Minnesota researcher makes the case to stop invasive carp at a lock and dam near Winona

If there was ever a place to draw a line in the sand to try to prevent invasive silver and bighead carp from spreading farther into Minnesota, it is at a barrier called Lock and Dam 5.

The dam, the fifth of a series built in the upper Mississippi River, rarely floods, giving the fish few opportunities to swim around it. Its floodgates are almost left in the water, even when the river flow is high, creating a current too strong for the carp to swim under. And it was built just a few miles downstream from another lock and dam, creating a pinch point — a small pool where the few carp that still do pass it can be aggressively fished out before they breed.

"This is the spot," said Peter Sorensen, researcher at the University of Minnesota. "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."

Sorensen made the case to state and federal agencies on Thursday to fortify the lock and dam against the carp, which can upend ecosystems, decimate native fish and injure boaters and water-skiers by leaping out of the water.

The proposal was met with some excitement and some skepticism. Nothing has been able to stop the carp yet. Some efforts to build better barriers to keep invasive fish out of Minnesota waters may have even made the problem worse.

The carp have been slowly creeping up the Mississippi since they were released in the American South in 1972. While bighead carp were purposely brought over from Asia for aquaculture, it's unclear exactly why silver carp were released into the river, said Duane Chapman, supervisory fish biologist for the U.S. Geological Survey.

They're native to Russia and are unbothered by Minnesota winters, he said. Silver carp are more feared than the bighead carp because while both can devastate ecosystems, silver carp jump when they're scared. At roughly 40 pounds, they can injure boaters, breaking people's jaws and noses.

Silver carp also tend to eventually outcompete bighead carp to become the dominant species of a water, Chapman said.

They are filter feeders that primarily eat plankton. As they've spread up the Mississippi, they've cleared out up to 90% of large zooplankton in the water, an important food source for young fish. They also eat phytoplankton, causing increases in chlorophyll, which turns the river green. Both the population and the average size of bass and crappie have fallen as silver carp have moved in, Chapman said.

"There have been substantial effects on the tourism industry," he said. "Nobody really wants to get hit in the head with a carp."

Researchers from the U meet every year with experts and agents from the Department of Natural Resources (DNR), U.S. Fish and Wildlife Service and the U.S. Geological Survey to talk about ways to keep carp from spreading. There was added urgency to the meeting this year, as record numbers of the fish were found in Minnesota waters in both 2019 and 2020. More than 80 carp were pulled out of the Mississippi River north of the Iowa border in 2020, enough to suggest that if they haven't established a permanent population in Minnesota yet they will soon.

Sorensen has been studying the carp for several years as they cross Minnesota's southernmost locks and dams.

Those dams flood too much to make for very effective barriers, Sorenson said.

Lock and Dam 5, however, near Winona, stops about 80% of the carp that try to cross it, he said. If the floodgates there were managed slightly differently, to keep a more constant pressure of water underneath them, and if lights and noise deterrents were added to scare off carp from following boats through the lock, it would block about 98% of carp from making it through, Sorensen found in a recently published study. If the state were to go one step further and have anglers weed out the carp that still make it past the dam, Minnesota would be able to prevent more than 99% of carp from breaching the area, Sorensen found.

"It looks like if we do all three methods, this is doable," he said. "No one thing is going to work."

But the carp have steadily made it past more than a dozen locks and dams as they've moved north, including one in Iowa that, on paper anyway, seems like a stronger barrier than Lock and Dam 5.

The state has been building dams and barriers for more than a century to try to stop the spread of common carp, which are slower than silver carp and can't jump nearly as high, said Luther Aadland, river scientist for the DNR.

The common carp eventually made it past nearly every barrier put up against it, Aadland said. Native fish, however, that rely on free-flowing rivers quickly died off.

"Some of the barriers we tried then actually created common-carp refuges, because they kept out all the native fish that would compete with them," Aadland said.

The best course of action would be to make the Mississippi and other Minnesota rivers as healthy as possible, to bolster the native species to fight off invaders, he said.

Giant, native catfish have been known to eat silver carp. Those catfish populations, however, have been nearly erased from the upper Mississippi ever since the dams were built, cutting off their habitat.

"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," he said.

Greg Stanley • 612-673-4882