WAHKON, Minn. — The Mille Lacs Band Department of Natural Resources is trying to learn what’s killing the lake’s juvenile walleye population.
Researchers with the Minnesota DNR have conducted seven walleye population estimates since 2002. Results range from 1.1 million fish in 2002 to a low of 249,000 in 2014. The most recent estimate, completed in 2018, projects the population to be 727,000 walleyes.
Adult walleye numbers have rebounded, but Carl Klimah, a fisheries biologist with the Mille Lacs Band DNR, is concerned about the number of juvenile walleye in the lake.
“Put simply, we have a ‘recruitment’ problem,” Klimah said. “There are not enough young walleye surviving until they’re 11-13 inches long. At that size, the risk of predation drops considerably and the fish is considered recruited into the population.”
He also noted that the current population estimate is dominated by a strong year class in 2013.
“We’re not sure why that year class was so strong,” Klimah said. “It was a cold spring and a late ice-out, which might mean more stained water and more zooplankton that aided the survival of walleye fry.”
What’s killing small walleyes?
Klimah said that many factors are likely responsible for the juvenile walleye decline.
“Mille Lacs Lake is clearer and warmer than ever before,” he added. “The lake began to clear in the 1990s, likely due to improved septic systems and an overall reduction in man-made pollutants entering the lake.”
Invasive species changed the lake even more. Zebra mussels filter vast amounts of plankton from the water, leaving less food for zooplankton and other aquatic organisms. Spiny water fleas are voracious predators that feast on other zooplankton — a critical food source for baitfish and small gamefish.
The clearer water has likely changed baitfish behavior, Klimah said, and made them harder for walleyes to catch. Walleyes have a predatory advantage in low light conditions, but that advantage is neutralized in clear water.
Kilmah added that increased water clarity and temperature have reduced the amount of suitable walleye habitat.
“Both juvenile and adult walleye prefer slightly turbid water about 68 degrees,” he said. “Increased water clarity and warming temperatures have reduced the amount of optimal habitat, forcing adults and juveniles into smaller areas.”
According to a study conducted in 2014 by Tyler Ahrenstorff with the Minnesota DNR, adult walleyes were the chief predator of juvenile walleyes in Mille Lacs.
“That’s a natural part of the system,” Klimah said. “It’s a strategy many fish species employ to prevent overpopulation. Cannibalism becomes a problem, though, when it reduces the overall walleye population.”
Follow the fish
In order to prove what’s happening, researchers needed to put ears in the lake. The Mille Lacs Band DNR, in partnership with the Great Lakes Indian Fish and Wildlife Commission and the U.S. Fish and Wildlife Service planted 61 telemetry receivers in a grid pattern across the lake and another 12 in streams that flow into or out of Mille Lacs.
Beginning in July 2018, crews began to capture and implant fish with small transmitters. Receivers can detect tagged fish up to half a mile in any direction, providing thorough coverage in each section of the lake and all types of bottom composition.
In total, researchers captured 70 adult walleye (18-28 inches), 70 juvenile walleye (7-11 inches), 20 northern pike (19-43 inches) and 20 yellow perch (5.5-10 inches). They will be back on the water this fall to implant transmitters in 20 tullibee.
“We’re using two sizes of transmitters,” Klimah said. “One for fish larger than 18 inches and one for smaller fish. The battery on the larger units lasts for two to three years. We expect half of the batteries in smaller units to be dead within eight months.”
To provide year-round tracking of juvenile walleyes, researchers split their tagging efforts across two seasons: 35 fish were tagged in the fall and 35 in the spring.
The receivers, along with sensors that record water temperature and light penetration, collect data for several months before being retrieved by fisheries crews.
“We bring the units back to our office to download data and replace the batteries,” Klimah said. “Each of the receivers is then returned to its original location to begin collecting again.”
Klimah and other researchers at the Band DNR eventually hope to document the movement and behavior of tagged walleyes, pike, perch and tullibee across the lake and throughout the year. This will allow them to identify the type of habitat preferred by juvenile walleyes that overlaps with their predators.
Last spring, researchers retrieved the receivers to collect the first round of data. Between July 2018 and May 2019, the receivers recorded almost 1.5 million individual interactions with tagged fish.
“That’s a lot of information to analyze,” Klimah said, “and it will likely take many months of work. Winter is our number crunching season. And by the time we’re done, it will be time to retrieve more data.”
Asked if a preliminary look at the data revealed any surprises, Klimah noted how shallow adult walleyes held during midsummer, often suspending over deeper water.
“We often find them suspended at 10 feet over water 23 to 39 deep,” he added, “often in water temperatures warmer than their comfort zone. Some adults stay near the area where they spawned, others move from one side of the lake to the other.”
Klimah hopes the study will provide resource managers more clarity on how to protect juvenile walleye.
“Good year classes are fewer and farther between,” he said. “Large lakes need a strong year class every three or four years. At Mille Lacs, we’re averaging seven years between productive hatches. That’s not sustainable.”