It’s a warm May evening in northern Minnesota. As the sky darkens and twilight settles in, a man driving a pickup truck slows and then comes to a stop where a culvert crosses under the road. Hopping out, he switches on his headlamp, peering down at the cold and clear water gurgling in the gravel streambed below. A quarter-mile downstream, the babbling brook opens into a small jack pine–lined lake, one of the man’s favorite fishing spots. He opens the tailgate and grabs a cooler that quietly hums from a battery-operated aerator attached to the lid.

The hum grows louder as the man opens the cooler, carries it over to the culvert, and pitches the water and a dozen black crappies into the stream below. He likes to catch the lake’s stocked rainbow trout with family and friends, but the addition of crappie, he thinks, will shorten the time between bites and provide something else to fish for throughout the year. His mission complete, he heads for home as the fish disperse downstream. In the next three years, he gets more than he bargained for, as the lake’s fish community is turned upside down. The introduced black crappies spawn and fill the lake with tiny young fish. Rainbow trout numbers decline, and eventually the fishery loses value and needs a costly reclamation process. The simple act of one man stocking a dozen fish from the side of the road directly led to the collapse of the very fishery the man was trying to improve.

This is a fictional scenario, but it’s not an implausible one: Minnesota has seen numerous cases where “bucket biologists” like this angler have introduced a fish species to a lake, doing it on the sly because they know the act is illegal. Other illicit releases are born less of deviousness than of ignorance—for example, anglers who pitch their leftover fishing bait into the lake or aquarium owners who dump their unwanted goldfish into a neighborhood pond.

Regardless of motive, signs of illegal fish stocking have turned up in every corner of the state, according to Minnesota Department of Natural Resources Fisheries supervisors and staff, sometimes with far-reaching effects that can be difficult and costly to reverse.

“Minnesotans are passionate about their fishing and fish,” says Assistant Regional Manager Brian Schultz. “But we do need everyone to understand that illegally moving fish into places they don’t belong can really harm Minnesota’s waters.”

Illegal stocking can upset the delicate balance of existing fish communities, spread fish disease, and bring other unintended consequences that can linger for years. Fish stocking, it turns out, is best left to the experts.

Carefully Planned Stocking

Because of the stakes involved, fish stocking is a highly regulated activity. The DNR has the authority to stock fish in the state’s public waters, sometimes working with neighboring states or federal agencies on border and shared waters. Tribal natural resource managers oversee stocking in waters under their jurisdiction and cooperate with the DNR on shared waters. Private lakes or ponds can be stocked only if permission is requested and granted by the DNR.

Legal fish stocking is done for a variety of reasons. Sometimes it’s done to supplement existing natural reproduction if natural reproduction alone is not sufficient or where spawning habitat is limited. Some stocking is done to reestablish populations, either in long-term projects returning fish from extirpation, like Minnesota’s efforts to restore lake sturgeon, or in short-term recoveries like the aftermath of a severe winterkill. And some stocking is to provide a fishing opportunity, from panfish for the family-oriented “Fishing in the Neighborhood” urban pond program to muskellunge in a small set of waters for trophy fishing.

In reviewing, approving, and executing stocking activities, the DNR operates according to well-established science and with careful planning. Stocking is often a part of lake and stream management plans created by biologists and managers using the best available data and the most up-to-date fisheries science. Stocked species are chosen according to available habitat and conditions, angler interest and desires, and local genetic strain. Numbers and life stage—small fry, fingerlings, or juveniles, for instance—are matched to the resource. Strategy and results are reviewed over time.

The lone twilight stocker with his cooler of fish doesn’t have any such resources, plans, or safeguards. Examples from waters across Minnesota show the damage that can be done when fish are illegally introduced. 

Vulnerable Fisheries

Some lakes are more sensitive than others to disruption by fish introduction. Stream trout lakes, where the DNR maintains stocked populations of trout for recreational angling, are especially vulnerable. 

“Stream trout lakes are a concern for us,” says DNR Finland Area Fisheries Supervisor Dean Paron. “We spend a lot of time and money to make these lakes for the most part fishless besides the trout. We know illegal stocking occurs, but we never observe it firsthand.” 

