Study shows how common carp became invasive

Researchers at the University found the fish don’t like clean and clear lakes.

by Allison Kronberg

The world’s second-most invasive fish, the common carp, has been the source of complaints about shallow lakes in southern Minnesota and the metro area since the species first came to the state about a century ago.

But much to the bewilderment of researchers and regulators, the fish has never become a nuisance in the northern Boundary Waters.

Research from the University of Minnesota looked at several factors in more than 550 lakes throughout the Midwest to try explaining what has allowed the species to become invasive in some places but not others.

The research, published in the conservation journal Diversity and Distributions last month, found that clean, clear lakes are much less likely to allow common carp to become invasive and that carp predators, like bluegill, play a smaller role in controlling the species.

“To be able to control [carp], you really need to understand why they are so abundant in the first place,” said Przemyslaw Bajer, the study’s lead author and a fisheries, wildlife and conservation biology research assistant professor.

Minnesota Department of Natural Resources fisheries research scientist and study co-author Tim Cross said the DNR has been trying to control common carp for decades because they can cause significant harm to lakes.

The fish, Bajer said, use their mouths to search through sediment at the bottom of lakes for insect larvae to eat, and then dispel the sediment they consume through their gills. In the process, they disturb the lake floor so much that plants can no longer grow there.

For hunters, this means waterfowl are less likely to stop at the lakes lacking plants for food. And for boaters and lake residents, it means a murkier-looking lake.

“They change the lakes enough that people notice,” Cross said.

Wildlife management senior at the University of Minnesota-Crookston Renee Tomala has been going on duck hunting trips with her father since she was about six years old.

She’s noticed duck populations varying greatly from year to year at different lakes, though she’s uncertain whether carp is the cause of that.

But Tomala has heard about the effects of carp on duck habitats in her classes, she said, and worries about the future of duck hunting if the species isn’t contained.

“Habitat loss and construction is already pushing down on wildlife numbers, and then you add invasive species,” she said. “It’s a huge thing to be concerned about — whether you’re a duck hunter or a bird watcher or anything. If [species] don’t have the right habitat, they’re not going to be there.”

To offset the carp problem, the Legislature began funding research on the species almost a decade ago, Bajer said.

Previous research has manipulated certain lakes to confirm that bluegill presence inhibits carp invasion, Bajer said. The next step is to manipulate new lakes to test if their hypothesis about water clarity can be confirmed.

Either way, Cross said, the research is more reason for the state to work toward cleaner lakes.

“We do recognize that carp do best in these lakes with poor water quality,” he said, “so by trying to clean up our lakes, we’re going to try to reduce carp problems.”