Walleye Abundance Research Articles

Do Water Level Fluctuations Influence Production of Walleye and Yellow Perch Young‐of‐the‐Year in Large Northern Lakes?

AbstractMany ecological processes depend on the regular rise and fall of water levels (WLs), and artificial manipulations to WL regimes can impair important ecosystem services. Previous research has suggested that differences in WL between late summer and early spring may alter the suitability of shoals used by Walleyes Sander vitreus for spawning. Other species, such as the Yellow Perch Perca flavescens, are unlikely to be affected in the same way by WL fluctuations because their spawning requirements are quite different. We used 11–23 years of data from six northern Minnesota lakes to assess the effects of WL fluctuations on the abundances of young‐of‐the‐year (age‐0) Walleyes and Yellow Perch. In two lakes (Rainy Lake and Lake Kabetogama), a change in WL management occurred in 2000, after which these lakes saw increased age‐0 Walleye abundance, while the other study lakes experienced decreases or no change. Rainy Lake and Lake Kabetogama also had increases in age‐0 Yellow Perch, but another study lake did also. We used partial least‐squares regression to assess whether WL metrics were associated with variation in age‐0 Walleye and Yellow Perch abundances, but WL metrics were seldom associated with age‐0 abundance for either species. Our analysis suggested a potential influence of WL regulation on age‐0 Walleye abundance, but we found no evidence that early spring access to spawning shoals was the mechanism by which this occurred.Received August 19, 2015; accepted July 15, 2016 Published online November 10, 2016

Examining the effects of climate change and species invasions on Ontario walleye populations: can walleye beat the heat?

AbstractAimThe combined effects of multiple environmental stressors continue to threaten global biodiversity, yet predicting how biotic interactions between native and invasive species may change across a landscape in a multiple stressor environment is relatively understudied. We aim to identify how the invasion of smallmouth bass (Micropterus dolomeiu) may influence native walleye (Sander vitreus) populations across Ontario lakes at the landscape scale in a changing climate.LocationOntario, Canada.MethodsUsing a database that included the abundance and occurrence of over 130 fish species, lake chemistry and lake morphology for 722 lakes, a redundancy analysis was conducted to identify environmental conditions preferred by walleye and smallmouth bass. Multiple linear regression models were then developed to identify the relationship between walleye and multiple stressors (including climate change and biotic interactions with invasive species). Using future scenarios of climate change, we were then able to project future walleye–smallmouth bass co‐occurrences.ResultsSmallmouth bass were found to prefer different environmental conditions than walleye; however, when walleye and smallmouth bass were found in the same lakes, walleye abundance was reduced almost threefold. Multiple regression models further suggested that there are fewer walleye in lakes with smallmouth bass. Subsequently, we predicted that under future scenarios of climate change the overlapping co‐occurrence of walleye and smallmouth bass may increase by 86–332% by the year 2070.Main conclusionsWe illustrate the importance of including multiple environmental stressors in statistical models when attempting to understand how native species will be impacted by invasive species and climate change. While independently climate change is anticipated to lower walleye abundances across Ontario, this change is expected to be exacerbated by invasions of warmwater predators.

Diet Overlap and Predation Between Largemouth Bass and Walleye in Wisconsin Lakes Using DNA Barcoding to Improve Taxonomic Resolution

AbstractOver the last decade, the abundance of Largemouth Bass Micropterus salmoides has increased in many northern Wisconsin lakes, causing concern among anglers and biologists regarding the potential for Largemouth Bass to negatively affect populations of Walleye Sander vitreus through predation or competition for prey. Our objectives were to determine whether (1) diet overlap and predation occurred between adult Walleyes and Largemouth Bass in four northern Wisconsin lakes and (2) the use of DNA barcoding to reduce unidentifiable fish in diet samples affected conclusions regarding diet overlap. A single occurrence of Walleye predation was observed in the diets of 945 Largemouth Bass. Moderate to high diet overlap was observed between Largemouth Bass and Walleyes throughout much of the study period. The use of DNA barcoding reduced the amount of unidentified fish in diets to <1% and showed that failure to identify fish or fish parts can affect conclusions regarding diet overlap. Largemouth Bass predation is probably not a primary factor affecting Walleye abundance in the lakes we selected, but observed diet overlap suggests the potential for competition between the two species.Received June 15, 2015; accepted January 18, 2016 Published online May 26, 2016

