What’s driving the Wisconsin walleye decline?

The study's lead author, Sarah Janssen, said that "If an angler is catching a walleye or something else offshore, and it fed near the contaminated zones, it's a potential that they have higher mercury and that mercury is from legacy contamination." (Minnesota DNR)

Arguably the most prized fish in Wisconsin, walleye hold a cultural significance that reaches far beyond being a thrilling fish to catch and a delicious fish to eat for the spear fishers and recreational anglers who harvest them.

But walleye populations have been declining for the better part of two decades.

While walleye at a Friday night fish fry haven’t come from Wisconsin in many years, they remain an important food source and tradition for Wisconsin tribes and part of an economically significant pastime – recreational fishing brings in more than $2 billion annually to the state.

Estimates say the sharp-tooth predator’s production dropped nearly 30 percent between 1990 and 2012 and takes 1.5 times as long to grow to the same size and weight as it did in 1990.

The Great Lakes Indian Fish and Wildlife Commission (GLIFWC) identified walleye as a moderately to extremely vulnerable species in Wisconsin in its recent vulnerability assessment report.

What’s driving their decline?

Researchers point to climate change as a pervasive culprit, but it’s a complicated story with a lot of question marks, Wisconsin Public Radio reported.

Lakes are complex ecosystems and Wisconsin is home to a diverse variety – lake size and depth, water clarity and surrounding tree cover all influence how lakes respond to climate change.

And temperature affects every corner of a lake ecosystem.

Researchers know Wisconsin lakes aren’t too warm for walleye, a cool-water fish, to survive. They suspect it’s a recruitment (surviving to maturity) issue that has more to do with food sources and what species has the competitive edge.

Challenges in early life

Just how rising temperatures and shorter winters are affecting walleye reproduction is an open research question, Gretchen Hansen, assistant professor in the Department of Fisheries, Wildlife and Conservation Biology at the University of Minnesota-Twin Cities, said, but it’s clear the first summer of a walleye’s life is the most precarious.

The walleye hatch early in the summer, but by the end of the summer large numbers have disappeared – what happened between those events has confounded researchers.

Timing is everything. Hansen said there are theories that young walleye aren’t matching up with their food source at the correct time because of changing lake ice patterns, driven by an increase in erratic warm and cold snaps during winter and spring.

David Bissonette, a Lac Courte Oreilles Tribe member and spear fisher, said spear fishers see lakes in a different way than others. They go out at night, with headlamps, in the springtime right after the ice melts.

When Bissonette was a kid, he associated walleye harvesting on Lake Chetac in Sawyer County with his cousin’s birthday, April 19, he said.

Walleye in Wisconsin generally spawn between mid-April and early May, and when the timing has been off, he hasn’t reached his walleye quota, he said.

Shift in balance of power

At the same time walleye are struggling, another predator species appears to be thriving: the largemouth bass.

Small changes in temperature have a big impact, said Hansen.

But exactly how those changes in temperature affect walleye habitat and reproduction are difficult to tease out. That’s the nature of climate change, she said, it isn’t straightforward.

Wisconsin has already warmed by about 2 degrees Fahrenheit since 1950, and is expected to rise an additional 2 to 8 degrees Fahrenheit by 2050. Increases in extreme and unpredictable weather events and seasonal variability have already shown themselves to be side effects of climate change.

Water temperatures are slower to rise, but some Wisconsin lakes have seen temperatures increase by more than 1.5 degrees Fahrenheit since 1980. And as water temperatures continue to climb, a relatively small number of lakes are expected to support walleye natural reproduction.

The same temperature changes that are making natural reproduction more risky for walleye are tipping the scales in favor of largemouth bass.

Walleye have a competitive advantage in low-light, stained water with limited plant growth. Largemouth bass thrive in warm water and prefer clear water with an abundance of aquatic vegetation.

And while it may not be intuitive, rising temperatures are tied to clearer lakes _ warmer and clearer lakes force walleye to swim deeper to be comfortable and reduce their habitat and range for food.

Largemouth bass will likely become more abundant as the climate changes, said Aaron Shultz, climate change inland fisheries biologist at GLIFWC.

But to date, there’s no direct evidence walleye’s decline comes down to competition between the two predator species.

Categories: News, Walleye

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