Measuring critical spawning habitat for walleye on selected lakes in the Namakan Reservoir and assessing how this habitat has been affected by water level management practices

Background/Question/Methods

The most recent water level management policies (rule-curves) in the Namakan Reservoir were established in 2000 by the International Joint Commission (IJC, Supplementary Order dated January 5, 2000) and were intended to strike a balance between benefits to plants and animals in the reservoir and human water needs (e.g., hydroelectric power generation, recreation, and navigation). Walleye (Sander vitreus) are prevalent in the reservoir, sensitive to water levels, and ecologically, culturally, and economically important, making them an excellent species for studying the effects of rule-curves. To compare the effects of two separate rule-curves (1970-2000 and 2000-present), I used spatial modeling to estimate seasonal habitat availability, water depth, and wave energy at various sites within the reservoir. The overall objective of the study was to determine if the most recent rule-curve has improved spawning habitat availability for walleye.

Results/Conclusions

The 2000 rule-curve change caused an increase in water level elevation over spawning sites compared to the previous rule-curve. Increasing water levels in the reservoir earlier during the spring walleye spawning period had varied responses. Gain or loss of available spawning habitat was dependent on the substrate distribution within a site, and the magnitude of the response was determined by the mean slope. During summer, the most recent rule-curve was shown to cause an increase in wave energy potential and air exposure, both factors which can promote reductions in the amount of periphyton and fine sediment over spawning substrates. Overall, the availability of spawning habitat has increased at known spawning sites due to the overlapping depth regimes at which spawning and periphyton removal are occurring; however, further study is required to elucidate disparate responses in walleye productivity in various lakes within the reservoir.