Improved Models for Predicting Walleye Abundance and Setting Safe Harvest Quotas in Northern Wisconsin Lakes

AbstractIn Wisconsin, the management of Walleyes Sander vitreus relies on a set of log‐linear regressions to predict Walleye abundance and to set safe harvest. The regression models predict mean Walleye abundance from lake area, but they ignore variability among years; they also predict equal Walleye populations in lakes with the same size and recruitment source. We evaluated three alternative models in terms of predictive accuracy and the risk of overharvest. We used 899 mark–recapture population estimates (collected between 1953 and 2013) from 219 lakes to develop and evaluate (1) a log‐linear mixed‐effects model that used all individual observations and estimated adult Walleye abundance from lake area and lake‐specific deviations from the overall intercept; (2) a mixed‐effects model that builds on model 1 by adding a linear fixed effect of sampling year; and (3) a mixed‐effects model that builds on model 1 by adding a random year effect. Walleye abundance was positively correlated with lake area in all models and was negatively correlated with sampling year (when included). Alternative models improved predictive accuracy by 17–22% over the current regression model. Restricting data to those collected during the most recent 20 years improved model responsiveness to new data and reduced the value of including a linear time trend. When all data were used for model construction, the relative risk of overharvest was lowest under the mixed‐effects model with a linear time trend; when the most recent 20 years of data were used, the risk was lowest under the mixed‐effects model with a random year effect. Accounting for variability among years would allow harvest to track changing Walleye populations and would allow management to be more adaptive. We recommend using the mixed‐effects model with a random year effect and restricting the data inputs to the most recent 20 years.Received May 20, 2015; accepted September 17, 2015

Factors influencing recruitment of walleye and white bass to three distinct early ontogenetic stages

AbstractDetermining the factors that influence recruitment to sequential ontogenetic stages is critical for understanding recruitment dynamics of fish and for effective management of sportfish, particularly in dynamic and unpredictable environments. We sampled walleye (Sander vitreus) and white bass (Morone chrysops) at 3 ontogenetic stages (age 0 during spring: ‘age‐0 larval’; age 0 during autumn: ‘age‐0 juvenile’; and age 1 during autumn: ‘age‐1 juvenile’) from 3 reservoirs. We developed multiple linear regression models to describe factors influencing age‐0 larval, age‐0 juvenile and age‐1 juvenile walleye and white bass abundance indices. Our models explained 40–80% (68 ± 9%; mean ± SE) and 71%–97% (81 ± 6%) of the variability in catch for walleye and white bass respectively. For walleye, gizzard shad were present in the candidate model sets for all three ontogenetic stages we assessed. For white bass, there was no unifying variable in all three stage‐specific candidate model sets, although walleye abundance was present in two of the three white bass candidate model sets. We were able to determine several factors affecting walleye and white bass year‐class strength at multiple ontogenetic stages; comprehensive analyses of factors influencing recruitment to multiple early ontogenetic stages are seemingly rare in the literature. Our models demonstrate the interdependency among early ontogenetic stages and the complexities involved with sportfish recruitment.

Sex-specific Changes in Walleye Abundance, Size Structure and Harvest Following Implementation of Regulation to Protect Broodstock

Abstract The popularity of walleye Sander vitreus fishing has resulted in the development of specialized regulations that are designed to protect these fisheries. In the case of Sherman Reservoir, Sherman County, Nebraska, the walleye population provides a sportfishing opportunity and serves as broodstock for the state. In 2009, for the primary purpose of protecting female broodstock, the regulation changed from a harvest limit of four walleye with 457-mm minimum length to allowing a reduced harvest limit of two walleye within a harvest slot (381–508 mm) and one walleye more than 711 mm. This study examined existing data sets to assess the percentage of spawning walleye protected with each regulation, sex-specific differences in relative abundance and size structure during broodstock collection and angler effort, total catch, and harvest of walleye. The new regulation has increased protection of female walleye by more than 90%, but decreased protection of male walleye by more than 60%. The relative abundance of female walleye caught per net during broodstock collection has more than doubled since the regulation was changed, but the size structure of female walleye collected during broodstock operations was similar. Correspondingly, the relative abundance of male walleye has declined since changing the regulation, but size structure remained similar. Effort and total catch of walleye by anglers were similar before and after the regulation was enacted, but harvest has increased by 130%. This regulation appears to protect female broodstock walleye, but it makes male walleye more vulnerable to angler harvest.

