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

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........

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