Walleye Fishery Research Articles

Diminishing productivity and hyperstable harvest in northern Wisconsin walleye fisheries

Diminishing productivity and hyperstable harvest in northern Wisconsin walleye fisheries

Using transcriptomics to examine the physiological status of wild-caught walleye (Sander vitreus)

Lake Winnipeg, Manitoba, is Canada’s second largest commercial inland freshwater fishery, and concern over collapse of the walleye fishery is growing. Molecular techniques have been increasingly used to study responses of wild organisms to environmental and anthropogenic stressors. The present study used transcriptomics to examine the physiology of wild-caught walleye ( Sander vitreus (Mitchell, 1818)) across Lake Winnipeg using non-lethal techniques. Gill transcriptomes of walleye sampled from the north and south basins of Lake Winnipeg, and the channel connecting them, exhibited distinct profiles implicating regionally specific biological responses. North basin walleye exhibited transcriptomic responses indicative of exposure to environmental stressors. Transcriptomic patterns suggested a shift to increased reliance on anaerobic metabolism and up-regulation of hypoxia-sensitive genes in north basin fish, possibly representing exposure to low-oxygen conditions. Exposure to environmental stressors may also have driven increases in gene transcripts associated with proteasomal catabolism, DNA repair, molecular chaperones, and ion regulation. North basin fish also exhibited transcriptomic patterns indicative of gill remodeling via regulation of the mTOR pathway, cytoskeleton reorganization, and fatty acid synthesis. Our results highlight the complexity of examining wild fish across environmental gradients but also the potential use of molecular techniques in elucidating organismal sensitivity to local environmental factors.

Stocking Practices and Lake Characteristics Influence Probability of Stocked Walleye Survival in Wisconsin’s Ceded Territory Lakes

AbstractAs culturally important fisheries for Walleye Sander vitreus exhibit declining densities and reductions in natural recruitment, stocking has increased in popularity as a management tool in Wisconsin’s Ceded Territory. Walleyes were stocked in an average of 160 lakes per year in the 1980s compared with 223 lakes per year in the 2010s. However, many Walleye fingerling stocking events are unsuccessful, with zero or few fish being detected as yearlings in electrofishing surveys the following year. We integrated 31 years of stocking and electrofishing data with lake habitat characteristics to identify factors that influence stocked fingerling survival, measured by the number of individuals stocked compared with those sampled in stocking evaluation surveys. Notably, 21% of stocking events exhibited zero returns in the following year. To handle the prominence of zeros, we used a zero‐inflated mixed‐effects model to test for effects of stocking practices and lake attributes along with lake‐level random effects. Our results suggest that the average length and stocking density are important stocking‐practice‐level characteristics, while water clarity, water temperature, and lake surface area are important lake‐level characteristics for predicting survival. Concerningly, we found a significant reduction in stocked fingerling survival through time. Managers should weigh these factors when determining whether stocking is worthwhile for the system in question and, if so, when selecting a stocking product. Although overall survival of stocked fingerlings is relatively low, better understanding stocking efficacy can help managers maximize utility of limited resources. Future work should assess stocking success in the context of recruitment to the fishery or return to creel. In light of recent work on Walleye fishery collapses and shifting environmental conditions, managers need to temper presumptions of Walleye stocking success and shepherd realistic expectations of stocking utility in regional Walleye rehabilitation efforts.

Modeled Walleye Regulation Outcomes in a Midwestern Reservoir with Varying Forage and Walleye Density

AbstractIn a put‐grow‐take fishery, management of Walleye Sander vitreus may require length‐based regulation and depends on population dynamics such as growth, which in turn depends on forage. Big Creek Lake, an important Iowa Walleye fishery, underwent forage changes twice within two decades. Concurrently, Walleye density changed due to increased stocking and reduced emigration, resulting in four scenarios: (1) low‐density Walleye with Gizzard Shad Dorosoma cepedianum forage (period A: 1998–2000), (2) low‐density Walleye with centrarchid or percid forage (period B: 2006–2008), (3) high‐density Walleye with centrarchid or percid forage (period C: 2013–2015), and (4) high‐density Walleye with Gizzard Shad forage (period D: 2019). We examined Walleye population dynamics and relative effects of four length‐based regulations during each period. Walleye in Big Creek Lake likely experienced density‐dependent effects when Walleye density was high and Gizzard Shad were absent, resulting in depressed growth, low condition, increased natural mortality, reduced exploitation, and ineffectiveness of regulations. At lower Walleye densities, the centrarchid or percid forage supported Walleye growth and condition equivalent to when Gizzard Shad were present. Fishing regulations for enhancing Walleye size distribution or yield differed over time due to shifting population dynamics and angler exploitation and were notably ineffective during period C. The preferred regulation for the current scenario (high‐density Walleye with Gizzard Shad) depends on angler preferences for higher catch or larger fish. Big Creek Lake’s Walleye fishery provided a useful demonstration of how rapidly a population can respond to changes in density and forage and, therefore, how dynamic fishery management needs to be.

