Bloomington, Minn. — Just what clearer waters on Lake Mille Lacs might mean for the lake’s walleyes is the subject of new research by the Minnesota DNR’s fisheries systems ecologist, Gretchen Hansen.
Hansen had the floor during the fisheries portion of the Minnesota DNR Roundtable at the DoubleTree hotel in Bloomington, where invited stakeholders spend the day discussing and listening to presentations on various conservation issues.
Hansen, presenting preliminary findings of research she has done since joining the Minnesota DNR in 2016, said that while water temps have largely stayed the same, the biggest change has been in “optical habitat.”
“Walleyes have a very clear preference for water that is stained or turbid,” Hansen said. “They don’t like clear water.”
The lake’s Secchi disc readings jumped in the late 1990s, with improved clarity of about 6 to 8 feet. That has left walleyes with less-suitable habitat, she said, noting that there was a big drop in key habitat as a result. Water clarity has continued to fluctuate with even more clarity at a new normal – and clearer – range since. “That is a big difference,” she said.
Hansen noted research in Ontario that showed walleyes have particular preferences for temperature and light conditions.
“The more overlap you have, the more walleyes you are going to get,” she said, noting that the lake’s thermal habitat has remained relatively suitable in the lake during the same period of time. “In general, it has not gotten too warm over time.”
There’s a theory that suggests the clearer water could be pushing young walleyes out to deeper water, where they are more susceptible to being eaten by northern pike and adult walleyes. Biologists have long been vexed as to why so few year-classes, despite good larval numbers, have recruited past age 1 on Mille Lacs.
DNR Fisheries Chief Don Pereira has often stated that clearer water in the lake is a trend that began before invasive zebra mussels were discovered in the lake. He’s in the past said those changes came about because of the federal Clean Water Act, and at the Roundtable, he pointed to improved septic systems around the lake for playing a role in the clearer waters.
Largemouths vs. walleyes
Hansen also reviewed a published research paper she authored when she was a fisheries research scientist for the Wisconsin DNR.
That paper investigated a trend seen in the Badger State where walleyes were declining in some of the same lakes where largemouth bass populations were increasing.
“Because they were happening at the same place at the same time, there was a thought like maybe the bass were eating all of the walleyes, a direct interaction,” Hansen said, stressing that correlation does not equal causation.
“It takes a little more careful study to tease apart the actual causation and look at what else might be going on to cause these things,” she said.
The research found there was a temperature threshold that favored largemouth bass and hurt walleye reproduction.
“There is this sort of tipping point where when it reaches a certain temperature, the probability of supporting walleye reproduction goes way down, and at the same time, the probability of supporting large numbers of largemouth bass went up.”
It’s a foreboding conclusion, but it gives fisheries managers some useful tools, considering the effects of climate change.
“We can look at these individual lakes, and not just on a statewide scale, and think about how we might want to manage these lakes given the changes we are seeing.”
Large lake research
Hansen gave an early preview of a large lakes study she’s heading up, looking at the effects invasive zebra mussels and spiny water fleas might be having in the food web of walleyes and yellow perch on Minnesota’s large lakes.
The research started last summer, with surveys sampling every part of the food web – from zooplankton to bugs and invertebrates to little and big fish.
Hansen, regarding zebra mussels, is trying to look at how those invasive crustaceans might be transferring energy up the food web.
Last summer, Hansen’s team surveyed Leech, Upper Red, and Mille Lacs lakes. This summer, it will get around to Lake of the Woods, Rainy, Kabetogama, Cass, and Winnibigoshish lakes.
“Then we will put it all together and look at how the food webs of these large lakes are being interrupted by zebra mussels and spiny water fleas,” she said.