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Could springtime flooding lead to spread of invasive carp?

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Mississippi River floodwaters cover parts of Upper Landing Park
Mississippi River floodwaters cover parts of Upper Landing Park along Shepard Road in St. Paul on Sunday, March 31, 2019.
Andrew Krueger | MPR News

This spring's widespread flooding across the Midwest has brought destruction and economic hardship in Iowa, Nebraska and Missouri. Minnesota, still in the midst of the spring flooding season, has seen rivers swell and tributaries spill their banks.

But there's another potential impact of flooding that has scientists on alert: It could increase the likelihood of invasive carp spreading into new territory.

No established populations of invasive carp have been found in Minnesota yet. We asked some experts if they're concerned that this year's floods could change that.

What are invasive carp, and why are they a problem?

Also known as Asian carp, the term "invasive carp" usually includes four species: silver carp, bighead carp, grass carp and black carp.

They were brought to the United States in the 1970s as a means to control algae in fish farms in the South. They escaped into Mississippi River and have been making their way upstream ever since.

Invasive carp have spread throughout the Mississippi, Ohio, Missouri and Illinois rivers and their tributaries. They are voracious eaters and pose an ecological threat to rivers and lakes by crowding out native fish species. 

There's a genuine concern that they could continue to move northward, perhaps eventually into the Great Lakes.

Runaway carp population growth
With fewer young fish in the water, larger fish like walleye and pike have less food available to them, and their numbers begin to dwindle. Adult carp grow too large for apex predators of the Mississippi to eat, creating runaway population growth.
William Lager | MPR News

And while individual bighead and silver carp have been caught in the Mississippi, Minnesota and St. Croix rivers, no breeding populations of invasive carp have been detected in Minnesota — meaning they're not established here yet.

How could flooding play a role in the spread of invasive carp?

In normal years, navigational locks and dams play a big role in slowing the movement of carp upstream. It's difficult for fish to get over the gates that control the water's flow.

But during a flood, those gates are lifted to let more water through, creating what's known as "open river condition," said Mark Cornish, a biologist with the U.S. Army Corps of Engineers in Rock Island, Ill. 

How a lock and dam works
A lock and dam enables vessels to cross dams or other obstacles by raising or lowering the water in the lock chamber.
William Lager | MPR News

"There's not that barrier anymore," Cornish said. "There's not that hydraulic pressure which pushes those fish back or keeps those fish from moving up freely. So it is possible that Asian carp could move upstream."

Cornish said some locks and dams in Illinois and Iowa rarely or never reach open river condition. While they may not entirely prevent carp from swimming upstream, they do impede the fish's movement, he said.

And in addition to being able to more easily swim upstream when the locks are open, carp and other fish can move easier in a flood because the rivers and lakes within a floodplain often become connected as they overrun their banks and shorelines.

That's a natural phenomenon that has benefits for native fish and other aquatic life, said Nick Frohnauer, invasive fish coordinator for the Minnesota Department of Natural Resources.

"It gets them out of the high-speed current, with lots of debris coming down the main channel, so it's calmer back there. They have protection," Frohnauer said. "It also opens up a whole new feeding area."

But that interconnection also could allow invasive species like carp to move into areas where they haven't been before — and stay there, when the floodwaters recede.

What is being done to stop the spread of invasive carp?

Great Lakes invasion
Two Asian carp are displayed in 2010, on Capitol Hill in Washington, during a hearing on preventing the induction of the carp, an aquatic invasive species into the Great Lakes.
Manuel Balce Ceneta | AP

Researchers are experimenting with different methods of deterring the fish from swimming upstream, such as barriers that use noise, light or bubbles.

Peter Sorensen, a carp expert at the University of Minnesota's Aquatic Invasive Species Research Center, said researchers need to take flooding into account when designing deterrent systems.

"Anyone that wants to control carp seriously has to recognize floods happen," Sorensen said. "It's part of the natural cycle, but I think we can to some extent work with it."

Meanwhile, the Army Corps of Engineers is working on a plan aimed at stopping invasive carp at the Brandon Road Lock and Dam near Joliet, Ill., considered a key bottleneck into preventing an invasion of the Great Lakes. In Minnesota, the Corps has modified the way it operates some of its locks and dams — and closed the Upper St. Anthony Falls lock and dam to prevent carp from moving upstream.

Jason George, one of the crew that caught the carp
Jason George holds up a bighead carp caught in the Minnesota River in 2016.
Courtesy Ron Miller

Are scientists worried that this year's flooding will mean more carp in more places?

So far, they're not overly concerned, especially in Minnesota. 

Experts say the timing of the floods is an important factor. Because it's early spring, northern rivers and lakes are still very cold, so carp and other invasive species aren't moving around much yet — and they're not spawning.

And even if one carp does show up in a new stretch of a river or tributary, if there's no sign that it's reproducing, it's not a reason to panic, Frohnauer said. 

"We just don't have a whole lot of invasive carp in the state," he said. "We typically catch less than 10 individuals a year. So while they may move to an area we haven't seen before, they're not established yet."

The DNR has run an extensive monitoring program for invasive carp for the past seven years. Researchers sample Minnesota rivers for all ages of carp, but have only found adults — never any eggs, larvae or juveniles.

Still, Sorensen cautioned it will be important for state and federal officials to be especially vigilant when monitoring for carp this year. 

"It's curious with these fish — they may sit there for years and years not doing anything, and then you get complacent," he said. "Then all of a sudden, they're there. A door was opened for them."