A bighead carp caught recently near Redwood Falls is the largest invasive carp caught in Minnesota and a clear indication that the unwanted fish is making its way deeper into the state.
"If there's one, there's more," said University of Minnesota Prof. Peter Sorensen, an expert on invasive carp. "It's just going to take one successful moment of reproduction and all those millions of eggs, then it's all over. And then there's nothing anyone can do, because we don't have a cure for an established population."
If they were to spread, invasive carp would compete with the state's native species for food and habitat.
The carp, recently captured by a bow angler, weighed 61.7 pounds and measured 47½ inches long. It was caught in a private gravel pit after it apparently found its way from the nearby Minnesota River when it spilled over into its flood plain, connecting the pit and the river.
The spot where the fish was caught is 80 miles upstream from the place where the only other bighead carp caught in the Minnesota River was found, said Nick Frohnauer, DNR invasive fish coordinator.
"What this says is that the fish are moving into the system," Frohnauer said, noting that the fish probably came up the Mississippi from Iowa or even from as far away as Missouri.
"It shows they can obviously get a long ways upstream."
The bighead is among three invasive carp species that have been caught over the past 25 years in Minnesota. Most made their way up the Mississippi from other states where their populations are booming.
Frohnauer said he and other experts are confident that the invasive carp aren't breeding because crews monitoring them haven't found any fish eggs, larvae or juvenile fish.
The carp caught near Redwood Falls was a "large female, capable of reproducing," he said. "But the good news is that it had all of its eggs and hadn't spawned this year." It will be tested to determine its age and whether it may have spawned in previous years.
The discovery of this second bighead carp in the Minnesota River is "ominous," because it's a perfect place for it to live and reproduce, the U's Sorensen said. The Minnesota is slow-flowing and polluted, with lots of nutrients and a flood plain to establish the fish's young. "If they started reproducing there, they probably would do very well," he said.
How to keep them out?
Frohnauer and Sorensen believe the state can take measures to keep the invasive carp downstream.
The Upper St. Anthony Falls lock on the Mississippi was closed two years ago in hopes of stemming the carp's migration upstream. But the river remains susceptible below the falls, Frohnauer said. "As for the Minnesota River, our concern is that there's a lot of river there," he said.
Sorensen and his team at the U are developing deterrents to keep the carp at bay. "People think of these fish as undefeatable beasts from another world," he said. "They're just large fish from China."
They're not great swimmers, he added, so he's working the with U.S. Army Corps of Engineers to adjust the water flow at locks and dams along the Mississippi in an effort to keep them from swimming through.
His team also is blasting sound through underwater speakers to frighten the carp away.
"These carp have a better sense of hearing than most fish," Sorensen said. "They're also a high-strung fish, so they're the perfect fish to scare away."
His team is currently experimenting at Lock and Dam 8 near the Iowa border with underwater speakers that blast the sound of motorboats. "It's 60 percent to 70 percent effective," Sorensen said.
But he thinks he can do better. More recent experiments in the lab show that a pulsing sound is more than 90 percent effective at repelling carp. "I have a two-year proposal that costs shy of $1 million to get speakers up at the Lock and Dam 5 near Lake Pepin," Sorensen said. "It would save most of the state, including the Minnesota River."
The main threat remains along the Mississippi's lock and dam system, Sorensen said.
"It's wide open, he said. "It's kind of like leaving your door open and going out for dinner. … All we're talking about is putting a lock on the door and closing it."