Midwest Fish & Wildlife Conference 2018

AUTHORS. Gretchen J.A. Hansen, Minnesota Department of Natural Resources; Jordan S. Read, US Geological Survey; Luke A. Winslow, Rensselaer Polytechnic Institute; Melissa Treml, Minnesota Department of Natural Resources; Patrick J. Schmalz, Minnesota Department of Natural Resources; Stephen R. Carpenter, University of Wisconsin-Madison

ABSTRACT. Water clarity and temperature are key lake attributes that influence numerous ecological processes. Walleye (Sander vitreus) are an economically important top predator fish with distinct temperature and light preferences. Walleye production in inland lakes is related to the area in which the optimal thermal and optical conditions for walleye exist concurrently, known as the thermal-optical habitat area (TOHA). Lake-specific estimates of TOHA require information on lake morphology, temperature, and clarity. We evaluated the role of changing clarity and temperature in explaining the decline of a high-profile walleye fishery in Lake Mille Lacs, Minnesota. TOHA was estimated using a thermodynamic simulation model of daily water temperatures and hourly light conditions from 1980-2016. We used the safe operating space concept to analyze how sustainable harvest and optimal population size of walleye depend on TOHA. Optical habitat area in Mille Lacs is directly related to Secchi depth, with maximum optical habitat available at Secchi depths between 2 and 3.5 m. Median Secchi depths consistently fell within this optimal range prior to 1996, but exceeded this range in over 50% of years from 1996 to 2016, resulting in reduced habitat for walleye. Maximum safe harvest levels of walleye declined with declining TOHA, suggesting that walleye harvest must be reduced to accommodate increasing water clarity and to a lesser extent, water temperature. Historic and projected future changes in water clarity and temperature are likely to impact sport fish populations, and future work is underway to expand these analysis to other inland lakes.