Modeling the Sustainability of Walleye Populations in Northern Wisconsin Lakes

Population sustainability of walleyes Sander vitreus in the face of angling and spearing fisheries has been a concern of fishery managers in northern Wisconsin since off-reservation tribal fishing rights were affirmed by federal court decisions. We evaluated sustainability of walleye populations across a range of densities that were subjected to various exploitation rates and allocations of angling and spearing harvest (the fisheries differ in size selectivity). We developed an age-structured population model for estimating extinction risk and time to extinction for a hypothetical walleye population at a specified exploitation rate, fishery allocation, and initial density. The model was parameterized from data obtained during surveys of walleye populations in northern Wisconsin lakes. Simulations covered a range of annual exploitation rates that included the currently accepted rate of 35% and a range of population densities that are presently included in regression models relating walleye abundance to lake surface area. ne risk of extinction began to increase above zero at exploitation rates of 61-65% for an unregulated angling fishery, 85-86% for an angling fishery with a 15-in minimum length limit, and 77-78% for a spearing fishery. The probability of decline began to increase above zero at exploitation rates of 61-65% for an unregulated angling fishery, 84-85% for an angling fishery with a 15-in minimum length limit, and 72-77% for a spearing fishery. As the exploitation rate increased, the average adult density decreased and the time to extinction decreased for all initial population densities. We conclude that the current maximum exploitation rate of 35% may be sustainable for all initial Population densities but that a controlled field experiment should be used to verify our modeling results before any changes to angling bag limits or spearing quotas are made.