Bayesian hierarchical modeling of larval walleye (Sander vitreus) abundance and mortality: Accounting for spatial and temporal variability on a large river

Larval fish are extremely variable in space and time while sampling of populations is generally restricted and incomplete. However, estimates of abundance and mortality are important for understanding population dynamics, habitat quality, and anthropogenic impacts. Acknowledging and addressing variability during sampling and data analysis are imperative to producing informative estimates. A combination of spatially and temporally distributed ichthyoplankton sampling and Bayesian hierarchical and state-space modeling was used to partition variance and estimate abundance and mortality of larval walleye (Sander vitreus) in the Maumee River during 2010 and 2011. System variability and degree of sampling coverage have a direct impact on the quality of abundance estimates. Small scale factors (i.e., within site and day-to-day) accounted for the most variation in larval walleye densities, therefore sampling should concentrate on capturing these sources. Bayesian state-space modeling can improve estimates by sharing information through time, properly accounting for uncertainty, and producing probability distribution based estimates. Larval fish are highly variable and difficult to sample; however, the application of Bayesian methods during the data analysis process can lead to improved estimates of abundance and informed management actions.

Using Ecopath Modeling to Describe Historical Conditions for a Large, Boreal Lake Ecosystem prior to European Settlement

AbstractTo help guide restoration efforts at a large‐lake ecosystem (Lac la Biche) in Alberta, Canada, I used Ecopath modeling software to create an energetically plausible model representing the system prior to European settlement. Over the last 200 years, Lac la Biche has shifted from a system dominated by predatory fish (e.g., Walleyes Sander vitreus and Northern Pike Esox lucius) to one dominated by forage fish and double‐crested cormorants Phalacrocorax auritus. In 2005, the Fisheries Management Branch of Alberta Environment and Sustainable Resource Development initiated a fisheries restoration program focused on increasing the abundance of Walleyes and average fish size in Lac la Biche; restoration would target a nondescript “historical” ecosystem configuration based on contemporary assumptions of what the system might have looked like. I used Ecopath to organize the main assumptions and information on the historical Lac la Biche ecosystem into an energetically plausible representation for the year 1800. From the modeling process, I learned that most of the assumptions regarding the trends in species biomass between contemporary and historical systems were appropriate guides for model balancing; however, the magnitude of the expected differences between 1800 and the present day were often larger than predicted, especially for infrequently sampled species (e.g., Yellow Perch Perca flavescens, White Suckers Catostomus commersonii, Spottail Shiners Notropis hudsonius, and Burbot Lota lota). Testing the validity of single‐species assumptions in an ecosystem context is important for improving our understanding of ecosystem structure and function. In a historical context, model creation based on assumptions and available information is important for developing ecosystem baselines that can provide context to and guide future fisheries management. Historical baselines are also important for highlighting ecosystem potential and productive capacity and for counteracting the effects of the shifting baseline syndrome.Received February 13, 2012; accepted August 2, 2013Published online December 23, 2013

The Relationship between Age‐0 Walleye Density and Adult Year‐Class Strength across Northern Wisconsin

AbstractWalleyes Sander vitreus are an important cultural and economic resource in northern Wisconsin, both as a recreational fishery and a tribal subsistence fishery. Understanding the recruitment of age‐0 walleyes to the adult population could be of great utility in effectively managing harvest and informing stocking efforts in this mixed‐use fishery. Our objective was to determine the relationship between abundance of age‐0 walleyes in the fall and the subsequent adult year‐class 4 years later and to explain additional residual variation related to environmental variables. Since 1986, fall electrofishing surveys have been conducted to estimate the densities of age‐0 walleyes and spring mark–recapture surveys to estimate spawning adult walleyes. We fit a gamma stock–recruitment curve to a data set of 104 paired surveys (age‐0 survey with adult survey conducted 4 years later) on 58 lakes and attempted to explain residual variation related to additional environmental variables. Our analysis suggests that the maximum number of age‐4 fish (3.36/ha) is not produced by the maximum number of age‐0 fish but by a somewhat intermediate density (243/ha) and that this relationship varies with the complexity of the shoreline. This density‐dependent relationship should help managers refine models forecasting walleye abundance, as well as optimize stocking rates.Received December 28, 2011; accepted April 27, 2012

