A newly released study found that walleye in Lake Mille Lacs consistently return to the same spots every year to spawn — preferably places with little or no shoreline development.
Researchers have tried to figure out why the central Minnesota lake’s walleye population declined in recent decades. The latest study suggests protecting certain places in the lake where walleye reproduce could boost their success.
Mille Lacs is one of the most heavily fished walleye lakes in Minnesota. It’s important both for recreational anglers and as a cultural and food resource for several Ojibwe tribes that retain harvesting rights under an 1837 treaty.
Previous studies found one of the primary causes of walleye’s decline is too few fish fry surviving to adulthood, said Kayla Lenz, a fisheries technician at the Great Lakes Indian Fish and Wildlife Commission. The commission partnered with the Mille Lacs Band of Ojibwe on the multi-year study.
Mille Lacs’ adult walleye “go out and spawn every year, but the percentage of the eggs that they lay that become adult fish is low,” said Lenz, who’s also a doctoral student at the University of Illinois Urbana-Champaign.
“So we want to make sure that these very early life stages have their greatest chance of success and survival,” she said.
Newly hatched walleye stay in spawning areas for a few weeks before they swim out into the larger lake, Lenz said. If the “nursery habitat” is good, they have a better chance of surviving longer, she said.
“The more fry that we can have, the more potential adult fish that we have,” Lenz said.
Tracking walleyes’ movement
Researchers placed a grid of more than 60 acoustic receivers at the bottom of Mille Lacs Lake in summer 2018. Then, they surgically implanted transmitters in about 70 adult walleye.
When the fish swam near one of the receivers, the transmitter recorded its location, depth, temperature and other information. The researchers collected data each spawning year from 2019 through 2021.
They found 96 percent of the walleye detected returned to the same spawning area year after year — “a really high percentage,” as Lenz put it.
Researchers also identified three “hot spots” in Mille Lacs that are especially important spawning grounds. Those tended to be places with hard bottoms of sand or gravel, so walleye eggs were less likely to get buried in muck.
They’re also spots with a lot of wind and wave action, which helps scatter the eggs in a wider area, Lenz said.
Undeveloped shoreline is key
One of the key takeaways is shoreline development and alteration reduces walleye spawning habitat, Lenz said.
In all three of the spawning hot spots, the lakeshore was virtually undisturbed with little to no human development, such as houses, docks or marinas. Those untouched stretches of shoreline are relatively rare on Mille Lacs.
“Human activities on shore can cause stressors and damage the environment and inhibit spawning success,” Lenz said. “So it seems like they're kind of searching for what areas can have fewer of those stressors.”
For example, removing trees and other vegetation along the shore can increase erosion. The dirt that washes into the lake can suffocate fish eggs, Lenz said.
Fertilizer applied to a lawn can wash into the lake and spur plant growth, which also can lessen walleye’s spawning success, she said.
If property owners are interested in helping protect spawning areas, they should maintain vegetation and limit their use of fertilizer and pesticides along the lakeshore, Lenz said.
They also should wait to install their dock until after walleye spawning season, which usually starts after the ice recedes and lasts for three to four weeks, she said.
Protecting spawning spots
The study recommended policy makers create conservation areas on or near the shoreline to protect walleye spawning areas in Mille Lacs.
To have abundant walleye for recreational anglers and tribes, “we need to help them get there and survive to adulthood,” Lenz said. “We can do that by helping to protect these spawning and nursery habitats.”
In a news release about the study, Kelly Applegate, natural resources commissioner for the Mille Lacs Band of Ojibwe, said the band cares deeply about the habitat of Ogaawag, the Ojibwe word for walleye.
“By learning more about important Ogaawag spawning sites, we can obtain valuable knowledge, allowing us to further build on our centuries-old reputation for successfully and sustainably managing our fisheries resources,” Applegate said.
The study was published in the journal Frontiers in Fish Science. Its findings are useful not only for Mille Lacs Lake, but other lakes as well, Lenz said.
She said researchers are working on the next phase of the study, which involves getting more detail about the tagged walleye’s movements and behavior in the spawning hot spots.
Researchers acknowledged they tagged a relatively small number of walleye compared to Mille Lacs’ overall population, so there are likely other popular spawning locations around the lake.
Minnesota Department of Natural Resources officials said Mille Lacs’ walleye population has shown signs of recovery in recent years, in part due to stricter fishing restrictions.
As a result, the department has eased those regulations slightly. Anglers on Mille Lacs can keep up to three walleye of a certain size this summer.
