NHFRA Publications
Research Findings the NHFRA has produced in recent years
The North Highland Fishery Research Area (NHFRA) is committed to producing objective research that is high-priority and high quality. Through this research, our scientists, biologists and technicians provide valuable insights that inform decision-making for Wisconsin fisheries and future management decisions within the fields of:
- Applied fisheries management and ecology
- Quantitative stock assessments
- Fish habitat
- Propagation science
- Population dynamics
- Predator-prey interactions
- Human dimensions
- Survival
- Climate resilience
As a large contributor towards fisheries research within the DNR and beyond, the NHFRA has been the foundation for countless long-term and experimental fisheries studies. By incorporating mandatory angler creel surveys for all who fish there and compiling that data over the years, their critical findings have become not only reflective of Wisconsin’s unique ecosystems but also relevant to a broad range of external stakeholders.
Below, you can find a list of scientific publications that have been a direct result of NHFRA research. Current and former DNR staff are indicated in bold, while an asterisk denotes graduate or post-doctoral students who have worked with the DNR.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.