Recent research by a team of scientists reveals that walleye decline in Mille Lacs is linked to loss of habitat and indicates that by adapting harvest policies to account for changing environmental conditions, walleye fisheries may be sustained.

The study focuses on Mille Lacs, Minnesota, where walleye populations have dramatically declined since the 1990s. Due to the lakes’ popularity and economic importance, strong social and political pressures exist to restore walleye in Mille Lacs to support previous levels of harvest.

“As the water has gotten clearer in Mille Lacs, the area of the lake with suitable walleye habitat has gotten smaller,” said CFANS Fisheries, Wildlife and Conservation Biology Assistant Professor Gretchen Hansen. “Because Mille Lacs is fairly shallow and uniform in depth, the walleye cannot retreat to deeper water as they can in some clear lakes and the lake can support fewer walleye. Therefore, harvest has to respond to avoid over-exploiting walleye under new habitat conditions.”

The study, recently published in Ecosphere, details how researchers linked changing walleye habitat to sustainable harvest by combining decades of monitoring data, hydrodynamic modeling, and statistical modeling. Results show that walleye habitat as measured by thermal and optical conditions have declined as water has gotten clearer in Mille Lacs, and identifies tools for linking harvest policies to habitat in order to sustain walleye populations even as conditions become less favorable.

“The good news is that we have identified tools for adapting harvest policies to maintain safe, sustainable walleye populations even in the face of environmental change,” Hansen said.

Altering harvest in response to changing conditions may allow Mille Lacs to retains its function as a walleye fishery. The model can be used to estimate water conditions that could be run annually and used to adjust harvest in response to changing water clarity and temperature. Continued monitoring of water clarity and temperature is relatively inexpensive and is already a part of standard monitoring of Mille Lacs and many other lakes.

Sparked by these findings, MN DNR and U of MN researchers are collaborating on a new project to assess walleye habitat in other Minnesota lakes and its sensitivity to changing water quality and temperature. This project is ongoing and scheduled to be completed by the end of 2020.

Research involved the work of a team including Luke A. Winslow, Rensselaer Polytechnic Institute; Jordan S. Read, U.S. Geological Survey; Melissa Treml, Minnesota DNR; Patrick Schmalz, Minnesota DNR; and Stephen R. Carpenter, Center for Limnology, University of Wisconsin-Madison.

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