Walleyes, the favorite game species in many Midwest lakes and Minnesota’s official state fish, are struggling to spawn successfully due to warming springs and highly variable ice-out dates, creating more bust years and fewer boom years for many walleye populations.

That was the finding of a University of Wisconsin study published this week in the journal Limnology and Oceanography Letters.

The problem stems from walleye being creatures of habits that developed over millennia and which can’t keep up with changing climatic conditions, especially increasingly earlier and variable ice-out dates.

The timing of walleye spawning has historically been tied to the thawing of frozen lakes each spring, said Martha Barta, a researcher at the University of Wisconsin-Madison and lead author of the study.

Fisheries biologists and volunteers survey walleye size during a spawning run on the St. Louis River in Minnesota in 2019. A new study has found that the generally earlier but highly variable ice-out dates that have occurred in recent decades are hurting walleye spawning success in Minnesota, Wisconsin and Michigan waters. (Steve Kuchera / Duluth News Tribune)

Within a few days of ice-out, walleyes begin laying and fertilizing eggs. That timing, in a normal year, sets tiny walleye fry up for success once they hatch. But Barta said the study found that “climate change is interrupting the historical pairing between ice-off and walleye spawning, and that threatens the persistence of walleye populations across the Upper Midwest.”

“We’ve known for several years that many walleye populations are not doing well. Now, this adds the phenology piece where we found that either a very early ice-out or very late ice-out in the spring, we didn’t see good walleye classes in the fall,’’ Zach Feiner, a co-author of the study and a fisheries biologist for both the University of Wisconsin-Madison and the Wisconsin Department of Natural Resources, told the News Tribune. “The more the ice-out varied from a normal year, the worse the walleye class was that year.”

How much variability is occurring? In 2012, many Northland lakes had their earliest ice-out date ever. In 2013, many of the same lakes saw their latest ice-out dates on record. And while ice-out dates have always shifted by a week or so early or late, the variability now is running four to six weeks from year to year, all while the long-term trend is earlier.

Barta and her team used data from walleye surveys from state natural resource departments and the Great Lakes Indian Fish and Wildlife Commission, as well as spring harvest counts from several Ojibwe tribal resource agencies, to track the fate of walleye populations on 194 lakes across Minnesota, Wisconsin and Michigan.

The researchers also had fall fish survey data from 122 of the lakes that showed how many walleye from that year were still around.

That data revealed “mismatches” in almost every lake. While the study found walleyes were slightly shifting their spawn earlier, the ice-off dates on Midwestern lakes were shifting at a rate three times faster than walleye spawning dates.

And that has the system mixed up.

“In an average ice-off year, you have this nice progression of events,” Feiner noted. “The ice goes off, you get light and warmer water that creates a bloom of small plant life called phytoplankton and then tiny animals called zooplankton emerge and eat the phytoplankton and, usually, the walleye spawning is timed for them to hatch when zooplankton are around in high abundance and can serve as fish food for the baby walleye.”

Now, lakes are on average thawing earlier, but the number of winters where lakes thaw late is also increasing. What’s being lost are the normal, “average” years when lakes thaw right about when they used to.

The problem with the increasing number of early and late ice-outs, Feiner said, is that the “progression of events is totally out of sync.” In an early ice-off year, for example, phytoplankton bloom early and begin to die back as zooplankton get going, which means there’s less food for zooplankton and their numbers are so low that “when the fish hatch, there aren’t enough zooplankton around and (newly hatched) walleye don’t have enough food to survive.”

A new study by the University of Wisconsin has found that walleyes don’t spawn well, producing poor year classes, when the ice-out date on a lake is highly variable from average. (John Myers / Duluth News Tribune)

There’s a similar dynamic in late ice-off years. This mix-up impacts the survival of newly hatched walleye through their first spring and summer of life, called recruitment. Losing one year of spawning success isn’t doomsday for overall walleye populations if the next year’s spawning success is good. But the increasing variability of spring thaws is “increasing the frequency of bust years, and we’re not seeing many or any boom years for a lot of walleye populations,” Feiner said.

While this is obviously bad news for walleye and the people who depend on them, the study underscores the need to identify and protect lakes that can offer refuge in bad years.

There is a need now to “find places where, through management of things we can control — like land use, fish harvest and invasive species — we can buffer or boost their resiliency to be able to handle stuff we can’t control — like climate change,” Feiner says. If fisheries managers can identify lakes where walleye populations are doing relatively well, they can at least try to keep conditions optimal so that the fish can take advantage during the increasingly rare years when ice-off and their spring spawn do line up.

Scientists involved in the study say they hope to move on to determine the impact of changing climate on spawning by several other species, including other spring spawners like perch, bass and muskie, but also fall spawning fish like lake trout that depend on ice for their eggs to over-winter.

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