Effects of Harvest, Natural Mortality, and Escapement on Reservoir Walleye Populations

AbstractWalleye Sander vitreus are commonly introduced to reservoirs and provide valued fisheries. Walleye are lost though harvest and natural mortality in all systems, but escapement may also reduce reservoir populations. However, the effects of harvest, natural mortality, and escapement loss on reservoir Walleye populations are not known. Our objective was to quantify Walleye harvest, natural mortality, and escapement in two Iowa reservoirs (Big Creek Lake and Brushy Creek Lake) to understand how these factors limit populations. We also assessed Walleye behaviors and compared them among escaped, harvested, and in‐lake Walleye to determine if behavior was associated with fate. Walleye were radio‐tagged beginning in October 2016 and tracked through May 2019. Telemetry was used to estimate annual core and home ranges, seasonal depth use, and movement rates. Detection histories were analyzed using a multistate capture–recapture model with environmental covariates to estimate weekly harvest, natural mortality, and escapement. Harvest was greater in Big Creek than Brushy Creek and was higher in April through July compared to other months. Natural mortality was positively related to water temperature but did not differ between systems. No Walleye escaped from Big Creek (fish barrier on spillway), but escapement at Brushy Creek (no fish barrier) was positively related to mean water levels during April. During 2017–2018, annual Walleye escapement ranged from 22% to 47% at Brushy Creek (0% at Big Creek), harvest ranged from 13% to 27%, and natural mortality ranged from 36% to 38%. Depth of habitat occupied by Walleye was shallower during spring and summer when escapement occurred, intermediate in fall, and deepest during winter. Escaped Walleye at Brushy Creek used greater depths and moved less than in‐lake Walleye; however, other behaviors did not differ among Walleye groups. Our results suggest that escapement can have a larger effect than harvest on reservoir Walleye populations in systems without physical barriers, and it should be taken into consideration when managing reservoir Walleye fisheries.

Mixed-stock analysis using Rapture genotyping to evaluate stock-specific exploitation of a walleye population despite weak genetic structure.

Mixed‐stock analyses using genetic markers have informed fisheries management in cases where strong genetic differentiation occurs among local spawning populations, yet many fisheries are supported by multiple, weakly differentiated stocks. Freshwater fisheries exemplify this problem, with many populations supported by multiple stocks of young evolutionary age and isolated across small spatial scales. Consequently, attempts to conduct genetic mixed‐stock analyses of inland fisheries have often been unsuccessful. Advances in genomic sequencing offer the ability to discriminate among populations with weak population structure, providing the necessary resolution to conduct mixed‐stock assignment among previously indistinguishable stocks. We used genomic data to conduct a mixed‐stock analysis of eastern Lake Erie's commercial and recreational walleye (Sander vitreus) fisheries and estimate the relative harvest of weakly differentiated stocks (pairwise F ST < 0.01). Using RAD‐capture (Rapture), we sequenced and genotyped individuals from western and eastern basin local spawning stocks at 12,081 loci with 95% reassignment accuracy, which was not possible in the past using microsatellite markers. A baseline assessment of 395 walleye from 11 spawning stocks identified three reporting groups and refined previous assessments of gene flow among walleye stocks. Genetic assignment of 1,075 walleye harvested in eastern Lake Erie's recreational and commercial fisheries indicated that western basin stocks constituted the majority of harvest during the peak walleye fishing season (July–September), whereas eastern basin individuals comprised much of the early season harvest (May–June). Clear spatial structure in harvest composition existed; catches in more easterly sites contained more individuals of eastern basin origin than did more westerly sites. Our study provides important stock contribution estimates for Lake Erie fishery management and demonstrates the utility of genomic data to facilitate mixed‐stock analysis in exploited fish populations having weak population structure or limited existing genetic resources.

