The sharp decline in the Mille Lacs walleye population that began in the late 1990s has coincided closely with significant increases in the lake’s water clarity, new research by the state Department of Natural Resources (DNR) has found.

DNR scientist Gretchen Hansen, who also is studying how invasive zebra mussels and spiny waterfleas are disrupting the food webs within Minnesota’s nine biggest walleye lakes, said the optical conditions in Mille Lacs have changed far more than the lake’s water temperatures. Mille Lacs has warmed since the ’90s, she said, but the water temps have remained suitable for walleyes.

Hansen said the shortage of low-light conditions alone leaves walleyes with less habitat. Similar research on walleye lakes in Canada also found correlations of reduced walleye numbers when water clarity increased. The fish thrive best in low-light, nutrient-rich, cool water.

Hansen presented the initial draft of her research at Friday’s DNR Roundtable in Bloomington, a large annual gathering of invited outdoors stakeholders. She cautioned that her main finding — compiled with historical data — is only a correlation, not an explanation for why the lake’s walleye abundance has crashed. With the population historically low and safe harvest quotas shrinking, it’s become illegal for state-licensed anglers to keep any walleyes during the regular, open-water fishing season.

“Today and to date, it seems like the optical habitat is where there’s been big changes,” Hansen said.

In the late ’90s, when Mille Lacs walleyes were plummeting in number, clarity readings were improving to the point where people could see 6 to 8 feet deeper into the water than they previously saw. Since then, clarity has increased by another 6 feet or so. “That’s a big deal,” Hansen said.

She and DNR fisheries chief Don Pereira said the water started clearing up before the lake became infested with invasive zebra mussels. The mussels themselves are powerful filters — stripping plankton and other nutrients out of the water — but Pereira said it’s his best guess that septic system upgrades around Mille Lacs and in the local watershed had a major effect on water clarity.

Hansen said no one is certain exactly how clearer water is contributing to the chief problem with the fishery: large numbers of baby walleyes dying before they reach the age of 3. One theory is that young offspring have been forced out of shallows that were once sufficiently low in light. The move to deeper water, where larger fish live, has made them more susceptible to predation and cannibalism, the theory goes.

Meanwhile, more studies are being launched to solve the mystery of the lake’s impaired fishery. This spring, the Mille Lacs Band of Ojibwe, other bands with fishing rights on Mille Lacs and the U.S. Fish and Wildlife Service will begin a walleye tracking study on the big, relatively shallow, lake. Under a grant from the Bureau of Indian Affairs, biologists will attach high-tech tags to juvenile and adult walleyes to help monitor movement.

The project is testing a theory that as lake water temperatures continue to rise and more light invades the depths, walleyes are being squeezed into ever-shrinking spaces that are suitable to them.