A new study from the University of Minnesota links the decline of walleye in Mille Lacs Lake to a loss of habitat that has resulted from clearer water.
Published Tuesday in the journal Ecosphere, the study suggests those changing conditions could be used to help determine the sustainable number of walleye that can be harvested from the lake in a given season.
Researchers wanted to understand what was causing walleye declines in Mille Lacs — and how agencies that manage the lake can sustain the fish's population, said Gretchen Hansen, an assistant professor at the university and the study's lead author. They used 30 years of data on Mille Lacs' water clarity and temperature to estimate how walleye habitat in the lake has changed.
Walleye prefer low light and cooler water. But in recent decades, Mille Lacs' water clarity has increased, most likey thanks to both septic system improvements around the lake and the invasion of zebra mussels, which are filter feeders that strain out microscopic algae.
But there's a downside to the improved water clarity: The study found as Mille Lacs has cleared, the area of suitable walleye habitat has gotten smaller, Hansen said.
"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."
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Mille Lacs is also fairly shallow, with a consistent depth throughout the lake. That means if conditions get too bright for walleye, they can't simply take refuge in a deeper, darker part of the lake, Hansen said.
The study also suggests that altering annual harvest levels based on changing water clarity and temperature could help sustain the lake's walleye population.
Linking harvest policies to habitat and environmental change is "sort of a new idea, and not something that is commonly done," Hansen said.
The state Department of Natural Resources, which sets the amount of fish anglers are permitted to harvest each year, uses the estimated number of fish in the lake to set harvest levels. Anglers on Mille Lacs have faced tighter restrictions on when and whether they're allowed to keep the walleye they catch in recent years.
This spring, the DNR announced that anglers on Mille Lacs can keep one walleye between 21 and 23 inches long or one walleye over 28 inches long, from the start of the fishing season on Saturday until May 31. Catch-and-release fishing for walleye returns on June 1.
The findings from the U of M study could be helpful for understanding how walleye in other Minnesota lakes are affected by changing habitat, depending on their size and depth.
Hansen said the university and the DNR 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.