Deep dive on Mille Lacs emphasizes downside of water clarity

A University of Minnesota study published Tuesday in the science journal Ecosphere offers a new research tool to assist in setting annual safe harvest levels for Mille Lacs walleyes.

The approach would factor in water clarity — a habitat condition that the study links to a dramatic decrease in the lake's walleye population. Mille Lacs lost turbidity starting in the 1990s, robbing the fish of dim spaces where they thrive best.

Gretchen Hansen, an assistant professor at the U of M who headed the study, said hindsight suggests that managers of the lake might have helped walleyes by tightening harvest regulations as soon as the water got clearer. On the other hand, harvest limits could have been set more liberally in some recent years if the lake's optical conditions were considered, she said.

The study's "big picture'' conclusion is that walleye fisheries may be sustained by adapting harvest policies to account for changing environmental conditions, she said.

"We don't have to sit there,'' Hansen said. "If conditions are different, we might have to manage differently.''

Minnesota Fisheries Chief Brad Parsons of the Department of Natural Resources said it makes sense to incorporate the "safe operating space'' habitat model provided in the new study — a research project that Hansen started in early 2017 while she worked at the DNR.

"I don't see why we couldn't use it to inform our decisions,'' Parsons said. "We need to utilize every tool as to what the harvest level can be.''

The study doesn't quantify how much walleye productivity has changed in Mille Lacs since routine measurements detected marked increases in water clarity and sharp reductions in phosphorus levels in Mille Lacs. A previous study linked those changes to septic system improvements around the lake.

Further clearing of the water happened after invasive zebra mussels entered Mille Lacs in 2005.

Hansen's study says water clarity peaked in 2013 and currently varies according to algae growth, sun angle, rainfall and other factors.

In some walleye lakes, increased water clarity hasn't cut deeply into populations of the fish. But Mille Lacs is relatively shallow despite its massive surface area. When the water there gets clearer, "the walleyes don't have anywhere to go,'' Hansen said.

The "safe operating space'' habitat model considers water clarity and water temperature in addition to walleye population. The study notes that cool, walleye-friendly water temps have existed in Mille Lacs for the past 30 years.

The model measures the area of a lake where good optical and thermal conditions for walleye overlap — a combination that is positively related to walleye production.

The model estimates walleye habitat area on a three-year rolling average and Hansen said it's possible to provide fisheries managers with meaningful input every fall when they are calculating the next season's harvest allocation. The model was built by quantifying water temperature and clarity for Mille Lacs for each day of 1980-2016.

The maximum safe harvest limit generated by the model can be different from the limit set by the standard method of annually estimating the lake's biomass of walleye and applying a harvestable percentage to that total.

According to the study, actual walleye harvest on Mille Lacs exceeded the "safe operating space'' model in 16 of 30 years. Walleye abundance declined following 14 of those 16 years, the study said.

Population increases were observed in 10 years and in eight of those years the previous years' harvest was below the model's safe levels.

In a separate Minnesota fisheries study, researchers have looked at the impacts of zebra mussels and spiny waterfleas on the growth of walleye and perch in multiple state lakes.

A presentation last week to the DNR's Citizen Walleye Advisory Committee said baby walleyes in year one are close to 1 inch smaller in late summer for invaded lakes compared to uninvaded lakes. The stunted growth could have "significant impacts'' on their survival, DNR research scientist Tyler Ahrenstorff told the committee.

Zebra mussels and spiny waterfleas disrupt the food chains of lakes they invade and the study found that food for baby walleyes is being reduced by as much as 50% in invaded lakes.

"We're seeing some reductions of 19 percent in the length of walleye growth,'' said Jim Justesen, a member of the committee.

The study comparing uninvaded lakes to invaded lakes found differences in the way perch grow, but overall perch growth didn't vary to the extreme it did for walleyes.