Grounding the flying fish: stopping invasive carp

Peter Sorensen and his team showed that a strategically placed wall of vibrating bubbles can shield Minnesota from these ruinous fish.

Fisherman holding a large silver carp in his hands in front of a lake with reeds

Outside Peter Sorensen’s office stands a life-size cutout of a large man holding a large fish by the gills.

The fish, a bighead carp, is close to five feet long and upwards of 80 pounds. A voracious eater, this carp can wipe out the base of an aquatic food chain.

Silver carp jumping in Fox River

Its cousin, the silver carp, can reach 40 pounds. When spooked—often by boat motors—silver carp routinely leap nine feet out of the water. They have left boaters with broken jaws, noses and ribs.

Not the kinds of fish you'd want in your waters. Unfortunately, both are beginning to invade Minnesota, says Sorensen, a professor emeritus in the Department of Fisheries, Wildlife and Conservation Biology. They now infest the Mississippi River watershed in states from Mississippi to Iowa.

Peter Sorensen, professor emeritus, standing in front of a tree
Peter Sorensen, 
Professor Emeritus,
Fisheries, Wildlife and Conservation Biology

“With our great waters, we in Minnesota and adjacent Wisconsin are like a shining ornament sitting above a huge area of infestation,” Sorensen muses. "But Minnesota could soon become their next home base.

“Experience shows it’s critical to stop them now. If we can, we’ll be the first state to do so.”

Minnesota’s best chance: a single, strategically placed wall of vibrating bubbles known as a bioacoustic fish fence, or BAFF, which can be placed in navigation locks. Sorensen and his team have identified this “bubbling sound deterrent” as the best carp defense—and it will soon become reality.

High stakes for Minnesota waters

Silver carp, bighead carp, and two other carp species were introduced in Arkansas in the 1970s to clean pond water. But they escaped and have now damaged numerous fisheries and ecosystems severely, if not irrevocably.

"Silver and bighead carp are efficient ‘filter feeders’—they vacuum out the ecosystem,” Sorensen says. “Native fish larvae, including walleye and other gamefish, are starved and/or eaten by these carp.”

Aerial view of lock and dam No. 5, Mississippi River.

Unhindered, they will spread into the watersheds of the area’s major rivers: the Minnesota; the Mississippi, up to Minneapolis; the St. Croix; and the Chippewa in Wisconsin.

The carp could wreck Minnesota’s fisheries, while silver carp alone could end boating and waterskiing—a blow to areas that depend on water sports. They would also wipe out the economic benefits these activities generate.

It would be game over.

Turning carp’s biology against them

From St. Louis to Minneapolis, a series of locks and dams in the Mississippi River reduce the movement of all fish, including carp.

“We identified Lock and Dam 5, north of Winona, Minnesota, as having exceptional potential to stop carp from moving northward,” Sorensen says.

Sorensen’s team worked with a British company to perfect a BAFF system designed for lock openings. His team crafted a three-part deterrence strategy over a decade of research involving lab experiments, mathematical modeling of flowing water, fish tracking in the Mississippi River, and ongoing field tests of a BAFF in Kentucky.

This work gave them a unique understanding of fish movement through locks and dams and how to stop carp.

The key part of the strategy is to use a BAFF to stop carp from moving upstream along with boats passing through the lock and dam’s navigation lock. The system features a tubular, bottom-lying “air diffuser” laid across the lock entrance. Air forced through it creates a curtain of bubbles from tiny holes along its length, and sound projected into it turns the curtain into an acoustic wall.

Experience shows that carp will be startled by the noisy bubble curtain and deterred from moving into the lock and passing upstream. Video courtesy U.S. Fish & Wildlife Service

Sorensen and his team discovered that this system causes bubbles to vibrate with a unique intensity and directionality that invasive carp detect and avoid.

“It’s because they have an exceptional sense of hearing—as evidenced by how silver carp respond to outboard motors,” Sorensen explains.

The system will also include flashing lights to further discourage fish from entering the lock.

Pop Quiz

Which of these carp could hit you in the head when you're boating? 

Bighead Carp

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Bighead Carp

No, though the bighead carp does eat a lot of plankton.

White Grass Carp

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Grass carp

No, though the grass carp does eat a lot of plants—and is considered a delicacy in China.

Silver carp

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Silver Carp

Yes! The silver carp, when spooked, could do some real damage if it becomes a missile.

The second part of the strategy involves using hydraulic modeling to adjust spillway gate openings at Lock and Dam 5 to reduce fish passage under them. For the third part, Sorensen strongly recommends that the state Department of Natural Resources (DNR) continue to remove as many invasive carp as possible, especially those deflected by the BAFF, to reduce pressure on the deterrent system.

“Our models show these measures should block 95-plus percent of the invasive carp,” he says. “It need not stop 100 percent, because carp can’t establish a breeding population with only a few individuals, and they only live about a decade.

“If any get through, we believe they’ll probably die of old age before reproducing.”

The newly enacted 2024 Legacy Bill includes funding for this solution and directs the DNR to implement it within five years. Sorensen says this should be enough time, although “the sooner the better, as carp continue to move northward from Iowa and, if not stopped, could still establish and breed.”

Project partners

This work was enabled by the Minnesota Environment and Natural Resources Trust Fund, US Army Corps of Engineers, U.S. Fish and Wildlife Service, and MN Dept. of Natural Resources. Its findings have been promoted by roughly a dozen environmental and fishing advocacy groups.