Simulated Equilibrium Walleye Population Density under Static and Dynamic Recreational Angling Effort

Abstract

Understanding the dynamics of angling effort and how fish populations are affected by angler effort is an important and often unappreciated component of fisheries management. Our objective was to determine the extent to which angling-induced mortality limits walleye Sander vitreus population density in northern Wisconsin lakes. We developed a simulation model to evaluate the effects on long-term equilibrium walleye population density caused by effort limitations, density-dependent angling effort, and angling regulations. Equilibrium density was limited by angling effort when effort was held constant but was generally lower when effort was density dependent. Equilibrium density ranged from 3.46 to 21.79 adults/ha when effort was density dependent, similar to the observed median density in lakes sustained by natural reproduction (8.7 adults/ha). Median equilibrium density was 23.3 adults/ha when effort was zero. Equilibrium density was higher when harvest was regulated by a minimum length limit as opposed to no length limit and with no postrelease mortality than with postrelease mortality. Population collapse was more likely and equilibrium density was lower when the population density and effort/ha relationship was strongest. Because the observed relationship between adult walleye population density and effort/ha was weaker, we conclude that open-access walleye fisheries in northern Wisconsin are generally self-regulating. However, populations likely persist at lower population densities than would be expected if angling effort were limited to lower levels than those currently observed. Our results are intuitive; however, they suggest that density increases may be on the order of 100% when effort is severely restricted compared with open-access fisheries with unlimited effort. Other fish species may experience different expected density increases with restricted angler effort depending on a number of factors, but our results suggest that population density changes can be dramatic and that similar modeling exercises may be useful to managers of other fish species if increasing density is of interest.

Received December 19, 2011; accepted June 13, 2012

ACKNOWLEDGMENTS

We thank Doug Beard for his initial discussions on this work. This work was supported by the National Marine Fisheries Service and the Federal Aid in Sportfish Restoration program. In addition, we thank the Wisconsin Department of Natural Resources and the Great Lakes Indian Fish and Wildlife Commission staff, who painstakingly collected the data utilized in this analysis. Finally, we thank Paul Conn and Jim Waters for their comments on earlier drafts of this manuscript.