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walleye.jpg

Sport fish

Description of Gretchen Hansen's sport fish research in Wisconsin.

Sport fish

Managing sport 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:


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.