Walleyes in northern Minnesota lakes infested by zebra mussels and spiny water fleas are growing at a slower rate than they did before the infestations.

That was the bad news in a recent study by University of Minnesota researchers who looked at nine popular walleye lakes that have been impacted by one or both of the aquatic invasive species.

Researchers (including Josh Dumke at the University of Minnesota Duluth’s Natural Resources Research Institute) looked at walleye growth rates in Lake of the Woods, Rainy, Kabetogama, Vermillion, Red, Cass, Winnibigoshish, Leech and Mille Lacs lakes using 35 years of data gathered by Minnesota Department of Natural Resources fisheries biologists.

The study found that walleyes in lakes infested with zebra mussels or spiny water fleas were 12% to 14% smaller after their first summer than before the lakes were infested. That means about half an inch to three-quarters of an inch for a young 6-inch walleye.

Mille Lacs Lake has both spiny water fleas and zebra mussels; two of the lakes in the study have just zebra mussels and four have just spiny water fleas.

Both spiny water fleas and zebra mussels eat at the bottom of the food chain by taking small zooplankton out of the water. It’s not clear what impact they are having higher up in the food chain, such as with popular game fish species. Researchers focused on walleye and perch because they eat zooplankton during the first year of their lives. The study did not find a consistent trend of smaller young yellow perch in infested lakes. And it's not clear if the decline in zooplankton is why the walleyes are smaller. It could be from other factors, such as clearer water caused by zebra mussels filtering lakes and making them more clear.

The study was published in the journal Biological Invasions on Jan. 24.