We used published information to determine optimum light and temperature conditions for walleye Sander vitreus (formerly Stizostedion vitreum) and then applied this simple niche definition to predict how water clarity, temperature, and bathymetry affect walleye habitat availability. Our model calculated thermal–optical habitat area (TOHA), the benthic area of a lake that supplies optimum light, and temperature conditions for walleye during an annual cycle. When water clarity is very low, little walleye habitat exists. As water clarity increases, TOHA for walleye initially increases and then declines exponentially. Optimum water clarity increases with maximum depth of the lake or, in the case of thermally stratified lakes, with thermocline depth. We tested this model by evaluating its ability to account for differences in the sustained yields of walleye fisheries on Ontario lakes. Our results demonstrate that (1) walleye harvest increases in proportion to TOHA times the square root of total dissolved solids, an index of nutrient level, and (2) optimum water clarity for walleye typically exists when Secchi depth is on the order of 2 m. These findings indicate that the increases in water clarity recently observed in the Great Lakes basin (as a result of phosphorus control and dreissenid mussel invasion) have reduced the supply of thermal–optical walleye habitat and, consequently have probably had negative effects on walleye production.

Defining objectives for ecological rehabilitation requires consideration of how an ecosystem responds to management. Validated quantitative models of physical, chemical, and biological processes are the best way to project such impacts; however, time, data, and model limitations often make these approaches impractical. An alternative is to encode expert knowledge about interactions among ecosystem components in a fuzzy cognitive map (FCM), which then translates that subjective, qualitative information into predictions of the effects of management on an ecosystem. Herein, we present the steps involved in constructing an FCM of an ecosystem, interpreting FCM output using multivariate statistics, and portraying the information in an easily communicated fashion. To illustrate these ideas, we rely on a complex (Ͼ160 variables) ecosystem model built for the Lake Erie watershed under the auspices of the Lake Erie Lakewide Management Plan (LaMP). Based on our experiences in building this model, we also offer recommendations for increasing the efficiency of the model-development and interpretation process. Use of the FCM method in this case promoted constructive interaction among dozens of scientists, managers, and the public, as well as providing insights concerning the potential effects of broad classes of management actions upon the Lake Erie ecosystem. The analysis focused the attention of participants on four broad alternatives for the Lake. One represents present conditions, and another results from a decrease in nutrient inputs but an increase in stresses from land use and human disturbance. The two others involve reduced stress from nutrients and land use, with one having relatively more nutrients and less human disturbance and fishing. The latter ecosystem alternative was tentatively endorsed by LaMP management, and all four alternatives will be reviewed by the public.

Several species interrelationships influenced by actural or preceived disturbances were described which managers should consider when manipulating fish populations and communities. For example, factors controlling homeostasis of adult northern pike (Esox lucius) and white suckers (Catostomus commersoni) in northern lakes may still operate despite walleye reductions, suggesting less niche overlap than we previously expected. In more northern centrarchid type communities, percid abundance and condition depend on how well northern pike and other predators control both white sucker and bluegill (Lepomis macrochirus) abundance. Evidence from walleye (Stizostedion vitreum vitreum) fry and fingerling plantings suggest that when intense interactions between species determine their abundance, these interactions occur during the very early life stages. Climatic changes may also be influential in determining abundance of several fish species common in north-temperate lakes. Rehabilitation of preferred species by removal of less desirable fishes can be successful in some ecosystems, but we warn that an undesirable compensatory response may also occur.

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