Hogback Lake in Paron’s Lake County area is one such lake, stocked by the DNR with rainbow trout and splake, a hybrid of lake trout and brook trout—and more recently stocked illicitly with problematic yellow perch. The waters of Hogback Lake now produce a lot of very small, slow-growing perch that compete with the rainbows and splake. 

“It’s been an inconvenience to the anglers,” says Paron, “as shore anglers can’t catch splake or rainbows because you can’t keep the small perch off your line.” If someone thought they were going to turn Hogback into a big perch factory, they were sorely mistaken. Paron can’t net the perch out and he can’t stock a predator to eat them, as any such predator would also feed on the trout. With no great solutions, Paron and his team will continue to monitor the fishery until it is time for a reclamation.

Eventually, introduced species can take over in designated trout lakes and the lakes must be reclaimed, an expensive and time-consuming process in which the lake’s fish are killed off with the chemical rotenone. Such reclamations require fleet, materials, and personnel. Reclaiming a 150-acre lake that averages 10 feet in depth would cost nearly $75,000 just for the rotenone, according to DNR Shoreland Habitat Manager John Hiebert. After a reclamation, anglers lose an entire year waiting for a lake to come back and be restocked.

Some fisheries are forever changed by new fish introductions. One is Grindstone Lake in Pine County, a rare deep lake in central Minnesota managed for lake trout. Grindstone’s lake trout population had long been self-sustaining—that is, until rainbow smelt were detected in 1963, possibly introduced as adults by an overzealous smelt netter or as viable eggs by someone cleaning buckets. However the smelt were introduced, they took off. The smelt displaced native cisco, a forage food for lake trout, and the change in diet caused the trout to produce eggs with low thiamine concentrations, delivering a big hit to natural reproduction. Natural lake trout stocks crashed, and today the population exists entirely on stocking.

At Grindstone Lake, the introduction of rainbow smelt led to the complete rearrangement of the coldwater fish community, adding tremendous costs to maintaining a lake trout population that was once entirely self-sufficient.

Illegal stocking isn’t a problem just in Minnesota; it happens across the country. In Yellowstone National Park, lake trout were illegally introduced in the late 1980s or early 1990s. As of November 2019, the National Park Service had spent $20 million over 25 years to remove more than 3.4 million lakers in efforts to protect and restore the native Yellowstone cutthroat trout. 

Elsewhere, northern snakeheads from Asia have prospered in parts of the Potomac River, and in the early 1950s, walleyes were illegally introduced in Washington state’s Lake Roosevelt. The descendants of these walleyes have thrived and made their way down the Columbia River, where they prey on native salmon smolts returning to the Pacific Ocean. Coast to coast, illegal stocking has serious impacts.

Fish Dumping Gone Wrong

Bait buckets and home fishbowls or aquariums are two other sources of new species introductions that can go awry.

Sometimes fish are introduced by pet owners who have fish that they no longer want to care for or that have outgrown their tanks. Ornamental or pet trade fish that have been found in Minnesota waters include goldfish, koi, pacu, and piranha.

A few goldfish might seem to some like a harmless addition to the local water body—but they’re not. Goldfish are in the minnow family and can work their way through city stormwater ponds and into lakes and streams downstream with big impacts, by rapidly reproducing, surviving harsh winters, and feeding in and stirring up the bottom like their close relatives, the common carp.

“It can be a pain to figure out what to do with a goldfish that you don’t want,” says city of Eagan water resources specialist Jessie Koehle—but she has experienced firsthand the problem with releasing them into the wild.

Several years ago, someone plopped goldfish into a pond at Eagan’s Central Park and the population spun out of control, muddying the pond’s waters and harming native plants. After two or three years of attempted goldfish removals and alum treatments, the fish remained, most of them 6 inches or less. Getting the orange invaders out took a lot of effort and expense. “We tried netting them out,” Koehle says, “and we got thousands of them, but we couldn’t get them all. Eventually we had to use rotenone to reclaim the pond, killing all the fish and starting over.” The pond’s water is again clear and native plants are thriving, but the whole episode could have been avoided if the goldfish owner had taken more care in disposing of the fish.