Use of a Restrictive Regulation to Manage Walleyes in a New Glacial Lake in South Dakota

AbstractAbove‐average precipitation in the 1990s resulted in many new fisheries across northeastern South Dakota and elsewhere in the Upper Midwest. Walleye Sander vitreus populations in these new fisheries can be extremely vulnerable to angler harvest. In an attempt to maintain a Walleye fishery in Reetz Lake, South Dakota, a new lake, a restrictive regulation of one Walleye ≥711 mm TL per day was implemented when the lake opened to public fishing (October 2001). To evaluate the regulation, we monitored the estimated number of Walleyes ≥508 mm, Walleye gill‐net catch per unit of effort (CPUE) of stock‐length fish (≥250 mm), size structure, condition, length at age at capture (ages 2–5), mortality, recruitment, and angler use during the period 2001–2019. The estimated number of Wall‐eyes ≥508 mm, gill‐net CPUE, and proportional size distribution of preferred‐length fish (PSD‐P; ≥510 mm) trended higher across years. Relative weight (Wr) for the quality–preferred (380–509 mm) and preferred–memorable (510–629 mm) length‐groups and the length at age at capture for ages 4 and 5 trended lower. Walleyes &lt;510 mm were in better condition than larger fish throughout the study. Estimated total mortality was 62% in 2004 but substantially declined in subsequent years before ranging from 6% to 21% for 2010–2019. The coefficient of variation of age‐3 gill‐net CPUE by cohort was 93%, indicating variable recruitment. Decreasing trends in Wr and growth were not surprising, as Walleye CPUE increased through time. Lengths at age remained at or above statewide and national averages. The values for number of angler‐hours and Walleye catch rates were high during the summer months when most anglers targeted Walleye. The regulation that restricted harvest at Reetz Lake to one Walleye ≥711 mm allowed an abundant Walleye population to establish and was successful at maintaining the fishery in terms of abundance, size structure, and angler use.

You can’t just use gold: Elevated turbidity alters successful lure color for recreational Walleye fishing

Increasing anthropogenic turbidity alters underwater visual environments, leading to disrupted perception of visual cues with a variety of consequences, such as diet shifts and reduced prey consumption. In this study, we used novel techniques, including a citizen science mobile phone application (app), to investigate the effects of altered water clarity on recreational fisheries. Our objectives were to determine if elevated turbidity (suspended sediments or algae) alters lure success in the recreational Walleye (Sander vitreus) fishery and if the behavior of recreational anglers shifts with algal blooms. We developed a mobile phone app to gather real time data on lure success across water clarity conditions in collaboration with Lake Erie charter captains. Citizen science data collected with the app showed that lure color success shifted with water color and clarity: white lures were most successful in clear water, yellow in sedimentary turbidity, and black in algal conditions. A survey of charter captains suggested that fishing practices and lure usage may change over the long term if algal blooms persist.

Production dynamics reveal hidden overharvest of inland recreational fisheries

Recreational fisheries are valued at $190B globally and constitute the predominant way in which people use wild fish stocks in developed countries, with inland systems contributing the main fraction of recreational fisheries. Although inland recreational fisheries are thought to be highly resilient and self-regulating, the rapid pace of environmental change is increasing the vulnerability of these fisheries to overharvest and collapse. Here we directly evaluate angler harvest relative to the biomass production of individual stocks for a major inland recreational fishery. Using an extensive 28-y dataset of the walleye (Sander vitreus) fisheries in northern Wisconsin, United States, we compare empirical biomass harvest (Y) and calculated production (P) and biomass (B) for 390 lake year combinations. Production overharvest occurs when harvest exceeds production in that year. Biomass and biomass turnover (P/B) declined by ∼30 and ∼20%, respectively, over time, while biomass harvest did not change, causing overharvest to increase. Our analysis revealed that ∼40% of populations were production-overharvested, a rate >10× higher than estimates based on population thresholds often used by fisheries managers. Our study highlights the need to adapt harvest to changes in production due to environmental change.

