Tag, bait, scoop: Researchers explore new ways to rid Minnesota lakes of pesky common carp

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On a hot day in early July, three aluminum flat-bottomed boats cruised across Lake Fremont, about 40 miles north of the Twin Cities, toward a strange contraption near the shore.
It’s a huge, rectangular net that is attached to posts sticking out of the water. It’s baited with cracked corn — a favorite snack for common carp.
“The fish can swim in easily and feed on the bait,” said Przemek Bajer, owner of a Twin Cities-based company called Carp Solutions and a professor at the University of Minnesota.

Common carp are a common problem in Minnesota lakes. The invasive species of fish are voracious eaters that root around the bottom of the lake for food — damaging plants and habitat, while muddying the water. Some lakes in central and southern Minnesota have thousands of these destructive fish.
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Bajer is part of a team of scientists working to develop methods to control common carp on Lake Fremont, and gauge whether reducing their numbers can restore natural habitat and help native species bounce back. The researchers’ goal is to use these management tools and technology to help other Minnesota lakes keep the invasive fish in check.
“Getting rid of carp can help improve water quality, can help improve the diversity and number of aquatic plants that are here, which then helps decrease other invasive species,” said Sarah Unruh, project manager of the Lab to Lakes Initiative at the Minnesota Aquatic Invasive Species Research Center, which is leading the four-year project.
Common carp are native to eastern Europe and Asia. They were introduced into Midwest waters as a game fish in the late 1800s. They spread rapidly and are now a problem species in 48 U.S. states, including Minnesota.
They don’t get as much attention as invasive silver and bighead carp. Silver carp in particular, when agitated by boat motors, are known for jumping high out of the water and striking boaters and waterskiers.
But common carp are also invasive and can be extremely destructive, damaging the habitat needed by native species such as bass and walleye.
“They can really change the dynamics of the lake,” Unruh said. “They can get the water all churned up. They can rip up plants from the bottom, which has cascading effects.”
Those effects include releasing phosphorus, which spurs algae growth.
“There seems to be some kind of threshold where below a certain number, carp are OK. They’re not that damaging,” Unruh said. “But Lake Fremont seems to have a number of carp that are causing a lot of problems in the lake.”
Researchers have calculated that threshold at about 90 pounds per acre. An assessment in the fall of 2023 calculated Lake Fremont’s carp numbers at about seven times that much — roughly 38,000 fish.
They’re taking a toll. Lake Fremont’s shallow waters are much cloudier than clear, with an unappealing green algaeic sheen on top.

The lake is considered impaired due to nutrient pollution, but within reach of its water quality goals, said Dan Cibulka, water resource specialist at the Sherburne County Soil and Water Conservation District.
“We began asking ourselves, ‘Where are our efforts best placed to clean up the lake?’” he said.
Cibulka reached out to the Lab to Lakes Initiative, which was looking for two Minnesota lakes to try research-based common carp control strategies. Lake Fremont was chosen, along with the Chisago Chain of Lakes in Chisago County.
Cibulka said the information researchers gather will help them create a longer-term plan for managing the lake’s carp, including ongoing removal and preventing future spawning.
“It’s probably not reasonable to say that we’re going to eradicate carp from Lake Fremont,” Cibulka said. “But we can reduce them greatly, and then hopefully, keep them at that very low threshold.”
Last fall, researchers tagged more than 150 common carp from Lake Fremont with passive integrated transponders, which have a microchip inside.

The PIT tags allow researchers to track where the big, yellowish fish move, spawn and congregate, and help them decide where to place the baited nets.
An antenna powered by solar panels tracks the tagged fish. With an app on his phone, Bajer can see how many tagged carp are in each net at any time.
“The remote antenna suggests that there may be five or so tagged fish in the net,” he said. “We’ll see if that’s the case.”
Once enough carp have filled the net, the team uses a remote-controlled trigger to raise the sides and trap the invasive fish.
Four workers wearing hip waders, hard hats and protective gloves jumped into the water. They used a floating boom to corral all the fish to one side.
Then, they attached the net to a crane, hoisted it up and rolled the carp into the boat, where they flopped around in the bottom until the workers tossed them into large plastic tubs filled with water and anaesthesia. Eventually, they’ll be composted.
Other types of fish don’t particularly like cracked corn, so there’s not much risk to other species, Bajer said. But, along with the carp, the net scooped up a few bluegill and a snapping turtle, which workers tossed back into the lake.
The crew will count the carp and calculate the ratio of tagged to untagged, Bajer said. That helps them estimate the total number of carp in the lake, and how many they need to remove to return the lake to a healthy level.
“The goal is to really bring the density down to a level you can tolerate,” Bajer said. Completely eradicating common carp from a lake is difficult and expensive, he said.
After getting the carp population under control, researchers will study how the lake’s ecosystem responds. Throughout the project, they will collect and analyze water samples, and study to what extent water quality improves, and native fish and plant species rebound.

“We’re interested in how quickly we might see changes in those native species populations,” said Solomon David, an assistant professor of aquatic ecology at the University of Minnesota. “Does the vegetation come back? Does the water clarity improve? Some of those things are going to take time, but we need to start with a baseline of information somewhere.”
David said researchers will look for differences in the numbers of native fish throughout their life cycle, and in their feeding habits. Common carp often destroy habitat near the shore that’s valuable for juvenile bluegill, yellow perch, bass and walleye, as well as nongame species such as gar and bowfin, he said.
“Are they doing better in terms of their life history, the life stages?” David said. “Do we see more young native species? Do we see more older native species?”
Lake Fremont residents who've seen the harmful effects of destructive common carp are hoping the management project brings positive changes to the lake.

“At times, there’s so many of them that they would roll up on the rocks,” said Mike Thieling, president of the Lake Fremont Improvement Association, who has lived on the lake for a decade.
Thieling hopes the carp management project will bring clearer water, fewer algae blooms and more native fish.
“It’s probably not going to be an overnight cure,” he said. “But we’re hoping that in the four years, it’ll really make a difference.”