Illegally introduced goldfish are causing an even bigger problem in the Grace Chain of Lakes in Carver County, because rather than just one pond, they have spread through the entire lake chain, which ultimately connects to the Minnesota River—and there may be hundreds of thousands of them. The Carver County Water Management Organization has contracted with WSB Engineering to research the problem and offer potential solutions. Last fall they netted 700 goldfish at the outlet of Lake Hazeltine, tagging 100 of them in an effort to track their movements. Their goal is ultimately to tag 500 fish and create a strategy to lower goldfish populations to the point where they’re not harming lake ecology.

Like aquarium owners, anglers who use live bait need to exercise caution. While commercial bait trappers and retail bait outlets do their best to screen minnows approved for use as bait, other species sometimes get mixed in with scoops of fathead minnows, golden shiners, and redbelly dace. Some of these species can compete directly or indirectly with other fish already present in a water body. That’s why anglers should always dispose of unused bait by throwing it in the trash instead of releasing it into a lake or stream.

The Pathogen Problem

Whenever outside fish are introduced into a water body, disease-causing pathogens may also be introduced, says Nick Phelps, an assistant professor at the University of Minnesota and director of the Minnesota Aquatic Invasive Species Research Center at the U. Phelps and his lab study such pathogen risks, and their research validates that anglers need to be careful in disposing of their unused bait. In a study in conjunction with the DNR, researchers tested golden shiners from 53 different bait shops  around the state. They found bacteria and parasites of concern, including Aeromonas salmonicida and Yersinia ruckeri, and eight new viruses that had never been described before. More work is needed to understand the risks these newfound viruses pose to fish in Minnesota. 

“When people release their bait into the wild, they are basically seeding that population and we don’t know what will ultimately happen. That’s the uncertainty part,” says Phelps.

“If we don’t know what the problem is, the problem could be happening and we just don’t know until it’s too late. There are viruses, bacteria, and parasites that could get inadvertently moved if people aren’t following the rules and safety protocols in place.”

Phelps and his fellow researchers recognize that bait sales, fish stocking, and the aquarium fish market are important to the state’s people and its economy—but also that fish pathogens could harm those activities along with treasured natural resources.

When the DNR stocks walleye, Phelps notes, it screens the hatchery fish for pathogens of concern. “They know what they are worried about and they look for it,” he says.

However, when illegal stocking occurs, there are no such protections and even more potential pathogens. “The consequences could be huge,” he says. “Once a disease gets into a system, it’s impossible to get it back out, more difficult than getting the fish itself out. The disease can persist in the environment for a long, long time.”

Stocking Trial and Error

Perhaps there is no better teacher than experience and the lessons learned in error. Fish stocking by government agencies before and around the turn of the 20th century brought an environmental philosophy that placed fish stocking on a pedestal. Some stocking was good and more was better; it was the means to recreational and commercial fishing prosperity. Fish stocking was done by the Minnesota Fish Commission, a precursor of the Department of Conservation and the modern Department of Natural Resources. Little to no scientific or technical training was applied to the commission’s stocking practices; American fisheries science was in its infancy.

From 1880 to 1890, the commission stocked common carp across lakes and rivers in Minnesota, an unfortunate introduction of an ecosystem-altering invasive species still problematic today on many of Minnesota’s waters. Atlantic and Pacific salmon were widely distributed until 1885; their long-term presence was thwarted by basic habitat requirements like water temperature. The commission sought to enhance nature’s offerings and frequently did so in partnership with the Northern Pacific Railway, dumping fish from rail cars at water crossings along railways stretching across the new state. Unplanned and poorly researched with little follow-up evaluation, most of these stockings either failed outright, had unknown outcomes, or, in the case of the carp, brought about significant ecological damage.

We’ve come a long way since then. Modern fisheries managers now have a long baseline of knowledge and outcomes, along with modern science and new technology, to guide their stocking decisions. These days, Minnesota’s fish stocking is done carefully, deliberately, and with long-term monitoring to protect the waters that we love. There are many reasons to avoid moving fish or dumping bait—more than 10,000 of them in fact.