Sex-based differences in spawning behavior account for male-biased harvest in Lake Erie walleye (Sander vitreus)

Sex- and size-specific reproductive behaviors can increase the vulnerability of certain demographic components of fish populations to exploitation, potentially leading to unsustainable harvest. Lake Erie’s largest walleye (Sander vitreus) spawning population, which aggregates on the Ohio reef complex during spring, is subject to angling. Information on the sex composition of harvest or how reproductive behavior might influence harvest is lacking. To address these uncertainties, we implanted 337 reef-spawning individuals with acoustic transmitters, and their spawning behavior on the reef complex was monitored for 4 years using acoustic telemetry. Males arrived on spawning grounds earlier and remained on them longer than females. These behavioral differences led us to predict that recreational angler harvest during the spawning season would be male-biased. Creel surveys confirmed this prediction, although sex composition of the harvest was influenced by angling technique. Collectively, these findings suggest that sex-based differences in reproductive behaviors bias the recreational harvest toward males on the reef complex during the spawning season. This male-biased harvest seems unlikely to pose an undue risk to Lake Erie’s walleye fishery.

Live versus artificial bait influences on walleye (Sander vitreus) angler effort and catch rates on Escanaba Lake, Wisconsin, 1993 - 2015

Walleye (Sander vitreus) support harvest-oriented fisheries that are generally assumed to be self-regulating, but many walleye populations have declined over time. Regulations (i.e., length or bag limits, closed seasons) commonly used to manage walleye primarily restrict angler effort and exploitation as opposed to influencing catch rates. We evaluated walleye angler compulsory creel survey data from Escanaba Lake, Wisconsin during 1993–2015 to test whether bait type (i.e., live versus artificial) influenced walleye angler effort, probability of a catching a walleye, total catch, catch rates (hereafter, catch per unit effort (CPUE)), and catchability. The number of walleye anglers that used live bait was significantly higher (26,250) than those that used artificial bait (2414). Angler effort differed significantly by bait type (mean live 3.4 ± 0.04 SE and mean artificial 2.9 ± 0.1 SE hrs/trip). The use of live bait, the use of a guide, angler effort, walleye density, and month significantly influenced the probability of a catching a walleye and the abundance of catch. CPUE was highest in April-May and declined into the summer. CPUE and catchability (q) were significantly higher for live bait anglers (CPUE mean 0.44 ± 0.04 SE fish/hr; q mean 0.02 ± 0.002 SE) compared to artificial bait walleye anglers (CPUE mean 0.19 ± 0.03 SE fish/hr; q mean 0.008 ± 0.002 SE). Our results suggest that angler catchability of walleye in Escanaba Lake was dependent on bait type. A live bait restriction may be a viable tool for reducing exploitation in open-access walleye fisheries during population rehabilitation efforts while maintaining angling opportunities.

Characterizing Angler Preferences for Largemouth Bass, Bluegill, and Walleye Fisheries in Wisconsin

AbstractManaging recreational fisheries in lake‐rich landscapes with diverse fish communities and anglers alike presents a social and biological challenge for managers. Understanding angler preferences is central to navigating these challenges and can aid in predicting shifts in angler behavior in response to management actions or changing fish populations. Species‐specific angler surveys do not incorporate tradeoffs inherent in multispecies fisheries, thus limiting their application to real‐world management issues. To better understand angler preferences in relation to realistic tradeoffs among different fishing opportunities, we conducted a survey of Wisconsin anglers in 2013–2014 that included questions requiring anglers to choose between fisheries providing different outcomes. We used stated‐preference modeling to quantify angler preferences associated with Bluegill Lepomis macrochirus, Largemouth Bass Micropterus salmoides, and Walleye Sander vitreus fisheries. Next, we conducted a latent class analysis using survey responses to classify anglers into subgroups. Finally, we assessed the sensitivity of angler choice to changes in fishing opportunities. Results of the stated‐preference models indicated that both residents and nonresidents largely prefer “quality” (i.e., moderate catch rate and size structure) over “action” or “trophy” Bluegill fisheries and that characteristics of Largemouth Bass fisheries have more value to nonresidents than to residents. Aspects of the Bluegill fishery were most important in choosing among hypothetical lakes for resident respondents. Five angler subgroups were identified that show the importance of Bluegill and Walleye but shed additional light on anglers’ commitment level and nonresidents’ preference for Largemouth Bass. In addition, simulations of changes in Walleye, Largemouth Bass, and Bluegill fisheries indicated that maintenance of quality Bluegill fisheries is important to ensuring continued angler participation, while the retention of high‐yield Walleye fisheries is likely paramount to a subgroup of anglers. Our results offer insight into angler preferences across the lake‐rich landscape of Wisconsin and demonstrate how angler behavior may shift in response to transitions in fishing opportunities.

