NHFRA Publications

Diminishing productivity and hyperstable harvest in northern Wisconsin walleye fisheries (2024)

Mrnak, J.T., Embke, H.S.*, Wilkinson, M.V., Shaw, S.L., Vander Zanden, J.M. & Sass, G.G.

Key Findings:

• After analyzing 32 years of data from the Ceded Territory of Wisconsin, it was found that productivity decreases and sustained harvest rates may be jointly contributing to observed walleye declines.
• Ceded Territory walleye lakes were classified into management groups of low, moderate or high vulnerability to harvest.
• Harvest declines may help to maintain or increase the adaptive capacity of walleye in the Ceded Territory.

Lagging spawning and increasing phenological extremes jeopardize walleye (Sander vitreus) in north-temperate lakes (2024)

Martha E. Barta, Greg G. Sass, Jeffrey R. Reed, Thomas A. Cichosz, Aaron D. Shultz, Mark Luehring, Zachary S. Feiner

Key Findings:

• This study assessed the spawning phenology of walleye in response to climate change.
• Ice-off phenology shifted earlier, about 3x faster than walleye spawning phenology over time.
• Spawning phenology deviations from historic averages increased in magnitude over time; large deviations were associated with poor offspring survival.

Whole-lake coarse woody habitat addition facilitates ecosystem regime restructuring in an oligotrophic lake (2024)

Preul-Stimetz, T.N., Shaw, S.L., Sass, G.G.

Key Findings:

• Coarse woody habitat (CWH) additions were conducted on Sanford lake in 2018 with Escanaba Lake serving as a reference system; Data collection began in 2015.
• Macrophyte density, chlorophyll-a and dissolved oxygen concentration increased in Sanford as a result of CWH addition, whereas water clarity and temperature did not change.
• The abundance of large zooplankters such as Daphnia declined as a result of CWH addition.

Stable isotopes reveal trophic ontogeny in cisco (Coregonus artedi) (2024)

Martin, B.E. & Mrnak, J.T.

Key Findings:

• Analyzed cisco 13C and 15N isotopes to assess how body size relates to trophic position.
• 15N and 13C isotopes showed cisco trophic position and pelagic reliance increased with body size.
• Larger cisco used deeper habitats than smaller cisco.
• Results suggest that cisco have significant within population variability in their trophic niche even though they are traditionally thought to only feed on zooplankton.

Evaluating the potential importance of individual identity, maternal traits, and environment as predictors of egg characteristics in walleye Sander vitreus (2024)

Preul-Stimetz, T.N., S.L. Shaw, Z.S. Feiner, and G.G. Sass

Key Findings:

• Egg size was linked to environmental stochasticity and maternal size over time.
• Fish identity was the strongest predictor of egg quality independent of other factors.
• Implications for genetic drivers of egg quality influenced by climate variation.

Density- and size-dependent mechanisms that modulate the outcome of stocking in a naturally recruiting freshwater piscivore (northern pike, Esox lucius): a replicated whole-lake experiment (2023)

Hühn, D., D. C. Gwinn, S. L. Shaw, J. Alos, M. S. Allen, T. Pagel, C. Skov and R. Arlinghaus. 

Key Findings:

• The study investigated the population-level outcome of stocking juvenile northern pike in naturally reproducing lentic stocks in 15 experimental lakes involving two stocking densities and unstocked controls.
• Stocked and wild fishes showed contrasting responses in terms of growth to variation in predator density, competitor density and the forage base.
• The authors concluded that whenever a natural northern pike population exists, stocking juvenile northern pike will not produce additive effects.

Evaluation of survival and cost to harvestable age of stocked fall fingerling walleye (Sander vitreus) in Northern Wisconsin Lakes (2023)

Elwer, B.M., J.A. VanDeHey, S.L. Shaw, L.W. Sikora, J.T. Mrnak, and G.G. Sass

Key Findings:

• The authors’ objectives were to test for differences in natural mortality of stocked walleye using several variables; estimate cost to harvestable age; and compare predicted adult walleye abundance in lakes with various stocking densities to Wisconsin walleye adult abundance standards.
• The results may inform science-based decision making and best-case scenario stocking rates to increase the probability of reestablishing walleye natural recruitment. 

Depensation in fish recruitment driven by context-dependent interactions with another predator (2023)

Dassow, C.D, G.G. Sass, S.L. Shaw, Z.S. Feiner, C. Nieman, and S.E. Jones

Key Findings:

• Using empirical estimates of depensation from 28 walleye populations in Wisconsin, the authors developed a predictive model to test for depensation on an additional 115 lakes without sufficient data to empirically test for depensation.
• The authors found that interactions between largemouth bass relative abundance, climate, land use and the fish community were correlated with the probability of depensation in walleye populations.
• This predictive framework could be used to prioritize lakes for different management actions based on depensation strength and average adult population size.

