Walleye are struggling to adapt to rapid seasonal changes

Walleye, a prized catch for freshwater sportfishing enthusiasts and a staple in Midwestern cuisine, also play a vital role in the cultural traditions of many Indigenous communities. 

However, according to a new study led by the University of Wisconsin-Madison, walleye populations are now facing challenges due to the increasing temperatures in the waters of the Midwest United States and Canada.

Inability of walleye to adapt

The experts shed light on one important aspect of the problem: walleye’s inability to adapt to rapidly changing seasonal conditions, especially the shift in winter’s timing. 

According to lead author Martha Barta, a research technician at the UW-Madison, the timing of walleye spawning (when the fish mate and lay their eggs) has historically been tied to the thawing of frozen lakes each spring.  Recently, due to climate change, walleye have been “unable to keep up with increasingly early and more variable ice-off dates.”

Disrupted spawning season

The study reveals that the crucial period following the melting of a lake’s ice, which signals the start of the walleye spawning season, is being disrupted by climate change. 

“Climate change is interrupting the historical pairing of ice-off and walleye spawning, and that threatens the persistence of walleye populations across the Upper Midwest,” Barta said, emphasizing the urgency of the situation.

Universal mismatches

To understand the impact of these changes, Barta and her team analyzed data on walleye populations from 194 lakes across Minnesota, Wisconsin, and Michigan. 

The researchers discovered almost universal “mismatches” between the timing of ice melt and spawning, with the rate of change in ice-off dates outpacing the slight adjustments in spawning times three times faster.

A delicate balance 

Zach Feiner, a fisheries scientist affiliated with both the UW–Madison Center for Limnology and the Wisconsin Department of Natural Resources, explained the delicate balance that normally exists. 

“In an average ice-off year, you have this nice progression of events,” said Feiner. “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.”

Increasingly unpredictable timing

However, the study found that the timing of these ecological events is becoming increasingly unpredictable, often leaving newly hatched walleye without the necessary zooplankton food sources. 

Feiner noted the “weird” nature of recent yearly thaws, with lakes thawing earlier on average but also experiencing an increase in late-thawing winters.

Broader implications for walleye

The implications of this research are significant not only for walleye but also for the broader ecosystem and human communities that rely on them. 

Feiner highlighted the importance of identifying lakes that can act as refuges during challenging years and the potential for management strategies to bolster walleye resilience in the face of climate change.

“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,” he explained.

Furthermore, the study prompts questions about the effects of changing winter patterns on other fish species that spawn in the spring, underscoring the need for expanded research to understand and mitigate the impacts of climate change on freshwater ecosystems.

The study is published in the journal Limnology and Oceanography Lessons.

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