Modeling the Sustainability of Walleye Populations in Northern Wisconsin Lakes

Abstract

Walleye populations in northern Wisconsin face exploitation from angling and spearing fisheries that are regulated using a total annual exploitation rate of 35%. However, the sustainability of the 35% exploitation rate has never been evaluated, so my primary objective was to evaluate the sustainability of walleye populations across a range of lake sizes ( abundance varies with lake size) that are then subjected to ranges of exploitation rates and allocations of angling and spearing harvest (the fisheries differ greatly in size selectivity). I also sought to determine if age at maturity and length-specific fecundity were density dependent for walleyes in Big Crooked Lake, Wisconsin, during 1997-2003, where walleye were purposely subjected to high exploitation as part of a field evaluation of walleye population sustainability under exploitation stress. I completed my secondary objective first to incorporate the results into my primary objective. To complete my secondary objective, I evaluated the effect of adult walleye Sander vitreus population density on age at 50% maturity and length-specific fecundity in Big Crooked Lake, Wisconsin, during 1997-2003. Abundance of adult walleye from mark-recapture surveys ranged from 2,046 fish (3 walleye/acre) to 4,901 fish (7 walleye/acre). Age at 50% maturity ranged from 3.89 years to 4.88 years, length of walleye sampled for fecundity ranged from 13.0 in to 24.7 in, and average fecundity of a 17-in walleye ranged from 41,061 eggs to 53,009 eggs. Age at 50% maturity increased significantly as adult walleye population density increased, whereas average fecundity of a 17-in walleye did not change significantly with density. Therefore, age at 50% maturity could be used as an indicator of population density and exploitation stress and thus could be used to set desired levels of harvest. To complete my primary objective, I 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 abundance. The age-structured population model was parameterized from intensive surveys of walleye populations in Escanaba Lake and extensive surveys of walleye populations in northern Wisconsin lakes. Simulations covered a range of annual exploitation rates that included the currently-accepted rate of 3 5% and a range of population sizes that are presently included in regression models that relate walleye abundance to lake surface area. The risk of extinction began to increase above zero at an exploitation rate of 56-61% for an unregulated angling fishery, 73-76% for an angling fishery with a 15 in minimum length limit, and 75-80% for a spearing fishery. The probability of decline began to increase above zero at an exploitation rate of 47% for an unregulated angling fishery, 60% for an angling fishery with a 15 in minimum length limit, and 60% for a spearing fishery. As the exploitation rate increased, the average adult abundance decreased and the time to extinction decreased for all lake sizes and initial population sizes. I conclude that the current maximum exploitation rate of 35% is sustainable for all lake sizes and initial population sizes and that angler daily bag limits and spearing quotas could be increased while still ensuring the sustainability of walleye populations. However, steps should be taken to implement a field experiment to verify the results of the simulation model before the results are used for management.Wisconsin Department of Natural Resources and by the U. S. Forest Service, Federal Aid in Sport Fish Restoration (grant F-95-P)