Eroding productivity of walleye populations in northern Wisconsin lakes

Managing fisheries through rapid environmental change requires diverse approaches for identifying and adapting to novel ecological conditions. For the Wisconsin Ceded Territory, we calculated 473 adult walleye (Sander vitreus) production (P), biomass (B), and P/B estimates for 1990–2012. Frequency distributions for production statistics were right-skewed, indicating the fishery is generally dominated by low production populations. Mean P, B, and P/B were significantly elevated in natural recruitment (NR) lakes compared with combination (NR + stocking) and stocked-only lakes. Furthermore, combination populations had significantly higher production compared with stocked-only lakes. In NR lakes, walleye productivity changed little over time; however, the proportion of NR populations has declined over time. In combination and stocked-only populations, there were significant temporal declines in P, B, and P/B, and the proportion of these lakes has increased through time. This study reveals the crucial link between fish recruitment potential and fish production, helping to explain why the regional walleye fishery is struggling. Causes for walleye recruitment and production declines remain unclear, but long-term shifts in fish habitats are likely involved (e.g., from climate change and indirect food web effects). Decreasing walleye production is an important and emerging fishery management challenge in the region and portends a need to adapt fisheries management systems collaboratively for future sustainability.

Characterizing the Angling and Tribal Spearing Walleye Fisheries in the Ceded Territory of Wisconsin, 1990–2015

AbstractAssessment of the Walleye Sander vitreus angling and tribal spearing fisheries in the Ceded Territory of Wisconsin (CTWI) is critical for the sustainability of this resource. Key to these assessments is an understanding of harvest demographics, exploitation, catch and harvest efficiency, and relationships between catch or harvest and adult density. We characterized the size distribution and mean length of harvested Walleyes, harvest, exploitation rate, and catch (angling) or harvest (spearing) rate for both fisheries during 1990–2015. Then, we evaluated catch and harvest rates in relation to adult density and tested for self‐regulation or hyperstability in each fishery. Size distribution and mean length of harvested Walleyes in both fisheries were statistically different but biologically similar. Anglers harvested significantly more Walleyes, and the mean exploitation rate was greater in the angling fishery. Spearfishers had significantly higher mean harvest rates compared with angler catch rates. Catch and harvest rates followed an asymptotic relationship with adult density, with the spear fishery showing more hyperstability than the angling fishery. In the CTWI, naturally reproducing Walleye populations are managed for densities ≥7.4 adults/ha. Our results suggest that maintaining adult Walleye densities near the point of diminishing returns of the asymptotic relationship (10–15 Walleyes/ha) will result in a sustainable fishery that also maximizes tribal harvest and angler catch. However, maintaining adult Walleye densities within this range in the unproductive lakes typical of the CTWI may be unrealistic. Due to the hyperstability observed in each fishery, active management of the spear fishery should continue and monitoring of the angling fishery should also continue given recent declines in natural recruitment and production observed in the CTWI in order to maintain Walleye populations in a “safe operating space.” An empirical understanding of CTWI Walleye angler and spearfisher effort dynamics is critically needed to mechanistically explain the observed hyperstability in each fishery.

