedly stressed that the way they stock nonnative fish today is nothing like it was in the past. The California Department of Fish and Wildlife, for example, said it does stock some brown trout, which are native to parts of Europe, Asia, and Africa, but all of them are bred to be sterile, meaning they can’t reproduce after being released. “Many game species are also native species, so funding we get to support native sport fish is also used for conservation,” the agency told Vox.

So, it’s complicated.

Under the existing funding structures, filling waterways with nonnative fish — some of which may or may not be invasive — helps conserve native species. But it also maintains, or possibly worsens, a broken ecosystem.

The obvious solution, said Michelle Lute, the executive director of Wildlife for All, is for state wildlife agencies to diversify their sources of income. Those agencies have long seen hunters and anglers as their only customers, Lute said, but there are all kinds of different outdoor users, such as rock climbers and birdwatchers. Those other users almost certainly care more about a healthy environment than about stocked fish, she said; the challenge is in turning those values into revenue for states.

Some states are making progress. In Oregon, for example, lawmakers recently voted to increase the state’s lodging tax and use the proceeds for wildlife conservation. Colorado, meanwhile, is raising money by selling special license plates to pay for efforts to minimize conflicts between ranchers and newly introduced wolves. Some lawmakers have also floated the idea of a “backpack tax” — taxing outdoor gear, like we do guns and fishing rods, and putting that money towards environmental agencies.

These funding approaches can help realign the priorities within wildlife agencies to better support native species and the values of their constituents outside the hunting and angling community, Lute said.

“Increased funding for conservation is good for everybody,” she told me. “Whether or not you wildlife-watch or hunt or fish, everyone benefits from a healthy ecosystem.”

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