Site Choice among Minnesota Walleye Anglers: The Influence of Resource Conditions, Regulations and Catch Orientation on Lake Preference

Understanding angler site choice preferences is important in the management of recreational fisheries to forecast angling demand and effort. This study investigated lake choice by recreational anglers fishing for walleye Sander vitreus in Minnesota and examined how choices were influenced by lake characteristics, angler demographics, and angler catch orientation. We collected data through a stated choice preference experiment using a survey administered to a sample of Minnesota resident (n = 1096) and nonresident (n = 535) anglers. Multinomial probit choice models were used to estimate preferences in lake choice. Lake characteristics included walleye abundance, walleye size, bag limit, slot limit, and distance from primary residence. Models included (1) lake characteristics only, (2) lake characteristics and angler demographics, and (3) lake characteristics with angler demographics and catch orientation factors. The coefficients of lake attributes had expected signs with greater preference for higher walleye abundance, larger walleye, bigger bag limits, absence of slot limits, and less driving time from home (P < 0.001 for all lake characteristics in the first model). Lake choice was influenced by the interaction of lake characteristics with age (negative with abundance of fish, P < 0.100; positive with distance from home, P < 0.001), metropolitan and out-of-state residency (positive with distance from home, P < 0.001), and strength of preference for walleye (positive with distance, P < 0.01). A stronger orientation to keep walleye was positively related to increased bag limits (P < 0.001) and negatively related to slot limits (P < 0.01). Study results have clear implications for managers—bag limits, relative to other lake characteristics, had a large influence on anglers’ lake choice for walleye fishing. Because of a stronger catch orientation among walleye anglers, low bag limits reduce lake preference. The results clarify the trade-offs that anglers make when selecting a place to fish for walleye and demonstrate how different management scenarios might influence angler participation. Received July 20, 2011; accepted January 4, 2012

Accounting for spatial population structure at scales relevant to life history improves stock assessment: The case for Lake Erie walleye Sander vitreus

Stock assessments commonly allow parameters to vary across fishery or jurisdictional boundaries, often by treating each region as a unit stock. However, animals generally disperse in response to spatial habitat features to satisfy particular life history requirements, and these features are often not congruent with fishery or jurisdictional boundaries. Thus, populations are often spatially structured at scales distinct from those acknowledged in assessments. Furthermore, when the spatial structure arises from dispersal of a common pool of recruits, redefining unit stock boundaries may not adequately capture these dynamics. Here we test the utility of spatially referencing parameters (vulnerability and catchability) in a statistical catch-at-age stock assessment model as a simple approach to account for life history variation of walleye ( Sander vitreus) when information on explicit movement rates is unavailable. We apply several alternative assessment models to Lake Erie walleye – a population identified as displaying age-specific differences in the extent of dispersal from spawning grounds – to investigate the importance of accounting for spatial heterogeneity at ecologically important scales in stock assessments. Comparisons of the most parsimonious assessment models (based on a deviance information criterion) with and without spatially referenced parameters (by basin) highlighted the importance of estimating regional vulnerability and catchability. There was strong statistical evidence that incorporating spatially referenced parameters at a scale relevant to walleye dispersal patterns improved model fit, and the change altered estimates of stock size and fishing mortality. For example, estimates of total age-2 and older walleye abundance in the most recent year decreased by 16% (34% for ages 7 and older) and fully selected fishing mortality increased by 70% after incorporating walleye spatial population structure. These results emphasize the importance of considering spatial aspects in stock assessments at scales relevant to the life history of the species or group of species under consideration.