Seasonal habitat use of yellow perch Perca flavescens in a north-temperate lake (2023)

Feucht, L.M, L.W. Sikora, G.P. Shay, G.G. Sass and J.T. Mrnak

Key Findings:

• We characterized yellow perch seasonal spatial distribution, size–structure and condition between littoral and pelagic habitats in Crystal Lake (Vilas Co., WI) during the open water period of 2021.
• The findings suggest that after spring spawning, larger yellow perch prefer pelagic habitats, while sub-adults and juveniles prefer littoral habitats, potentially resulting from increased predator refugia and/or resource availability.

Response of yellow perch to water level fluctuations in north-temperate lakes (2023)

Shay, G.P., G.G. Sass, and J.T. Mrnak

Key Findings:

• This study looked at population-level responses of yellow perch to natural water level fluctuations in four oligotrophic lakes in northern Wisconsin.
• The lack of statistically significant findings potentially suggests a buffering mechanism of the lakes due to their small surface area to volume ratios, relative lack of nutrients and(or) littoral structural habitat.
• Natural water level fluctuations may not be an environmental concern for yellow perch populations in similar lakes.

A spring population survey impacts nest site fidelity but not reproductive timing in male smallmouth bass (Micropterus dolomieu) (2023)

Stegens, E. D.D. Wiegman, L.M. Angeloni. J.R. Baylis, R.A.S. Laroche, S.P. Newman, S.P. Egan, G.G. Sass, K.L. Weinersmith 

Key Findings:

• Spring electrofishing surveys for smallmouth bass have the potential to displace breeding males from preferred nesting habitats.
• Electrofishing and fyke netting did not influence timing of reproduction.
• If displacement negatively influences fitness, spring electrofishing surveys would not be recommended for assessing smallmouth bass populations.

Do angler catch and harvest rates differ between open water and ice anglers in Wisconsin? (2023)

Greg G. Sass, Samuel T. LaMarche, Zachary S. Feiner 

Key Findings:

• WDNR point-intercept creel survey data was used to test for seasonal fisheries differences for black crappie, bluegill, northern pike, walleye and yellow perch targeted by open water and ice anglers in Wisconsin lakes.
• Our results suggest that ice angling should not be assumed to have a negligible influence on fish populations in north-temperate lakes; fisheries-dependent and -independent monitoring data should be jointly considered when assessing fish population status.

Shrinking body size and climate warming: many freshwater salmonids do not follow the rule (2023)

Solakas, M.A., Feiner, Z.S., Al-Chokachy, R., Budy, P.E., DeWeber,J.T., Sarvala, J., Sass, G.G., Tolentino, S.A., Walsworth, T.E., and Jensen. O.P.

Key Findings:

• This study examines whether declining body length in fish species is a response to a warming climate.
• Populations of 12 freshwater salmonid species in the Northern Hemisphere were examined; more than two-thirds of the populations increased in body length over time, with high variability among populations and species.
• Physically larger populations are experiencing greater increases in length than smaller populations.
• Results suggest that declining body length in response to climate change is not universal and may be largely influenced by local factors.

Synthesizing Professional Opinion and Published Science to Build a Conceptual Model of Walleye Recruitment (2023)

Krabbenhoft, C.A., Ludsin, S.A., Marschall, E.A., Budnik, R.R., Almeida, L.Z., Cahill, C.L., Embke, H.S.*, Feiner, Z.S., Schmalz, P.J., Thorstensen, M.J., Weber, M.J., Wuellner, M.R. and Hansen, G.J.A.

Key Findings:

• Environmental drivers of walleye recruitment differ by geographic region, lake surface area and life stage.
• A conceptual model was created to illustrate the abiotic and biotic characteristics that affect walleye recruitment.
• Walleye may serve as an example of finding common recruitment drivers and potentially applying to other systems.

Density-dependent and environmental influences on juvenile walleye survivorship in northern Wisconsin lakes (2022) 

Zebro, L.R., J.T. Mrnak, S.L. Shaw, S.R. Chipps, and G.G. Sass

Key Findings:

• Walleye (Sander vitreus) natural recruitment has declined in northern Wisconsin lakes over time.
• Elevated age-0 to age-1 walleye mortality at low age-0 CPE supports previous findings of depensatory recruitment dynamics in northern Wisconsin walleye populations.
• Additional research is needed to address elevated juvenile walleye mortality at low adult stock sizes and/or with declining natural recruitment to inform management decisions.

