FOR IMMEDIATE RELEASE: 2/26/24 – Walleye are one of the most sought-after species in freshwater sport fishing, a delicacy on Midwestern menus and a critically important part of the culture of many Indigenous communities.
They are also struggling to survive in the warming waters of the Midwestern U.S. and Canada.
According to a new study published today in the journal Limnology and Oceanography Letters, part of the problem is that walleye are creatures of habit and our seasons, especially winter, are changing so fast that this iconic species of freshwater fish can’t keep up.
The timing of walleye spawning has historically been tied to the thawing of frozen lakes each spring, says the report’s lead author, Martha Barta, a research intern at the University of Wisconsin-Madison. Now, thanks to our changing climate, the species has been “unable to keep up with increasingly early and more variable ice-off dates.”
Within a few days of ice-off, walleye begin laying eggs and fertilizing them. That timing, in a normal year, sets baby fish up for success once they hatch. But, according to Barta, “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.”
Barta and her team used data from walleye surveys from various state natural resource departments and the Great Lakes Indian Fish and Wildlife Commission, as well as spring harvest counts from several Ojibwe Tribal Nations, to track the fate of walleye populations on 194 lakes across Minnesota, Wisconsin and Michigan. That data revealed “mismatches” in almost every single lake. While there has been a slight shift to earlier spring spawning dates for walleye, the ice-off dates on Midwestern lakes was shifting at a rate three times faster.
More and more, the timing is all wrong for walleye, explains Zach Feiner, a fisheries scientist with both the UW-Madison Center for Limnology and the Wisconsin Department of Natural Resources.
“In an average ice off year, you have this nice progression of events,” Feiner says. “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.”
But lately, Feiner says, the timing of yearly thaws has gotten “weird.” Lakes are, on average, thawing earlier, but the number of winters where lakes thaw late is also increasing. What’s being lost are the nice, normal, “average” years when lakes thaw right about when they used to.
The problem with the “weird” years, Feiner says, 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 walleye don’t have enough food to survive,” Feiner says.
In late ice off years, there’s a similar dynamic, where everything that is supposed to flow in a nice, orderly progression gets all mixed up.
This mix up impacts walleye recruitment, or the survival of baby walleye through their first spring and summer of life. On a year-to-year basis, this isn’t necessarily a problem, as adult walleye can always spawn again the next year, when conditions may be more favorable and more of their offspring can survive and increase the population. But, Feiner says, the heightened 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.”
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.
Then there is also the question of what our “weird” winters mean for other species of fishes.
“Most of our big time sportfish species [in the Midwest], like walleye, perch, pike, bass, bluegill and muskies spawn in springtime,” Feiner says. What’s more, other species like lake trout and whitefish spawn in the fall and their eggs overwinter under the ice.
While his team started with walleye, because of the extensive research being done on them, :the next thing I want to do is start to build a bigger dataset for these other species and see if this is a general pattern or if there are species more resilient to the changes we’re seeing,” Feiner says.
ARTICLE AVAILABLE AT: https://aslopubs.onlinelibrary.wiley.com/doi/full/10.1002/lol2.10383
MEDIA CONTACTS:
Zach Feiner (corresponding author) – zsfeiner@wisc.edu
Adam Hinterthuer (Center for Limnology communications) – hinterthuer@wisc.edu, 608-630-5737