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Study links Mille Lacs Lake walleye decline to water clarity

BRAINERD, Minn. (AP) — A University of Minnesota study links the decline of walleye in Mille Lacs Lake to a loss of habitat resulting from clearer water, which doesn’t favor walleye habitat.
The study was published in the journal Ecosphere. Researchers used 30 years of data on the lake’s water clarity and temperature to estimate how walleye habitat has changed, assistant professor and lead author Gretchen Hansen told Minnesota Public Radio.
Walleye prefer low light and cooler water. But in recent decades, Mille Lacs’ water clarity has increased, most likely due to septic system improvements around the lake and the invasion of zebra mussels, which are filter feeders that strain out microscopic algae. The change has reduced walleye habitat, according to Hansen.
“As the water has gotten clearer, more light can penetrate into the water,” she said. “And walleye, being low-light specialists, don’t really like that.”
The study suggests that altering annual harvest levels based on changing water clarity and temperature could help sustain the walleye population. State officials currently base harvest limits on the estimated number of fish in the lake
Linking harvest policies to habitat and environmental change is “sort of a new idea, and not something that is commonly done,” Hansen said.
The university and the state Department of Natural Resources are starting a new project looking at thousands of lakes across the state and how sensitive walleye habitat in those lakes is to changing water clarity and temperature. That study should be completed by the end of 2020, Hansen said.
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Information from: Minnesota Public Radio News, http://www.mprnews.org