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Genetic relationships among lake whitefish ( Coregonus clupeaformis ) spawning aggregates in Lake Michigan were assessed and used to predict a stock or management unit (MU) model for the resource. We hypothesized that distinct spawning aggregates represented potential MUs and that differences at molecular markers underlie population differentiation. Genetic stock identification using 11 microsatellite loci indicated the presence of six genetic MUs. Resolved MUs corresponded to geographically proximate spawning aggregates clustering into genetic groups. Within MUs, analyses suggested that all but one delineated MU was a stable grouping (i.e., no between-population differences), with the exception being the Hog Island – Traverse Bay grouping. Elk Rapids was the most genetically divergent population within Lake Michigan. However, low Fst values suggested that moderate to high levels of gene flow occur or have occurred in the past between MUs. Significant tests of isolation by distance and low pairwise Fst values potentially led to conflicting results between traditional analyses and a Bayesian approach. This data set could provide baseline data from which a comprehensive mixed-stock analysis could be performed, allowing for more efficient and effective management of this economically and socially important resource.

Maternal influences on age-0 walleye (Sander vitreus (Mitchill, 1818)) recruit abundance and survival from egg to fall were observed in Escanaba Lake, Wisconsin, in 1957–2015. Annual egg production best explained variation in age-0 recruitment, compared with female relative abundance, and adult abundance (sexes combined). Age-0 recruitment was not significantly correlated with any temperature metric tested or our index of yellow perch (Perca flavescens (Mitchill, 1814)) abundance. Survival of walleye from egg to fall age-0 was positively correlated with the percent contribution of large females (>55.9 cm) to annual egg production. Mean size diversity of females by length class did not influence age-0 recruit abundance or survival over time. Evidence for maternal effects via size- and age-specific influences on fecundity and age-0 walleye survival suggest that exploitation may influence natural recruitment by altering adult female size structure. Given recent declines observed in walleye natural recruitment in the upper Midwestern USA, understanding the roles of maternal drivers and exploitation on recruitment is critical for sustainable walleye management.

The recently developed empirical percentile (EmP) method, a technique for deriving standard weight (W s ) equations, putatively reduces the length-related biases that often plague such equations. To determine whether the EmP method is superior to the regression line-percentile (RLP) method in reducing length-related biases, we developed new W s equations by applying both methods to two morphologically distinct species, walleye Sander vitreus and black crappie Pomoxis nigromaculatus. We also investigated diagnostic approaches to provide quality control for weight-length data. We evaluated the new W s equations with filtered independent data to determine which equation reduced length bias the most. We suggest a protocol for evaluating length-related bias using an independent data set. Our results showed that for randomly selected walleye populations, the RLP method did not have any length-related biases when relative weight (W r ) was plotted as a function of length. However, the W r values calculated from the EmP W s equations were length biased when the latter were applied to those same populations. Both methods generated W s equations that were length biased when W r was plotted as a function of length for black crappies. Further, the absolute difference in W r between the RLP and EmP methods indicates that there is little difference between the methods as far as their relevance to management is concerned.. Based on these results, we believe that revising existing W s equations using the EmP method is unnecessary and that the RLP technique should remain the standard for developing W s equations pending the development of an approach that clearly eliminates methodological length bias.

Bullheads Ameiurus spp. are found throughout much of the United States and are infrequently studied species. Although limited information has been published on the Black Bullhead A. melas and Yellow Bullhead A. natalis, it has been shown that bullheads can dominate the fish biomass in some north temperate U.S. lakes, resulting in a fish community exclusive of Walleye Sander vitreus and Yellow Perch Perca flavescens. Recently, recruitment and abundances of popular coolwater sport fishes, such as Walleye and Yellow Perch, have been declining in some northern Wisconsin lakes. These observed declines, coupled with high bullhead densities, led fisheries biologists to conduct whole‐lake bullhead removals on four northern Wisconsin lakes. Removal of Black and Yellow bullheads from these lakes resulted in substantial changes in the fish communities. The CPUE of naturally reproduced age‐0 Walleye increased in Lake Metonga and Patten Lake following the removal of bullheads. Additionally, survival of stocked Walleye increased in Crane and Pickerel lakes after bullhead removals, resulting in higher CPUE of age‐0 and age‐1 Walleye. Subsequently, adult Walleye abundance has increased or remained steady in all bullhead removal lakes. Significant declines in abundance of Bluegill Lepomis macrochirus and increases in abundance of Yellow Perch and Black Crappie Pomoxis nigromaculatus were also observed after bullhead removals, further illustrating a shift in fish community composition. Our observations suggest that when at high densities, bullheads can play a significant role in structuring fish communities and the removal of bullheads can increase recruitment, survival, and abundance of sport fishes like Walleye and Yellow Perch.

Walleye Sander vitreus and sauger Sander canadensis naturally hybridize in many populations, but the factors influencing the recruitment of hybrids are unknown. We determined relative year‐class strength for 1,454 saugers, walleyes, and naturally produced hybrids from three Missouri River reservoirs. Hybridization rates varied from 4% in Lakes Sharpe and Francis Case to 21% in Lewis and Clark Lake. Hybrids comprised multiple year‐classes in each system, indicating that hybridization does not occur in erratic pulses but at a consistent low‐level recruitment rate. The year‐class strength of hybrids was not correlated with that of either walleyes or saugers. Our models suggest that higher than average recruitment of walleyes, saugers, and hybrids can be expected during years with warmer spring and early summer water temperatures in Lake Sharpe and Lewis and Clark Lake and during years when inflow (from main‐stem discharge for hybrids and tributary inputs for saugers and walleyes) is reduced in Lewis and Clark Lake.

Stable isotope analysis of fishes is often performed using muscle or organ tissues that require sacrificing animals. Non‐lethal sampling provides an alternative for evaluating isotopic composition for species of concern or individuals of exceptional value. Stable isotope values of white muscle (lethal) were compared with those from fins and scales (non‐lethal) in walleye, Sander vitreus (Mitchill), from multiple systems, size classes and across a range of isotopic values. Isotopic variability was also compared among populations to determine the potential of non‐lethal tissues for diet‐variability analyses. Muscle‐derived isotope values were enriched compared with fins and depleted relative to scales. A split‐sample validation technique and linear regression found that isotopic composition of walleye fins and scales was significantly related to that in muscle tissue for both δ13C and δ15N (r2 = 0.79–0.93). However, isotopic variability was significantly different between tissue types in two of six populations for δ15N and three of six populations for δ13C. Although species and population specific, these findings indicate that isotopic measures obtained from non‐lethal tissues are indicative of those obtained from muscle.

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