Species differences, but not habitat, influence catch rate hyperstability across a recreational fishery landscape. (2022)

Mosley, C., C. Dassow, J. Caffarelli, A. Ross, G.G. Sass, S.L. Shaw. C.T. Solomon, and S.E. Jones

Key Findings:

• Catch rates remain high even as fish stocks decline (i.e., hyperstability of catch rates) leading to delayed management intervention and overexploitation.
• The authors found significant differences in the strength of hyperstability amongst species but did not identify a consistent influence of habitat on hyperstability of catch rates.
• Angler preferences and behavior may explain some of the variance in non-proportional catch rates.

RAD decision-making tool: an application to walleye (Sander vitreus) in Wisconsin (2022)

Dassow, C.J., A.W. Latzka, A.J. Lynch, G.G. Sass, R.W. Tingley III, and C. Paukert

Key Findings:

• A decision-support tool was developed and applied to the walleye recreational fishery in Wisconsin, as an example of how to link the Resist-Accept-Direct framework to real-world management of a large recreational fishery.
• The tool and broadscale results can inform decisions about whether to resist, accept, or direct for specific walleye populations.

Structural Habitat in Lakes and Reservoirs: Physical and Biological Considerations for Implementation (2022)

Sass, G.G., S.L. Shaw, C.C. Fenstermacher, A.P. Porreca, and J.J. Parkos

Key Findings:

• A framework is needed to guide management expectations when using coarse woody habitat (CWH) additions in different ecosystems.
• Rates of response to CWH additions varied among fish species studied.
• Influences can differ between CWH additions in an oligotrophic natural lake and an eutrophic, turbid reservoir, as well as by longitudinal position within a reservoir.

Similar environmental conditions are associated with walleye and yellow perch recruitment success in Wisconsin lakes (2022)

Brandt, E.J.*, Z.S. Feiner, A.W. Latzka, and D.A. Isermann

Key Findings:

• Important predictors of walleye and yellow perch recruitment include winter conditions, growing degree days, spring temperature variation, peak summer temperature, and Secchi depth.
• Results indicate that landscape-level patterns in recruitment success for the two species are likely similar.
• Declines in yellow perch could influence prey availability and survival of age-0 walleye as well as influence angler harvest. This may lead to management implications such as changes in stocking rates or increased harvest regulations.

Resist-Accept-Direct (RAD) considerations for climate change adaptation in fisheries: the Wisconsin experience (2022)

Feiner, Z.S., A.D. Shultz, G.G. Sass, A. Trudeau*, M.G. Mitro, C.J. Dassow, A.W. Latzka, D.A. Isermann, B.M. Maitland*, J.J. Homola, H.S. Embke*, and M. Preul

Key Findings:

• Current RAD strategies focus mainly on Resist actions; they may need to shift towards Accept and Direct actions in the future.
• Wisconsin’s diverse inland fisheries were used as a case study, reviewing management strategies for fisheries in response to climate change within the RAD framework.
• Co-production of policy and an adaptive management framework between state and tribal authorities could lead to more appropriate lake management action.

A perspective on the ecological and evolutionary consequences of phenological variability in north-temperate lakes (2022)

Feiner, Z.S., H.A. Dugan, N.R. Lottig, G.G. Sass, and G.A. Gerrish

Key Findings:

• The average date of ice-off is shifting earlier and becoming more variable.
• This shift alters limnological conditions and yields food web responses that have impacts on fish spawn timing and recruitment success.
• An understanding of how genetics plays into the response of aquatic communities to increasingly variable ecosystems is needed.

Black crappie influences on walleye natural recruitment in northern Wisconsin lakes (2022)

Broda, S., Z.S. Feiner, J.T. Mrnak, S.L. Shaw, and G.G. Sass

Key Findings:

• In Wisconsin Lakes, age-0 walleye relative abundance was always low, while black crappie relative abundance was high.
• Results suggest that black crappie may negatively influence walleye recruitment.
• These findings only reveal a pattern; more research is needed to inform the co-management of both species.

Resisting ecosystem transformation through an intensive whole-lake fish removal experiment (2022)

Embke, H.S.*, S.R. Carpenter, D.A. Isermann, G. Coppola, T.D. Beard, A.J. Lynch, G.G. Sass, Z.S. Feiner, and M.J. Vander Zanden

Key Findings:

• Examined whether intensive management actions such as the experimental removal of ~285,000 centrarchids (bass and sunfish species) over four years would improve walleye recruitment.
• As centrarchids were removed, yellow perch increased in abundance, but there was no evidence of walleye recruitment.
• Findings provide a platform for management discussions to move beyond resist strategies in the Resist-Accept-Direct (RAD) climate adaptation framework to navigate ecosystem change.

