Mimicking Nature: How water level management on a large reservoir affects walleye (Sander vitreus) spawning habitat

Abstract

The Namakan Reservoir, located on the Minnesota-Ontario border, has been successively managed under two water-level management policies (‘rule curves’) in recent decades. To compare the effects of the rule curves on walleye spawning, I modeled wave energy, ice scour, and habitat availability at 44 spawning locations. Wave energy increased (18% for observed water levels, 6% for modeled water levels) and ice scour decreased 11% (both P < 0.01) over spawning habitat during the most recent rule curve. Observed water level data suggested available spawning habitat on Lake Kabetogama increased 95% (P < 0.01), but availability on Namakan and Sand Point Lakes was unaffected. However, when controlling for weather events, habitat availability increased significantly (Kabetogama = 179%, Namakan = 72%, Sand Point = 93%, P < 0.01) on all three lakes. These findings suggest that the most recent rule curve is likely to improve reproductive success for walleye in the reservoir.