Study Says “Hidden Overharvest” From Fishing Plays a Role in Wisconsin Walleye Declines

by Adam Hinterthuer

Holly Embke, a CFL graduate student working in Jake Vander Zanden's lab, led the study
Holly Embke, a CFL graduate student working in Jake Vander Zanden’s lab, led the study

Over the last few decades, annual walleye production in many of Wisconsin’s 900 or so “walleye lakes” has declined by 35 percent. Walleye stocks also now take one and a half times longer to replenish themselves. State fisheries managers have responded by changing angler regulations to protect large female walleye and stocking hatchery-raised fish in struggling lakes, among other things, but these efforts didn’t reverse the broader walleye decline.

Despite this trend, the fish remains as popular as ever with the public. And, while fewer individual fish are being caught, the percentage of walleye that state and tribal resource managers allow to be harvested each year has stayed about the same.

Considering the cultural and economic importance of this inland fishery, it’s time to reassess current regulations, says CFL graduate student, Holly Embke.

Embke was the lead author of a study published this November in the Proceedings of the National Academy of Sciences. That study 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 thirty years, resource managers have focused on fish abundance and not fishery productivity when calculating harvest limits.

WDNR fisheries biologist (and CFL alum), Dan Oele, takes measurements of a walleye during a spring sampling trip. Photo: Adam Hinterthuer
WDNR fisheries biologist (and CFL alum), Dan Oele, takes measurements of a walleye during a spring sampling trip.
Photo: Adam Hinterthuer

One way to think of it, Embke says, is in terms of a bank account. “Abundance tells you the money in the bank while production tells you the interest rate,” she says. If you start taking more money out of your account than the interest rate contributes each year, your savings shrink. Do this several years in a row, and those annual withdrawals begin to have an outsized impact on what little money is left in the bank.
Using data that are collected already by Wisconsin Department of Natural Resources (WDNR) and Great Lakes Indian Fish and Wildlife Commission (GLIFWC) researchers, 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 4 percent estimates generated when fisheries managers consider abundance alone.

What’s more, Embke says, the study found great variation in walleye production from lake to lake. Some lakes remain walleye strongholds and can handle current fishing pressures, while others can’t sustain even current average harvest rates of 15 to 20 percent, much less the 35 percent harvest benchmark. By considering production, fisheries managers may be better equipped to set limits for individual lakes.

These results, the researchers write, “highlight the urgent need for improved governance, assessment, and regulation of recreational fisheries in the face of rapid environmental change.”

The good news is that data fisheries managers already collect can be plugged in to Embke’s method for estimating production and help chart a way forward. 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.

Full paper – Production dynamics reveal hidden overharvest of inland recreational fisheries