Researcher recommends steps to stop more invasive carp from reaching St. Croix River

Scientist says a lock and dam on the Mississippi River could control destructive fish species.




4 minute read

Electrofishing invasive carp at a dam in Nashville, Tennessee. (U.S. Fish and Wildlife Service photo)

A University of Minnesota scientist is urging authorities to fortify a lock and dam on the Mississippi River near Winona as a line of defense against invasive carp. Dr. Peter Sorensen says recent research shows Lock and Dam Number 5 is an excellent candidate for measures that would almost eliminate the chance of carp sneaking through, the Star Tribune recently reported.

The invasive fish, primarily bighead and silver carp, have decimated fisheries and tourism in rivers in Illinois and nearby. They are voracious eaters that grow quickly, out-competing native fish for food. Studies have shown smaller bass and crappies in rivers with significant carp populations.

One species, silver carp, leap out of the water when motorized boats pass nearby, frequently crashing into boats, damaging the craft and harming people on board.

Sorensen, who works at at the University of Minnesota’s Aquatic Invasive Species Research Center recently published an article in a peer-reviewed journal explaining why Lock and Dam Number 5 could be an effective barrier.

“This is the spot,” Sorensen told the Star Tribune. “If no one does anything and they get past Lock and Dam 5, then they will be in Lake Pepin. Then there’s nothing to stop them from getting into the St. Croix [River]. Then you’re losing half the state.”

Control steps greater than sum of carp

Map by Greg Seitz/St. Croix 360

Lock and Dam No. 5 includes a concrete structure more than a quarter-mile in length. Thousands of barges carrying tons and tons of farm products go through the lock headed south each year. Similar numbers come upstream with industrial chemicals and other cargo. It is also used often by recreational boaters.

It also seems to be the perfect place to make a stand against carp. For one thing, there’s a relatively short stretch of river above it before the next dam. Sorensen’s new paper says such a situation is important for controlling carp because it provides a pool that can be closely monitored for the presence of carp, and allow for harvesting any fish that sneak through Lock and Dam No. 5.

Sorensen has also tested several lock and dam modifications to deter the fish. They include a “bubble curtain,” shooting out a wall of aerated water that carp don’t like. In coordination, underwater speakers can blast sounds that irritate them. (Sensitive hearing is why silver carp leap from the water when boats pass.) And the dam’s spillways, which allows for extra water to be released, can be modified to always flow at speeds fast enough to prevent carp from swimming against it.

Sorensen’s paper says each method can reduce the number of carp that slip past, but combining them should deter nearly all of the fish.

“Our simulations demonstrate that upstream passage of invasive silver carp in the [Upper Mississippi River] can be reduced to only 1–2% of current rates through an integrated approach that uses consecutive [locks and dams] and some combination of three tractable control techniques,” Sorensen wrote in the paper, with his co-author Daniel Patrick Zielinski of the Great Lakes Fisheries Commission.

With only a few of the unwanted fish typically found each year in the St. Croix River, and no reproduction believed to occur yet, such a significant reduction of fish passage should drastically reduce the risk to the river.

Differing goals

Electrofishing carp in the Fox River of Illinois. (Ryan Hagerty/USFWS)

One DNR expert says he finds the plan problematic. Luther Aadland, an agency river scientist, points out that it depends on making locks and dams even more difficult for fish to pass through. Many species of fish must migrate to successfully reproduce and survive in a river.

In an era when dams are being torn down around the country, sometimes to help fish, the new proposal would amount to a hardening one of them.

“My fear is that one of the biggest damages of the silver carp, on top of all the problems they create, will be that they shut down any work to provide native fish passage on the Mississippi River,” Aadland said.

Species like catfish have been nearly eliminated from the Mississippi because of dams that block their movement. Such catfish are believed to eat silver carp, and restoring the species could be a natural method of reducing carp.

“While some migratory fishes have disappeared from the Upper Mississippi River since [locks and dams] were installed, analyses of the current fish population structure suggest that [locks and dams] likely have little effect on the remaining populations of native fishes, although their effects on newly arriving invasive carp appear quite substantial,” the paper says.

Sorensen says his studies have shown that invasive carp are far more sensitive to the barriers than native fish. Native species should be much less affected.

In 2016, Sorensen told St. Croix 360 that restoring a healthy native fishery and ecosystem was an important part of protecting the rivers from invasive carp.

“I don’t want to just stop carp, I want to improve the darn river,” Sorensen said. “We’ve got something to protect here, if you can protect and enhance, that’s even better.”

Sorensen was developing the idea for the three-pronged carp barrier at the time. The latest published research is notable for helping predict the effectiveness and chances of success.

State officials and scientists discussed the proposal in a recent meeting about carp. The legislature and several agencies now can decide whether or not to pursue it.

Report your carp catch

Invasive carp captures must be reported to the DNR immediately. Take a photo and make arrangements with the DNR to transport the carp to the nearest fisheries office. Call 651-587-2781
or email

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Researcher recommends steps to stop more invasive carp from reaching St. Croix River