Water Level Regulations and Fisheries in Rainy Lake and the Namakan Reservoir

Abstract

The difference between the yearly maximum and minimum water levels (YMXR) is an index of lake dynamics: shoals are exposed and inundated, nutrients are oxidized and reduced, and the diversity and density of the aquatic plant community are affected. Shoals and emergent macrophytes provide spawning habitat for fish. The 5-yr moving variance of YMXR fluctuates regularly with periods of about 11.2 yr (periodicity of sunspot cycles). This reflects the effects of within-year consecutive periods of storms and dry spells. Water level regulations resulted in changes in both amplitudes and frequencies of YMXR compared with natural fluctuations. We established links between fluctuations in YMXR and fluctuations in fish populations. Water level regulations, through their effects on YMXR, corresponded to changes in interspecific interactions on Rainy Lake and the Namakan Reservoir. In both, walleye's (Stizostedion vitreum) fluctuations were synchronized with both those of lake whitefish (Coregonus clupeaformis) and northern pike (Esox lucius) more than those of either species with the other two. On the Namakan Reservoir, YMXR fluctuations were accentuated by water level regulation; on Rainy Lake, they were dampened. Regulations should consider frequencies and amplitudes of changes in water level and their effect on fish populations.