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Inside a cramped shed alongside a lock and dam on the Mississippi River south of La Crosse, Wis., fisheries biologist Jeff Whitty flipped a switch.

A few yards away, at the foot of the lock, a dozen submerged speakers began blasting a repetitive drumbeat — a Pacman-like whomp, whomp, whomp — while underwater strobe lights began to pulse with bright flashes.

Researchers were about to discover what fish think about the best — and perhaps last — hope for keeping destructive Asian carp out of Minnesota’s rivers and lakes.

“We know the fish are here,” said Peter Sorensen, a University of Minnesota invasive species researcher who designed the system and is leading the project. “This is the main conduit — the way in [to Minnesota]. We are in a position to prevent the arrival of one of the worst fish invaders the country has seen in a century.”

The $1 million noise and light system was installed in May at Lock and Dam No. 8, about 50 miles from the Iowa border and 100 miles north of where the carp have taken hold in large numbers. The Mississippi is essentially the only entry point for carp to colonize in Minnesota, and if the state can close that choke point it’s unlikely the fish will be able to establish a population, Sorensen said.

In laboratory tests, the skittish creatures were scared off by bright lights and loud noises. Now Sorensen and his team are testing whether the barrier works in the wild.

Two species of damaging Asian carp have been working their way up the Mississippi since the 1970s, when they were brought to the American South to clean algae from fish farms and sewage treatment ponds. Bighead carp, which grow to more than 100 pounds, disrupt local ecosystems by eating up to 20% of their body weight every day. Silver carp are smaller, growing to about 20 pounds, and out-compete native fish for food. They gather in schools and leap out of the water en masse when they’re scared, which on occasion has resulted in injuries to boaters, anglers and water skiers.

Goal: Stop them from breeding

Small numbers of the fish have been caught in Minnesota every spring for several years, but biologists say there is no evidence yet that they are reproducing here. This year, with high levels of flooding, a record 12 Asian carp were caught in the Mississippi and St. Croix rivers near the Twin Cities.

It’s just a matter of time before those sporadic adult fish, strong enough to make it past a series of dams, find one another and begin breeding, Sorensen said.

“This is really about stopping them from having young,” he said. “As soon as they’re reproducing, it’s game over.”

Sorensen and his team tried a cruder version of the noise system in 2012 on the same southernmost lock and dam, but it didn’t work. This time the equipment is better and the sound has been proved effective. The noise will be paired with the strobe lights, and the barrier will operate all day, every day, Sorensen said.

To test the system, researchers with the U and the U.S. Fish and Wildlife Service are monitoring the reactions of common carp, a similar but less destructive species that has been in Minnesota’s waters since the 1800s. They capture fish a few miles upstream from the dam, insert pea-sized radio transmitters just beneath their skin, and then release them below the dam.

Tracking their journey

The fish are hard-wired by nature to try to return upstream, Sorensen said. Using a series of radar receivers placed above, below and along every part of the lock and dam, the researchers can track whether the fish are being turned away by Sorensen’s barrier or finding a way to breach it.

Sorensen has also been working with officials in Kentucky to build a similar system along the infested Tennessee River. That system, which also generates air bubbles that reflect the strobe lights and form what seems to carp like a frightening and loud underwater wall, will go live in early November.

In the lab, the noise and light system alone stopped about 80% of carp, Sorensen said. When air bubbles were added, the deterrent stopped 99%.

The barrier test, which is funded with money from Minnesota’s Environment and Natural Resources Trust Fund, will continue through the spring of 2021. At that point, Sorensen’s team will share the results, along with those from Kentucky, with state biologists and lawmakers.

Yet even if Sorensen’s barrier proves effective, it might be too late to keep invasive carp out of Minnesota entirely.

There are essentially two ways for the fish to get past a lock and dam. The first, and most common, is through the navigation lock, whose doors swing open as boats and barges pass up or down the river. The fish simply follow the boats through the open doors and move up the river.

The second occurs during major floods, when the U.S. Army Corps of Engineers opens the dam’s floodgates to release more water downstream and relieve pressure upstream. When that happens, fish can swim under or around the dam’s gates.

The U’s sound and light barrier may deter carp from the navigation locks, but it won’t stop them from swimming under the gates when the river floods.

This year, flooding on the Mississippi was so severe that the gates at Lock and Dam 8 were raised for more than two months. As a result, for much of the spring, any carp that were scared off by the sound system could have simply gone upriver under the gates.

Sorensen says a better hope lies at Lock and Dam No. 4, near Lake Pepin, where flooding has been less severe and the gates were raised for less than a week.

“Because the gates … are only opened a few days a year, we could make it almost impassable,” he said.

Now he just needs results from Lock and Dam No. 8.

“If it were to prove effective, I would say we should immediately build something on 4 and 5,” Sorensen said.

“Immediately.”

Greg Stanley • 612-673-4882