Carp: Minnesota lakes’ hidden threat

They’ve been called garbage fish, bottom feeders, scum suckers, sewer bass and inland bone fish, but one thing they are almost never called is welcome.

Common carp have been in the United States since the 1800’s– long enough that the Minnesota Department of Natural Resources (DNR) doesn’t technically consider them invasive at this point. The U.S. government brought the fish over from Europe in the early 20th century as a game fish. Today, local, regional and state governments are spending millions of dollars a year to eliminate them.

“The government brought them in for immigrants because it was a popular fish in Europe,” said Lake Minnetonka Association Executive Director Eric Evenson. “People soon realized carp were fished there because it was one of a few choices. Here, you have great native fish and people don’t eat carp.”

Peter Sorenson specializes in common carp research at the University of Minnesota Aquatic Invasive Species Research Center. He works with a group of researchers to try and find better ways to manage existing populations and their effects on native species. The biggest issue with carp, according to Sorenson, is their feeding habits.

“They dig into the bottom of lakes to find food,” Sorenson said. “They can go a foot into the sediment, which no native fish can do. While they’re doing that, they uproot the native plants, which is devastating,”

Not only is that behavior directly killing off native plants, but it also destroys the habitat for native animal species, in effect disrupting the entire food chain. The Minnehaha Creek Watershed District estimates that on Halsted Bay in Lake Minnetonka, there are about 622,650 pounds of carp. In the Six Mile Creek Subwatershed, which includes Halsted Bay, Parley Lake, Lake Wasserman and Mud Lake, among others, there are closer to 876,000 pounds of carp. With that many fish constantly hunting for whatever food they can find, Sorenson said it’s not a pretty picture below the water’s surface.

“If you were to go down there, it would be like looking out the window and seeing a tornado every day,” he said. “It’s just devastating. Nothing can survive that kind of turn over.”

Sorenson said that the large population of fish in Lake Minnetonka stems from favorable feeding and breeding conditions in the Minnehaha Creek Watershed, especially with the poor water quality in the Six Mile Creek area.

The Minnehaha Creek Watershed District did a three year study on the carp populations in the Six Mile Creek area that ran from 2014 through 2016. Moving forward they have plans to implement physical barriers to keep the carp from traveling easily between the lake chains, as well as aerating lakes in the winter. The aeration process reintroduces oxygen into bodies of water that ice over, which helps keep populations of native fish like blue gill and sunfish healthy and ready to feed on carp eggs.

“It’s kind of tools in a tool box,” MCWD AIS Program Manager Eric Fieldseth. “We have different options to utilize as it makes sense in different locations.”

Managing those populations isn’t easy, but it is possible. Sorenson said he and his colleague Przemek Bajer have found success using native species to help keep carp nurseries in check. Sunfish are natural predators for fish eggs and in the right conditions, Sorenson said they can be very effective at managing new generations of carp.

With carp living up to 60 years, eliminating the populations before they can mature into full grown fish can have huge effects on monitoring the bottom feeder populations for future generations.

“If you try to do something else before you deal with the carp, the carp will just mess it up,” Fieldseth said. “We want to provide a healthier lake for the fish who live in it and the people who recreate on it.”

Common carp are not the only fish threatening Midwestern ecosystems. The most recent carp-shaped threat is heading upstream in the form of bighead, silver, black and grass carp– collectively referred to by the DNR as invasive carp.

These carp were also imported by the government, but came over more recently as biological agents to deal with problems in North American ecosystems. The fish were thought to be a safer alternative to pesticides and chemicals at a time when the American public was getting more concerned about the use of chemicals on the environment. This change came in large part because of the book “Silent Spring” that was published in 1962 which highlighted human’s negative effect on wildlife.

“A lot of these came over in the 60’s and 70’s during the ‘Silent Spring’ era when people were really starting to learn about how chemicals were affecting the environments,” DNR Invasive Fish Coordinator Nick Fronhauer said. “The intentions were good, but not enough research on what would happen in 20 years.”

The silver and big head carp were brought in to take care of over abundance of phytoplankton in water ways. Black carp feed on snails and were intended to get a handle on small freshwater mollusks that spread disease among fish populations. Grass carp were meant to help control unruly aquatic vegetation.

Today, they are spreading quickly and doing much more damage than good to the aquatic ecosystems they’re now changing.

“Carp are what we call ecosystem engineers,” Sorenson said. “They disrupt the entire food chain and change ecosystems into completely different environments.”

Right now, Fronhauer said that they only see about 10 of these invasive carp in Minnesota waterways each year. His job is to keep it that way and so far, he says it’s been successful. Right now, they’re only seeing isolated fish swim upstream into Minnesota waterways. Fronhauer said they are likely jumping off from the established schools in Iowa, Missouri and in the Illinois River in an attempt to create new colonies in the land of 10,000 lakes.

“Prevention is the cheapest solution in the long run, and the one with the least negative impacts,” he said.

He asks that Minnesota residents get involved in the efforts as well and report any sightings of invasive carp, which he’s said have happened in the St. Croix, Missouri and Mississippi Rivers. Sightings and captures can be reported to the DNR via email at [email protected] or via phone at 651-587-2781.