Stock structure, dynamics, demographics, and movements of walleyes spawning in four tributaries to Green Bay

Abstract To test assumptions related to the current conceptual model for walleye Sander vitreus management in Green Bay, we evaluated whether: 1) spawning aggregations in the Fox, Menominee, Oconto, and Peshtigo rivers represent genetically distinct stocks; 2) population dynamics and demographics vary among walleye spawning at these locations; 3) walleye spawning in these rivers contribute to the fishery in northern Green Bay, and 4) walleye spawning in these rivers exhibit spawning site fidelity or if they stray among rivers. Genetic differentiation among the four tributaries was low and sex-specific total length (TL), mean TL at age 5, and age-class diversity were generally similar among rivers and observed differences were not consistent. Movements of walleye inferred from angler tag returns suggest that walleye spawning (and tagged) in the four tributaries typically remain within southern Green Bay; however, this assertion may be confounded by the distribution of angling effort that provides tag recoveries. Straying rates among rivers ranged from 0 to 23% and were likely sufficient to preclude genetic differentiation among stocks. Collectively, results suggest that walleye spawning in the Fox, Menominee, Oconto, and Peshtigo rivers do not function as separate stocks and do not significantly contribute to the fishery outside of southern Green Bay. The primary assumption of the current conceptual model that remains to be tested is whether the walleye fishery in southern Green Bay is supported primarily by fish spawning in these four rivers, or if there are substantial contributions from fish spawning at other unknown locations.

Otolith chemistry as a fisheries management tool after flooding: The case of Missouri River gizzard shad

AbstractReduced river–floodplain connectivity can decrease fisheries production and cause ecological and socioeconomic consequences. In 2011, the largest flood on record in the Missouri River since 1898 nearly eliminated connectivity between an embayment (Hipple Lake) and Lake Sharpe, impeding movement of walleye (Sander vitreus) and a forage fish, gizzard shad (Dorosoma cepedianum). Thus, we used otolith chemistry to quantify Hipple Lake's natal contribution to Lake Sharpe's gizzard shad population and forecast effects of connectivity loss on the reservoir's socioeconomically important walleye fishery. Fish were classified to natal habitats with 79–89% accuracy, with most gizzard shad (64%) hatching in floodplain habitats (i.e., embayments, tributaries, canals, and stilling basins). Hipple Lake contributed 12% of gizzard shad to Lake Sharpe, more than a tributary (4%) and embayment (0%) but less than a canal (27%) and stilling basin (21%). Hipple Lake (178 acres) covers 0.31% of Lake Sharpe (56,884 acres), so its natal contribution is 38 times what would be expected if contribution was linearly related to area. Sediment and water management to maintain connectivity between Lake Sharpe and Hipple Lake and other floodplain habitats is important for continued gizzard shad production and prey supply for the walleye fishery. Otolith chemistry facilitates assessment of gizzard shad natal contributions in different habitats, serving as a fisheries management tool to inform floodplain habitat protection and rehabilitation after floods.

Exploring optimal walleye exploitation rates for northern Wisconsin Ceded Territory lakes using a hierarchical Bayesian age-structured model

We assessed population dynamics of walleye (Sander vitreus) in multiple Ceded Territory lakes, which support recreational and tribal fisheries, using a hierarchical Bayesian age-structured model. We used distributions of parameter estimates to develop a dynamic simulation model to forecast performances of walleye fisheries across these lakes under alternative recreational and tribal fishing scenarios. Application of a hierarchical approach allowed us to obtain more accurate estimates of stock–recruitment relationships, natural mortality, maturity and selectivity schedules, and growth parameters for individual lakes, especially for those with relatively uninformative data, and to characterize their variability among lakes. Using standing spawning stock biomass, recreational and tribal harvest, and probability of population collapse as performance metrics, our simulations suggest that northern Wisconsin walleye populations can sustain a regional optimal exploitation rate of about 20% on average given the existing recreational and tribal gear selectivities. However, lake-specific optimal exploitation rates may be higher or lower depending on estimated lake productivities, suggesting that effective management of the Ceded Territory walleye fisheries should account for variability in population dynamics among lakes.