Long‐Term and Interannual Dynamics of Walleye and Yellow Perch in Saginaw Bay, Lake Huron

AbstractWalleye Sander vitreus and yellow perch Perca flavescens are well‐studied, ecologically important fish species that co‐occur in a wide range of systems and experience complex interactions; on the one hand, they are physiologically and ecologically similar and therefore may respond analogously to environmental variation, while on the other hand they interact antagonistically as competitors, predators, and prey. In Saginaw Bay, Lake Huron, walleyes and yellow perch have supported a combination of commercial and recreational fisheries and have been exposed to a series of ecosystem‐level stressors and management actions that may have impacted these populations via multiple pathways. We used dynamic factor analysis and correlation analysis of walleye and yellow perch annual fall trawl catch data to elucidate the overall trends in walleye and yellow perch populations in Saginaw Bay. The results suggest that walleyes and yellow perch trend differently; while the relative abundance of age‐0–2 walleyes generally increased and their mean length decreased from 1980 to 2008, the trends in yellow perch abundance and length differed among the young‐of‐year, yearling, and age‐2 age‐classes. Moreover, interannual negative associations between yellow perch relative abundance and mean size are suggestive of compensatory, density‐dependent control on growth, while similar evidence is lacking for walleyes. Correlation analyses indicate that walleye year‐class strength is set by the fall of age 0, while the age at which recruitment is set for yellow perch is less clear and may not occur until the fall of age 1. Thus, we suggest that future studies evaluating environmental determinants of year‐class strength in Saginaw Bay evaluate age‐0 walleyes and age‐1 yellow perch. Finally, while age‐0 yellow perch and age‐0 walleyes appear to respond similarly to annual environmental conditions, the recent increase in walleye abundance and decrease in the mean size of age‐0 yellow perch may have contributed to the reduced abundance of adult yellow perch via walleye predation and overwinter mortality.

Changes in relative abundance of adult walleye and egg density following the addition of walleye spawning habitat in a midwest irrigation reservoir

Cobble substrate for walleye (Sander vitreus) spawning was added at Sherman Reservoir, Nebraska, in January 2008. We evaluated changes in relative adult walleye abundance and egg density in response to the cobble substrate and estimated site fidelity of adult walleye during the spawning seasons of 2007, 2008, and 2009. A 15% increase in electrofishing catch-per-unit-effort (CPUE) of adult walleye was observed on the cobble substrate the first season it was available (2008), but a 5% decrease in electrofishing CPUE was observed the second season (2009) compared to the 2007 season. While gill nets were not run during the first season the cobble was available, a 113% increase in gill net CPUE was observed the second season. During the first season the cobble was available, a 215% increase in egg density was observed, but no increase in egg density was found in the second season. Site fidelity of adult walleye within a spawning season was 94% and was 81% between seasons. These results indicate that the addition of the cobble substrate increased the relative abundance of adult walleye and density of eggs on the cobble substrate.

Role of organic fertilizers in walleye (Sander vitreus) production in plastic-lined culture ponds

This study used six 0.04 ha plastic-lined ponds to compare the effects of a fertilization regime using a one-time initial application of an organic fertilizer (alfalfa pellets) with the current regime of weekly applications of organic fertilizers on the abundance and distribution of aquatic invertebrates and walleye, Sander vitreus, fingerling production. Walleye, 3–4 days post hatch, were stocked on 1 May 2002 and harvested on 6–7 June 2002. Throughout the growing season, a ratio of 7:1 total nitrate-nitrogen to total phosphorus was maintained in all ponds regardless of the treatment. Once fry were stocked, ponds in Treatment #1 were fertilized weekly with organic fertilizer (alfalfa pellets; 112 kg ha−1 week−1) for a total of 795 kg ha−1 pond−1. Ponds in Treatment #2 only received an initial application of alfalfa pellets (112 kg ha−1). Ponds in Treatment #1 had significantly higher ammonia and nitrate levels as well as higher chironomid larvae but not zooplankton compared with the other treatment. At harvest, walleye in the Treatment #1 ponds were significantly longer and heavier; however, the survival and relative weight were not significantly different. These results suggest that weekly applications of organics are important for the benthic food base and growth of fingerling walleye reared in plastic-lined ponds.

