Evaluating Walleye Length Limits in the Face of Population Variability: Case Histories from Western Minnesota

Abstract

Population and fishery responses to the implementation of minimum length limits (356 and 381 mm total length) for walleyes Sander vitreus were evaluated for two Minnesota populations (Big Stone Lake and Lac Qui Parle) and were compared with population trends observed in a single reference lake (Lake Traverse) between 1991 and 2004. High variation in walleye recruitment (coefficient of variation (CV = 100 × SD/mean) in mean gill-net catch per unit effort of age-2 walleyes > 98%) was observed in all study populations and also in recruitment indices from 20 populations across North America (mean CV = 112%), indicating that high variability in recruitment is common. Walleye growth rates were also variable; mean length at age 3 varied by more than 120 mm among year-classes in all three study populations. Observed trends in population age- and size-structure indices estimated from gill nets appeared to be largely related to growth and recruitment patterns both before and after the regulations were in place. No direct evidence existed to indicate that adult walleye abundance, size structure, or age structure was improved after implementation of length limits or that the regulations reduced annual variation in size structure. In light of recruitment variation, meaningful evaluation of walleye length limits will require long-term annual sampling efforts designed to monitor the fate of multiple year-classes of similar magnitudes during both preregulation and postregulation periods. Lastly, managers must select meaningful metrics to measure the effects of length limits. Observed improvements in fishery-related metrics, such as size structure of harvested fish, may merely reflect changes in angler behavior rather than actual improvements in the population. Also, fishery-based metrics may be difficult to measure given the budgetary and logistical constraints associated with creel surveys.