Female sex ratio bias in extended growth hatchery Walleye Sander vitreus produced in Wisconsin (2022)

Sass, G.G., S.L. Shaw, J.A. Gorne, D. Godard, N. Nietlisbach, D. Giehtbrock, A. Sikora*, G. Muench, L. Tate, L. Wawronowicz, and H.-M. Hsu

Key Findings:

• Extended growth hatchery (EGW) walleye raised in Wisconsin under similar incubation and rearing protocols may be skewed toward females.
• If sex ratios of EGW are skewed towards females, the use of EGW may hinder efforts to rehabilitate natural recruitment.
• An experimental and observational framework was used to test for factors influencing the high percentage of females in the hatchery product to inform protocols to better balance sex ratios.

Applying panarchy theory for aquatic invasive species management: a case study on invasive rainbow smelt Osmerus mordax (2022)

Mrnak, J.T., L. Sikora, M.J. Vander Zanden, and G.G. Sass

Key Findings:

• Invasive rainbow smelt causes negative effects on some native ecosystems.
• The success of invasive rainbow smelt management depended on whether enough rainbow smelt was removed and, if desired, native species filled their place.
• The probability of successful invasive species control and(or) native species restoration may depend on management intervention during the release or collapse phase of the nested adaptive cycle.

Aquatic community responses to the cessation of invasive Eurasian watermilfoil chemical treatments in a north-temperate USA lake (2022)

Schleppenbach, B.T.*, G. Matzke, S.L Shaw, and G.G. Sass

Key Findings:

• Zooplankton density increased immediately post-chemical treatment and then stabilized.
• There were immediate increases in recruitment post-chemical treatment for largemouth bass, smallmouth bass, bluegill, and yellow perch.
• Results suggest long-term 2,4-D treatments may negatively influence fish through lethal and sublethal mechanisms.

Demographic and life history characteristics of black bullheads Ameiurus melas in a north temperate USA lake (2022)

Sikora, L.W., J.T. Mrnak, R. Henningsen, J.A. VanDeHey, and G.G. Sass

Key Findings:

• Results suggested that black bullheads exhibited relatively fast growth rates, early ages at maturity, moderate fecundity, and a diverse omnivorous diet.
• Black bullheads have the potential to dominate fish community biomass in their native and introduced range.
• Results from this study may inform the management of black bullhead as a native and invasive species.

Catch-and-release angling effects on Lake Sturgeon in Wisconsin, USA (2022)

Shaw, S. L., Z. Lawson, J. Gerbyshak, N. Nye and M. Donofrio

Key Findings:

• Water temperature had a positive effect on sturgeon reflex impairment and its need for recovery time. Recovery time was correlated with how long the fish was out of water.
• There was no evidence of sturgeon mortality related to the angling events.
• Managers considering catch-and-release fisheries for lake sturgeon should consider population-specific relative abundance and catch rates.

Social Fish-Tancing in Wisconsin: The effects of the COVID-19 Pandemic on statewide license sales and fishing effort in northern inland lakes (2022)

Trudeau, A., B. Beardmore, G.A. Gerrish, G.G. Sass, and O.P. Jensen

Key Findings:

• In the summer of 2020, fishing license sales in Wisconsin, USA, increased while vehicle counts at lake access points varied considerably by lake.
• Lakes with greater proportions of public shoreline experienced pandemic-associated increases in lake visitors.
• Results suggest that the distribution of recreational fishing in Wisconsin potentially placed additional harvest pressures on hot-spot inland lakes.

Sportfish behavioral responses to a littoral coarse woody habitat addition in a north-temperate lake (2021)

Smith, Q.C.*, G.G. Sass, T.R. Hrabik, S.L. Shaw, and J.K. Raabe

Key Findings:

• The effects of coarse woody habitat (CWH) enhancements on sportfish behavior were studied by adding 160 trees to the littoral zone of Sanford Lake, Vilas County in 2018.
• Radio telemetry data collected pre-manipulation (2017) and post-manipulation (2018-2019) were used to construct annual home range estimates for muskellunge, smallmouth bass and walleye.
• Home ranges increased over the course of three years, suggesting a behavioral response to the CWH addition that may be species-specific.

