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More than 50 invasive Asian carp were caught this week in the Mississippi River above Minnesota’s southernmost dam, causing biologists and state wardens to fear that the threshold they’ve long worried about may have finally been crossed: Asian carp may have begun to spawn in Minnesota.

A total of 39 silver carp and 11 grass carp were found in the nets of two commercial fishing boats over the weekend near La Crosse, Wis., according to the Minnesota Department of Natural Resources (DNR). Another silver carp was also caught 20 miles upstream.

While individual carp, or even a handful of them, have been found in the river before, this was by far the largest population of the invasive fish ever seen in Minnesota. For years, the state’s silver lining has been that there has been no evidence that those lone carp had spawned or established permanent populations in Minnesota. But with so many caught at once, it’s possible the carp may have established a foothold that will be difficult, if not impossible, to remove.

“We just don’t know” if the fish are spawning, said Heidi Wolf, DNR invasive species unit supervisor. “We’re worried about it, but we don’t know.”

The DNR is working with the U.S. Fish and Wildlife Service to determine the age, size and gender of the 51 fish that were caught and will try to test whether the fish have undergone a spawning event, Wolf said.

Over the next few weeks, DNR crews from Minnesota and Wisconsin, along with the Fish and Wildlife Service, the U.S. Geological Survey and commercial fishermen, will set nets in the river near La Crosse to learn more about the population and to try to fish it down, she said.

A finding of this size was predictable and inevitable, said Peter Sorensen, a University of Minnesota invasive species researcher who is leading efforts to build carp barriers along the river.

“I’m not the least bit surprised,” Sorensen said. “It’s time for the state and the DNR to get serious and reconfigure their strategy.”

Sorensen has been testing a light and sound deterrent system at the state’s southernmost lock and dam — Minnesota’s first barrier and line of defense for invasive species — over the past year. The 51 invasive carp were caught just above that dam. The problem is that part of the river floods so frequently it will never make for a good barrier, he said.

When the Mississippi River is running high, the dam gates are raised, allowing fish to swim freely upriver. With more frequent flooding brought on by climate change, and consistently higher water levels in the river, the gates were raised and wide open for about 90 days last year. The carp could have easily swum upstream any one of those days, Sorensen said.

There is still time and hope, Sorensen said, to slow the spread and limit the damage, even if the fish are now freely spawning above the dam in Minnesota.

The fish don’t move particularly fast — they’re more steam engine than bullet train.

Farther north, near Lake Pepin, there are two locks and dams along the Mississippi that make for excellent barriers, Sorensen said. The river flows and the dams are designed in a such a way that the gates are rarely raised, even during floods and high water.

It could take five years for the carp to breach those barriers, he said. It would take much longer, even decades, if the state would install noise, bubble or light deterrents and turn those dams into a last line of defense.

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. They can grow up to 100 pounds and eat up to 20% of their body weight every day, completely disrupting ecosystems. Silver carp, which grow to about 20 pounds, can be especially disruptive as they gather in schools and leap out of the water en masse when they’re scared, injuring boaters, anglers and water skiers.

“These carp are part of our lives now, but they can be controlled and dealt with if we do sensible things,” Sorensen said.

Greg Stanley • 612-673-4882