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We quantified the relationship between the population density (number/acre) of age‐0 walleyes Sander vitreus (formerly Stizostedion vitreum) and electrofishing catch per effort (CPE; number/mi) in 19 Wisconsin lakes to update a 1982 analysis by Serns, who used linear regression through the origin to develop a model from a small data set that has been widely used to estimate age‐0 walleye density from electrofishing CPE. We added new data, explicitly tested for the linearity of the relationship, and accounted for the effect of measurement errors. We found that electrofishing CPE was nonlinearly related to the population density of age‐0 walleyes, which indicated that the catchability of age‐0 walleyes to electrofishing declined with population density. The measurement errors in electrofishing CPE were more than nine times as great as those in age‐0 walleye population density, so that the parameters of the relationship between electrofishing CPE and age‐0 walleye density were accurately estimated by ordinary‐least‐squares linear regression. Among lakes, the variation in the catchability of age‐0 walleyes to electrofishing was positively related to the variation in specific conductivity but not to the variation in other physical features (shoreline complexity or littoral area) or chemical features (alkalinity or pH). Within lakes, the variation in the catchability of age‐0 walleyes to electrofishing was negatively related to the variation in temperature at the time of sampling. We recommend that electrofishing CPE only be used as a crude index of age‐0 walleye population density.

Many of Wisconsin's native populations of muskellunge Esox masquinongy exhibit declining reproductive success and failing natural recruitment. As a result, self‐sustaining populations of muskellunge are diminishing. This study focused on spawning habitat factors that influence egg development and survival and, consequently, the reproductive success of muskellunge. Muskellunge spawning habitat characteristics in lakes with self‐sustaining populations were compared with spawning habitat characteristics in lakes that were once self‐sustaining but are now maintained by stocking. The hatching success of artificially fertilized eggs was assessed under natural lake conditions. Spawning sites were typically marshy areas in water less than 1 m deep. Characteristics of the spawning habitat influenced successful reproduction. Spawning areas in stocked lakes had low dissolved oxygen (DO; 1.2–5.4 mg/L) at the substrate–water interface, whereas self‐sustaining lakes had more variable DO (0.5–9.6 mg/L) with some microhabitats having high DO. Organic carbon content, texture of spawning substrate, and water temperature at the substrate did not differ between self‐sustaining lakes and lakes supported by stocking. Fallen logs, stumps, and other wood in spawning areas may increase egg survival. Muskellunge egg survival over natural substrate was low (0.0–1.3%), even in lakes with self‐sustaining populations. Collections of eggs and observations of fry indicated that major mortality occurred after egg deposition but before fry reached nursery habitats several weeks after hatching.

We determined the physical, chemical, biological, and land use characteristics that distinguish northern Wisconsin lakes with self‐sustaining populations of muskellunge Esox masquinongy from lakes where stocking is required to maintain populations. Lakes that supported self‐sustaining muskellunge populations were characterized by fewer shoreline alterations and by spawning habitats with softer, organic‐nitrogen‐rich sediments. Lakes that required stocking had extensively developed shorelines. The direction of water level change during the spawning period, percentage of spawning area sediment covered by woody debris, number of deadfall trees per kilometer of shoreline, and percentage of shoreline that was totally developed were the most important variables for classifying the level of muskellunge reproduction a lake could support. A linear discriminant function correctly classified 83% of the lakes with self‐sustaining muskellunge populations and 89% of the lakes requiring stocking to sustain or enhance muskellunge populations. Lake managers wishing to use muskellunge stocking programs to reestablish self‐sustaining populations should critically review each candidate lake by considering our model and that of Dombeck et al. (1986).

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