Demographic rate variability of bighead and silver carp along an invasion gradient (2021)

Erickson, R.A., Kallis, J.L., Coulter, A.A., Coulter, D.P., MacNamara, R., Lamer, J.T., Bouska, W.W., Irons, K.S., Solomon, L., Stump, A.J., Weber, M.J., Brey, M.K, Sullivan, C.J., Sass, G.G., Garvey, J.E., & Glover, D.C.

Key Findings:

• Length-weight relations and growth curves varied among the subpopulations studied, whereas maturity curves did not.
• Findings demonstrated spatial variability in demographic rates for bighead and silver carp across a broad geographic area in relation to invasion status and river conditions.
• General subpopulation management options are provided to explain this observed spatial variability in demographic rates.

Fish community changes associated with bullhead removals in four northern Wisconsin lakes (2021)

Sikora, L.W., VanDeHey, J.A., Sass, G.G., Matzke, G., & Preul, M.

Key Findings:

• Adult walleye abundance has increased or remained steady in all bullhead removal lakes.
• Declines in the abundance of bluegill and increases in the abundance of yellow perch and black crappie were observed after bullhead removals.
• Observations suggest that removing bullheads can increase recruitment, survival, and abundance of sport fishes like walleye and yellow perch.

Genomic and environmental influences on resilience in a cold-water fish near the edge of its range (2021)

Ackiss, A.S., M.R. Magee, G.G. Sass, K. Turnquist, P.B. McIntyre, and W.A. Larson

Key Findings:

• High levels of genetic differentiation among populations were punctuated by a phylogeographic break and residual patterns of isolation-by-distance.
• Neutral and non-neutral genetic diversity were most strongly correlated with lake surface area.
• Differences among lakes in the availability of estimated oxy-thermal habitat left no clear population genomic signature.

Hydroacoustic surveys underestimate yellow perch population size: the importance of considering habitat use (2021)

Mrnak, J.T., Sikora, L.W., Vander Zanden, M.J., Hrabik, T.R., & Sass, G.G.

Key Findings:

• When using spring mark-recapture surveys, the mean population abundance estimate was 4.0–8.5 times greater than when using summer hydroacoustic estimates.
• It was assumed that the spring mark–recapture survey sampled the entire adult population, while summer hydroacoustics sampled the post-spawn pelagic component.
• The study highlights the importance of evaluating gear bias and considering multiple habitat preferences within a lake.

Environmental influences on walleye Sander vitreus and muskellunge Esox masquinongy angler catch in Escanaba Lake 2003 – 2015 (2021)

S.L. Shaw, K.M. Renik, and G.G. Sass

Key Findings:

• Fishing trip success and catch rates for both species were most strongly influenced by angler-related variables.
• Walleye catch success was positively associated with environmental factors such as lower light, lower air temperatures, lunar position (overhead or underfoot), and lunar phase (full moon or gibbous).
• Muskellunge catch success was positively associated with environmental factors such as mean daily solar radiation and increased air temperature.

Empirical evidence for dispensation in freshwater fisheries (2021)

Sass, G.G., Feiner, Z.S., & Shaw, S.L.

Key Findings:

• Of the walleye populations evaluated, about half exhibited depensatory recruitment dynamics (low juvenile survival at low adult stock sizes).
• There were few clear cases of compensation, with most populations exhibiting weak density dependence.
• Suggests that depensation could explain declines in walleye populations in the Ceded Territory of Wisconsin and may be implicated in other invisible collapses of freshwater fisheries.

Plastic abundance and demographic responses of walleye to elevated exploitation in a north-temperate lake (2021)

Sass, G.G., S.L. Shaw, L.W. Sikora, M. Lorenzoni, and M. Luehring

Key Findings:

• Results suggest walleye compensatory age-0 to age-1 survival and declines in length at maturity interacted to offset elevated-exploitation effects on adult density.
• Density-dependent growth responses were most evident in juveniles.
• This level of long-term exploitation is likely unsustainable for most walleye populations due to findings of previous exploitation studies and observations of dispensatory recruitment dynamics.

Celebrating 75 years of the Northern Highland Fishery Research Area: the past, present and future (2021)

Sass, G.G., S.L. Shaw, and K.M. Renik

Key Findings:

• The Northern Highland Fisheries Research Area (NHFRA) has been operating since 1946.
• Over the years, the NHFRA has evaluated experimental regulations, provided data critical to developing a walleye management plan for a joint tribal subsistence and recreational fishery, and has been a training ground for early career fisheries professionals.
• Highlights the importance of adaptation, long-term data, establishing sentinel lakes for observations of environmental change, field stations, and partnerships for successful fisheries management.