Abundance, growth, and life history characteristics of sympatric walleye (Sander vitreus) and sauger (Sander canadensis) in Lake Winnipeg, Manitoba

Walleye (Sander vitreus) and sauger (Sander canadensis) are the two most economically valuable and sought-after species in the Lake Winnipeg fishery, the second largest freshwater commercial fishery in Canada. We compared recent temporal trends (1979–2003) in population and life history characteristics of these species across the three main management zones of the lake (north basin, channel region, and south basin). Walleye abundance (gill net catch-per-unit-effort) increased relative to sauger abundance, particularly in the north and south basins, and the ages and sizes of the sampled fish exhibited greater spatial and temporal variabilities in sauger than in walleye. Walleye of the south basin and channel region exhibit a bimodal growth pattern, previously unreported for walleye populations. Growth rates of both walleye and sauger increased, and ages and sizes of maturity generally decreased from 1979 to 2003. However, walleye showed much greater flexibility in these traits, both spatially and temporally, than sauger. Sauger utilize a reproductive strategy of younger age and smaller size at maturity and higher fecundity with smaller, more lipid-rich eggs relative to walleye. Recent environmental and/or harvesting conditions on the lake appear to favour walleye over sauger, and differences in their life histories could make the sauger population more vulnerable to the impacts of commercial harvest than the walleye population.

Evaluation of harassment of migrating double-crested cormorants to limit depredation on selected sport fisheries in Michigan

Diverse management techniques have been used to mitigate conflicts between humans and double-crested cormorants ( Phalacrocorax auritus) including harassment methods supplemented by lethal take. In this study we evaluated impacts of programs to harass spring migrating cormorants on the walleye ( Sander vitreus) fishery in Brevoort Lake and the yellow perch ( Perca flavescens) and walleye fisheries at Drummond Island. Cormorant foraging declined significantly ( p < 0.05) at both locations subsequent to initiation of harassment programs. Overall harassment deterred 90% of cormorant foraging attempts while taking less than 6% lethally on average at each site. Yellow perch were a predominate prey item in number and biomass at both locations. Walleye made up a small proportion of the diet at both locations. However, both walleye and yellow perch abundance increased significantly ( p < 0.05) at Drummond Island. Walleye abundance at age 3 increased to record levels in 2008 following 3 years of cormorant management at Brevoort Lake. The estimated cormorant consumption of age-1 walleye in the absence of management at Brevoort Lake during 2005 would account for 55% of the record 2006 age-1 walleye population. These results support the hypothesis that cormorant predation on spawning aggregations of sportfish was a significant mortality factor and cormorant management reduced sportfish mortality and increased abundance at both locations. Continuation of harassment programs and fishery assessments will determine whether improvement of targeted sport fisheries through control of spring migrating cormorants is sustainable.

Modeling the Sustainability of Walleye Populations in Northern Wisconsin Lakes

AbstractPopulation sustainability of walleyes Sander vitreus in the face of angling and spearing fisheries has been a concern of fishery managers in northern Wisconsin since off‐reservation tribal fishing rights were affirmed by federal court decisions. We evaluated sustainability of walleye populations across a range of densities that were subjected to various exploitation rates and allocations of angling and spearing harvest (the fisheries differ in size selectivity). We developed an age‐structured population model for estimating extinction risk and time to extinction for a hypothetical walleye population at a specified exploitation rate, fishery allocation, and initial density. The model was parameterized from data obtained during surveys of walleye populations in northern Wisconsin lakes. Simulations covered a range of annual exploitation rates that included the currently accepted rate of 35% and a range of population densities that are presently included in regression models relating walleye abundance to lake surface area. The risk of extinction began to increase above zero at exploitation rates of 61–65% for an unregulated angling fishery, 85–86% for an angling fishery with a 15‐in minimum length limit, and 77–78% for a spearing fishery. The probability of decline began to increase above zero at exploitation rates of 61–65% for an unregulated angling fishery, 84–85% for an angling fishery with a 15‐in minimum length limit, and 72–77% for a spearing fishery. As the exploitation rate increased, the average adult density decreased and the time to extinction decreased for all initial population densities. We conclude that the current maximum exploitation rate of 35% may be sustainable for all initial population densities but that a controlled field experiment should be used to verify our modeling results before any changes to angling bag limits or spearing quotas are made.

