Here's why the DNR put an invasive carp back in the St. Croix

It's part of the fight to keep the fish out of the state

The fight to protect Minnesota waters from invasive carp normally means killing off the fish whenever one shows up. Right now, though, state biologists are trying something different.

When they caught a 37 pound bighead carp in the St. Croix River last week, they did a little surgery on it and returned it to the water. 

Now the fish has a four-inch long transmitter implanted in it. And every day scientists are tracking where that fish goes. The Department of Natural Resources says learning more about the range and behavior of the invasive fish will help them get better at catching them and keeping them out of Minnesota's rivers and lakes. 

After a few weeks they'll catch it again (that'll be pretty easy since they know exactly where it is). That time they'll kill it and cut it open to see if it's a female with eggs.

Special permission from the Legislature

Releasing an invasive species into the water is against the law in Minnesota, so the DNR had to get special permission for this from the Legislature. 

The agency says the bighead they're tracking on the St. Croix now is their first use of the new technique. 

Biologists tested the catch-implant-release-track-recapture system last summer on a Mississippi River fish similar to invasive carp (the bigmouth buffalo) to make sure the concept worked. 

How will this help?

Invasive carp – which include bighead, silver, and grass carp – are pretty rare in Minnesota, with just a few reported per year. As far as we know they aren't breeding in the state. That's good because in places where they get established invasive carp damage the native fish by hogging up most of the food and space.

Nick Frohnauer, the DNR's invasive fish coordinator, tells MPR News tracking the invaders that do show up will teach researchers more about the fish. 

"Previously when we captured a fish we only knew that the fish was there at that specific time," he says. "We didn't know if it was hanging out there or passing through, where it came from or where it was going. This will help us gain that."

The DNR already has a network of 70 acoustic receivers, mostly in the Mississippi, and they're expanding that with money from the environmental trust fund that operates with State Lottery proceeds.

Anyone who catches an invasive carp while fishing should contact the DNR at 651-587-2781 and arrange to either bring it to a fisheries office or have it picked up. 

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