The amount of usable walleye habitat in Mille Lacs Lake has declined dramatically in recent decades because the big central Minnesota lake is getting much clearer, likely due to a reduction in human-caused pollution and an invasion of zebra mussels.
That was the finding of a University of Minnesota study published Tuesday in the journal Ecosphere which found the now yearslong decline in walleyes in the big lake is being spurred by that reduction of suitable habitat.
It's not that the water is gone, it's just too light, too bright, for walleyes to thrive, said Gretchen Hansen, University of Minnesota assistant professor who headed the research. Hansen said the lake's water clarity jumped dramatically in the 1990s, even before zebra mussels invaded the lake. That's probably due to a reduction in human-caused pollution like fertilizer runoff and better septic systems and sewage treatment along the lake.
After zebra mussels arrived in 2005, they filtered even more nutrients out of the lake, making it even clearer. Water clarity was around 6 feet from the 1930s, the earliest records, to the early 1980s. Now it's doubled to 12 feet.
"As the water has gotten clearer in Mille Lacs, the area of the lake with suitable walleye habitat has gotten smaller," said Hansen, assistant professor of fisheries, wildlife and conservation biology at the university. "Because Mille Lacs is fairly shallow and uniform in depth, the walleye cannot retreat to deeper water as they can in some clear lakes and the lake can support fewer walleye. Therefore, harvest has to respond to avoid over-exploiting walleye under new habitat conditions."
Walleyes thrive in darker water thanks to their low-light vision that allows them to outcompete other predator species for food.
"We know they thrive in darker water conditions. But what's happening in the lighter water, what they are doing and why that's impacting the population, we don't know," Hansen said. "We don't know what they are doing differently now than when the lake was darker."
Managers can use formulas to measure the available habitat and use that to help make decisions on harvest limits and fishing regulations, Hansen said.
"The good news is that we have identified tools for adapting harvest policies to maintain safe, sustainable walleye populations even in the face of environmental change," Hansen said.
The study confirmed what many people had suspected - that because of increased water clarity, the lake can no longer support the number of walleyes it did in the 1970s or '80s, said Tom Heinrich, Mille Lacs fisheries supervisor for the Minnesota Department of Natural Resources. "And that's just something we have to live with."
"There's an optimal water clarity level for walleyes. If it's too bright, or too murky, walleyes lose their advantage," Heinrich said. "When you see this increase in water clarity, even in other lakes across the country, walleyes go down and bass go up. That's what happened here."
In addition to decreasing habitat and zebra mussels, Mille Lacs walleyes also have faced invasions of Eurasian watermilfoil, spiny water fleas as well as a changing zooplankton community and a reduction in tullibee, a favorite walleye food.
Minnesota's walleye fishing season starts Saturday.
The Mille Lacs walleye population has increased slightly thanks to a good hatch in 2013 and, after three years of catch-and-release-only summer fishing, anglers on the lake will be able to keep one walleye per day between 21- and 23-inches long from Saturday through May 31. After that, walleye fishing on the big central Minnesota lake will revert to catch-and-release only, which it has been all open water seasons since 2016.
The lake is jointly managed by the Minnesota Department of Natural Resources and multiple Anishinaabe bands that have fishing rights across a large portion of east-central Minnesota under treaties with the U.S. government.
The state and the bands with harvest rights in Mille Lacs Lake agreed on a 2019 safe harvest level of 150,000 pounds of walleye, resulting in a state allocation of 87,800 pounds. Under the catch-and-release-only regulation last year, the walleye angler kill totaled just over 47,000 pounds.
Researchers are collaborating on a new project to assess walleye habitat in other Minnesota lakes and its sensitivity to changing water quality and temperature. This project is ongoing and scheduled to be completed by the end of 2020.
Researchers from Rensselaer Polytechnic Institute, the U.S. Geological Survey, Minnesota DNR and the University of Wisconsin-Madison collaborated on the Mille Lacs study.