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This story appeared in the Spring 2020 Letters & Science magazine.

Over the last few decades, walleye in Wisconsin have been on a downward trend. As lakes in the upper Midwest warm due to climate change, this cool-water species has found itself with less habitat in which to thrive. Yet the percentage of walleye that state and tribal resource managers allow to be harvested each year has stayed about the same.

In the late 1980s, the Wisconsin Department of Natural Resources (DNR) and the Great Lakes Indian Fish & Wildlife Commission worked together to set sustainable harvest limits on walleye for both recreational and tribal fisheries.

Today, more than one million recreational anglers account for about 90 percent of the total annual harvest on the state’s 900 “walleye lakes.” The other 10 percent comes from the 450 tribal members who spear walleye on roughly 175 lakes each spring.

L&S Center for Limnology graduate student Holly Embke was lead author of a study published in November in the Proceedings of the National Academy of Sciences, which found that “40 percent of walleye populations are overharvested, which is ten times higher than the estimates fisheries managers currently use,” she says.

A big reason for this “hidden overharvest,” says Embke, is that, for the last 30 years, resource managers have focused on fish abundance and not fishery productivity when calculating harvest limits.

For her study, Embke says she wanted to take a more nuanced approach.

“We wanted to ask not only how many fish are in a lake, but also consider how fast they’re growing, how big they are, and how many are produced every year,” she says.

Using data that state and tribal researchers had already collected, Embke and her colleagues calculated how walleye biomass had changed over a 28-year period in 179 lakes. Measuring biomass is akin to throwing all of the walleye in a lake on a scale and recording the overall weight. Production is a reading of how much biomass grows each year, an indication of a population’s ability to replenish its losses.

By comparing walleye production to the total fishery harvest in these study lakes, they found that overharvest is ten times higher than the four percent estimates generated when fisheries managers consider abundance alone.

By better understanding the resilience of Wisconsin walleye populations and by acknowledging the role that anglers play in reducing stocks, the future of this iconic fishery just may have a fighting chance.