Diet of walleye and sauger and morphological characteristics of their prey in Lake Winnipeg

Abstract Lake Winnipeg has the second largest commercial walleye ( Sander vitreus ) fishery in North America. The lake has experienced and will continue to undergo comprehensive changes such as eutrophication, invasion of non-indigenous aquatic species, and climate change. The North and South Basins of the lake differ in many ways, e.g. mean water depth and temperature, turbidity, and fish community. The study objectives were to establish a baseline for diet of walleye and sauger ( Sander canadensis ) in Lake Winnipeg, to determine whether the diets currently differ between basins and among seasons, and to investigate whether the non-indigenous rainbow smelt ( Osmerus mordax ) contributes substantially to their diet. Walleye and sauger diets in the South Basin had more diverse species composition, whereas in the North Basin, the diet consisted almost entirely of the introduced rainbow smelt (as well as troutperch ( Percopsis omiscomaycus ) for sauger). Some seasonal variation was also documented and found to be attributable to the emergence of young-of-year fish and mayfly larvae. The predominant incorporation of the abundant and large rainbow smelt in the diets of walleye and sauger in the North Basin may lead to increased growth and condition of fish in this basin. These data provide a baseline for comparison in future investigations of walleye and sauger diets as changes occur in the food web of the lake with invasion of zebra mussels ( Dreissena polymorpha ) and spiny water fleas ( Bythotrephes longimanus ).

Assessing environmental stressors on a commercial walleye fishery from a large northern ecosystem (Tathlina Lake) using water chemistry and paleolimnology

Abstract Tathlina Lake (Northwest Territories, Canada) is a large, shallow ecosystem in the rapidly warming northern boreal forest. This lake is of considerable cultural and economic significance as it supports a commercially important walleye ( Sander vitreus ) fishery that has experienced large fluctuations since the 1940s, the causes of which are poorly understood. Here we used paleolimnology to describe long-term environmental changes in the lake that may have contributed to recent collapses in walleye populations. The sub-fossil remains of diatoms and chironomids were used to assess changes in turbidity, nutrients, and oxygen levels, all of which are important to walleye. Minimal changes have occurred in diatom assemblages from the early 1920s to present, suggesting that turbidity and nutrients have not changed markedly in the lake. Hypoxia-tolerant chironomid taxa were found throughout the sediment record, and our modern water chemistry showed that oxygen levels were supersaturated in the summer, but close to the lower tolerance limit of walleye in winter. An increase in sedimentary chlorophyll- a since ~ 1940 suggests Tathlina Lake is affected by recent climate warming. Our findings indicate that walleye populations are likely regularly exposed to hypoxic winter conditions, which may increase the sensitivity of the population to other interacting stressors that occur with recent climate warming. Long-term records of environmental change in large, shallow northern lakes are rare, and paleolimnology provides a framework to reconstruct missing monitoring data, especially in lakes that are economically and culturally important to northern communities.

Landscape Prediction and Mapping of Game Fish Biomass, an Ecosystem Service of Michigan Rivers

AbstractThe increased integration of ecosystem service concepts into natural resource management places renewed emphasis on prediction and mapping of fish biomass as a major provisioning service of rivers. The goals of this study were to predict and map patterns of fish biomass as a proxy for the availability of catchable fish for anglers in rivers and to identify the strongest landscape constraints on fish productivity. We examined hypotheses about fish responses to total phosphorus (TP), as TP is a growth‐limiting nutrient known to cause increases (subsidy response) and/or decreases (stress response) in fish biomass depending on its concentration and the species being considered. Boosted regression trees were used to define nonlinear functions that predicted the standing crops of Brook Trout Salvelinus fontinalis, Brown Trout Salmo trutta, Smallmouth Bass Micropterus dolomieu, panfishes (seven centrarchid species), and Walleye Sander vitreus by using landscape and modeled local‐scale predictors. Fitted models were highly significant and explained 22–56% of the variation in validation data sets. Nonlinear and threshold responses were apparent for numerous predictors, including TP concentration, which had significant effects on all except the Walleye fishery. Brook Trout and Smallmouth Bass exhibited both subsidy and stress responses, panfish biomass exhibited a subsidy response only, and Brown Trout exhibited a stress response. Maps of reach‐specific standing crop predictions showed patterns of predicted fish biomass that corresponded to spatial patterns in catchment area, water temperature, land cover, and nutrient availability. Maps illustrated predictions of higher trout biomass in coldwater streams draining glacial till in northern Michigan, higher Smallmouth Bass and panfish biomasses in warmwater systems of southern Michigan, and high Walleye biomass in large main‐stem rivers throughout the state. Our results allow fisheries managers to examine the biomass potential of streams, describe geographic patterns of fisheries, explore possible nutrient management targets, and identify habitats that are candidates for species management.Received May 20, 2014; accepted November 6, 2014