Forecasting Walleye Abundance in Northern Wisconsin Lakes

AbstractSeventeen variations of a modified statistical catch‐at‐age model were applied to annual spawning adult mark–recapture population estimates (PEs) for walleye Sander vitreus from seven mixed‐fishery lakes in northern Wisconsin. In addition, 15 variations were applied to annual PEs for walleyes age 3 and older from Escanaba Lake. The model variations included different mortality, selectivity, and catchability assumptions, as well as different data weightings, averaging of forecasts, and naïve models using limited data. Population forecasts were examined retrospectively for each model variation to evaluate whether they were accurate enough to use as a basis for setting harvest quotas. Models were ranked according to how closely the forecasts matched the PEs and the abundance estimates fitted using all available data. A model that used estimated mortality and averaging of forecasts performed best for the large mixed‐fishery lakes, and a model that used fixed mortality and downweighted age composition data performed best for the small mixed‐fishery lakes. These model forecasts can be considered for setting harvest quotas on the mixed‐fishery lakes since they performed better than the naïve models, but the improvements were modest. For the large mixed‐fishery lakes, the mean absolute deviation and mean deviation of the forecasts from the PEs were 38% and 13%, respectively, for the naïve model using the previous year's PE as the forecast and 31% and 3%, respectively, for the best model; for the small mixed‐fishery lakes, these deviations were 34% and 16%, respectively, for this naïve model and 36% and 12%, respectively, for the best model. A naïve model using the previous year's fitted abundance as the forecast performed best for Escanaba Lake. Model forecasts performed poorly for Escanaba Lake, mostly because of the age composition of the population being estimated.

Optimal Stocking Densities of Walleye Fingerlings in Minnesota Lakes

AbstractA generalized additive model was developed that described the effects of fingerling stocking density on the abundance of walleyes Sander vitreus in 551 lakes in Minnesota. Walleye abundance was measured as a catch per effort (CPE) from 1,511 standard Minnesota Department of Natural Resources gill‐net assessments conducted during 1986–2004. In addition to fingerling stocking, the effects of lake morphometry, productivity, and fish community variables on walleye CPE were also described using a generalized additive model. Lake morphometry variables had sharply nonlinear effects on walleye abundance. Walleye abundance declined in lakes with areas less than 1,000 acres and increased rapidly in lakes with depths of less than 30 ft. The lake productivity variable, total alkalinity, was positively related to walleye abundance. Abundance of northern pike Esox lucius had a negative effect and abundance of yellow perch Perca flavescens a positive effect on walleye abundance. The response of walleye abundance to fingerling stocking was asymptotic: peak walleye abundance occurred at stocking densities greater than 0.65 lb·littoral acre−1·year−1. A fingerling stocking density of 0.50 lb·littoral acre−1·year−1 is recommended as a balance between increased abundance and relative costs of stocking for most lakes in Minnesota.

Environmental and Ecological Conditions Surrounding the Production of Large Year Classes of Walleye ( Sander vitreus) in Saginaw Bay, Lake Huron

The Saginaw Bay walleye population ( Sander vitreus) has not fully recovered from a collapse that began in the 1940s and has been dependent on stocking with only limited natural reproduction. Beginning in 2003, and through at least 2005, reproductive success of walleye surged to unprecedented levels. The increase was concurrent with ecological changes in Lake Huron and we sought to quantitatively model which factors most influenced this new dynamic. We developed Ricker stock-recruitment models for both wild and stock fish and evaluated them with second-order Akaike's information criterion to find the best model. Independent variables included adult alewife ( Alosa pseudoharengus) abundance, spring water temperatures, chlorophyll a levels and total phosphorus levels. In all, 14 models were evaluated for production of wild age-0 walleyes and eight models for stocked age-0 walleyes. For wild walleyes, adult alewife abundance was the dominant factor, accounting for 58% of the variability in age-0 abundance. Production of wild age-0 fish increased when adult alewives were scarce. The only other plausible factor was spring water temperature. Predictably, alewife abundance was not important to stocked fish; instead temperature and adult walleye abundance were more significant variables. The surge in reproductive success for walleyes during 2003–2005 was most likely due to large declines in adult alewives in Lake Huron. While relatively strong year classes (age-1 and up) have been produced as a result of increased age-0 production during 2003–2005, the overall magnitude has not been as great as the initial age-0 abundance originally suggested. It appears that over-winter mortality is higher than in the past and may stem from higher predation or slower growth (lower condition for enduring winter thermal stress). From this it appears that low alewife abundance does not assure strong walleye year classes in Saginaw Bay but may be a prerequisite for them.