Date Published: June 7, 2018
Publisher: Public Library of Science
Author(s): James H. Larson, Ryan P. Maki, Benjamin A. Vondra, Kevin E. Peterson, Judi Hewitt.
Water level (WL) fluctuations in lakes influence many aspects of ecosystem processes. Concern about the potential impact of WL fluctuations on fisheries was one of the factors that motivated the decision in 2000 to alter the management of WL in the Rainy-Namakan reservoir complex (on the border between the U.S. state of Minnesota and the Canadian province of Ontario). We used a Before-After, Control-Impact (BACI) framework to identify potential impacts of the change in WL management to Walleye, Northern Pike and Yellow Perch catch per unit effort (CPUE). The CPUE of these species from 1990–1999 and from 2005–2014 were compared in four impact lakes (Lake Kabetogama, Namakan Lake, Rainy Lake and Sand Point Lake) and two control lakes (Lake of the Woods and Lake Vermilion) using a simple Bayesian model. Changes in fish CPUE in the impact lakes were often similar to changes that occurred in at least one control lake. The only change that was not similar to changes in control lakes was an increase of Yellow Perch in Lake Kabetogama. The two control lakes often differed substantially from each other, such that if only one had been available our conclusions about the role of WL management on fisheries would be very different. In general, identifying cause-and-effect relationships in observational field data is very difficult, and the BACI analysis used here does not specify a causative mechanism, so co-occurring environmental and management changes may obscure the effect of WL management.
Humans have frequently altered the water-level fluctuations in natural and artificial water bodies, and these water-level fluctuations may influence fisheries through a wide variety of mechanisms . For Walleye and Northern Pike, the suitability and accessibility of spawning and nursery areas may be impacted by water level regimes [2,3]. For generalist species such as Yellow Perch (Perca flavescens), higher water levels may simply indicate an increase in available habitat . Water levels may also impact fish species indirectly by altering the composition or productivity of primary producers [5–7] or the invertebrate community [8–10].
The primary objective of this study was to identify whether a change in water level management in the Rainy-Namakan complex was associated with changes in an index of Walleye, Northern Pike and Yellow Perch abundance. When specific mechanisms that might drive changes in fish abundance are well understood and amenable to modeling, the specific effects of water level fluctuations can be evaluated using specific statistical models . However, water level fluctuations have an array of ecological effects, and it is difficult or impossible to model the aggregate effect of these changes. In the before-after, control-impact analytical framework (BACI), evidence for the impact of a known change can be evaluated without mechanistic modeling . In the BACI analysis completed here, the impact was the change in water level management that occurred in 2000 and altered water levels in Lake Kabetogama, Namakan Lake, Sand Point Lake, and Rainy Lake . This analysis suggests that Yellow Perch CPUE was positively impacted by this change in water level management in Lake Kabetogama.