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walleye.jpg

Fish community change

Description of Gretchen Hansen's research on changing fish communities.

Fish community change in lakes

Managing fisheries requires understanding of spatial and temporal patterns in populations and food webs. My research focuses on identifying long term trends, testing hypotheses for explaining those trends, and developing predictive models of population abundance to be used in a management context. Current work includes:


Water Clarity and temperature as determinants of walleye abundance

walleye swimming

Walleye are an important sport fish in Minnesota and are managed in over 1,400 lakes. Declining walleye populations in Midwestern lakes are a major concern, and walleye lakes must be managed with limited resources. Walleye prefer low water clarity and cool water temperatures. Water clarity and temperature combine to determine the amount of suitable walleye habitat in a lake, known as its thermal-optical habitat area. If clarity and/or temperature change in a lake, walleye habitat is likely to be effected, although the magnitude of the effect depends on lake depth and starting conditions.  

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Successful management of natural resources requires that local action must adapt to larger-scale environmental changes in order to maintain ecosystems within the bounds of acceptable conditions. This idea, known as the safe operating space, suggests that local action can influence ecosystem response to global change if management can identify the change and adapt. We applied these concepts to understanding walleye declines and potential management responses in Mille Lacs, a popular walleye fishery in Minnesota.

  • As the water has gotten clearer in Mille Lacs, the area of the lake with suitable walleye habitat has gotten smaller. Because Mille Lacs is fairly shallow and uniform in depth, the walleye cannot retreat to deeper water as they can in some clear lakes.

  • Temperature is also important in determining walleye habitat, although to date water clarity has been the more important factor in determining walleye habitat in Mille Lacs. This could change as temperatures warm, with potentially further reductions in walleye habitat.

  • Changing environmental conditions means that harvest levels that used to be sustainable under previous conditions may not be sustainable under new conditions.

  • Our research suggests that harvest policies that adapt to changes in water clarity and temperature can help sustain walleye populations over the long-term. Harvest may have to decrease when light and temperature conditions are poor for walleye.

  • Theory suggests that in the long run, adapting to changing environmental conditions is likely to allow for more harvest of walleye compared to harvesting too many as conditions become unfavorable, which could cause a population collapse.

    Hansen, G. J. A., L.A. Winslow, J. S. Read, M. Treml, P. J. Schmalz, and S.R. Carpenter.  2019. Water clarity and temperature effects on walleye safe harvest: an empirical test of the safe operating space concept. Ecosphere 10(5): e02737. 10.1002/ecs2.2737

Current work led by post-doc Dr. Kelsey Vitense will expand these findings to lakes throughout Minnesota to evaluate the status and sensitivity of walleye habitat to changes in water temperature and clarity.


Predicting walleye recruitment

Predicted probability of natural walleye recruitment success in Wisconsin lakes based on lake size, water temperature growing degree days, conductance, and shoreline complexity.

Predicted probability of natural walleye recruitment success in Wisconsin lakes based on lake size, water temperature growing degree days, conductance, and shoreline complexity.

Recruitment is one of the most variable and most influential vital rates of any fish population. Identifying factors that control recruitment is a long standing goal of fisheries science. Furthermore, forecasting recruitment success on a lake-specific basis allows managers to develop strategies that account for spatial variability in sport fish populations and adapt to changing conditions.

Walleye recruitment in Wisconsin's inland lakes is predictable with a high degree of accuracy (83% success rate) based on lake morphology and lake temperature (specifically, growing degree days). We have developed this model for use in identifying lakes where natural reproduction of walleye is most likely. Current work focuses on expanding this model to predict walleye recruitment success in the future, and predicting the abundance of other sport fish species such as largemouth bass.

Hansen, G. J. A., S.R. Carpenter, J. W. Gaeta, J. M. Hennessy, and M. J. Vander Zanden.  2015. Predicting walleye recruitment as a tool for prioritizing management actions. Canadian Journal of Fisheries and Aquatic Sciences 72(5): 661-672. DOI 10.1139/cjfas-2014-0513


Safe harvest of wisconsin's walleye

Ratio of safe harvest quota for walleye under alternative models to that under the current model, as a function of the number of observed population estimates available for a lake. Two panels show results using all available data or only those collected in the past 20 years.

Ratio of safe harvest quota for walleye under alternative models to that under the current model, as a function of the number of observed population estimates available for a lake. Two panels show results using all available data or only those collected in the past 20 years.

Walleye in northern Wisconsin support a joint fishery comprised of recreational angling and tribal spearing and netting. Harvest is managed through the annual application of safe harvest quotas to each of hundreds of lakes, which set the maximum allowable catch at 35% of the adult walleye population. Because direct estimates of adult walleye population size are not available for every lake and every year, managers rely on a statistical model to predict adult walleye abundance on a lake-specific basis to set these safe harvest levels. Together with Wisconsin DNR fisheries managers, I have developed a new statistical model for predicting adult walleye populations that accounts for variability both within and among lakes, and allows harvest to more closely track changing walleye abundance over time. Under this new model, harvest is more conservative in lakes lacking direct observations of walleye abundance, and more liberal in lakes with many direct estimates showing a healthy walleye population.

Hansen, G. J. A., J. M. Hennessy, T. Cichosz, and S.W. Hewett.  2015. Improved models for predicting Walleye abundance and setting safe harvest quotas in northern Wisconsin lakes. North American Journal of Fisheries Management 35: 1263-1277. DOI 10.1080/02755947.2015.1099580

Smallmouth bass in northern Wisconsin. Photo credit Gretchen Hansen.