Evaluating Methods of Estimating Walleye Angling Exploitation in Northern Wisconsin Lakes
Deroba, Jonathan J.
University of Wisconsin-Stevens Point, College of Natural Resources
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Creel Surveys are used by the WDNR and GLIFWC to estimate walleye (Sander vitreus) angling exploitation rate, but are expensive and time consuming, and the assumptions of these methods have not been tested. The objectives of this thesis were to determine: (1) if angler use and success varied among hours, months, and years during 1991-2002 in northern Wisconsin lakes: (2) if creel survey efficiency could be improved by reallocating sampling effort within months to minimize bias and maximize precision of estimates of walleye angling effort, harvest, harvest rate, and exploitation rate in northern Wisconsin lakes; and (3) if estimates of exploitation rate of walleyes in Wisconsin were biased by lack of mark recognition or fin regeneration. To achieve my first objective, I examined trends in the average number of complete-trip interviews, effort per acre, numbers of walleyes harvested per angler hour, and numbers of walleyes harvested per acre among hours, seasons, and years during 1991-2002 in northern Wisconsin lakes. The average number of complete-trip interviews within days was higher on weekends than weekdays during the open-water and ice-fishing seasons, and trends within days differed between seasons. The average number of walleyes harvested per hour and trends within days were similar on weekends and weekdays and during open-water and ice-fishing seasons. Average angling effort per acre was higher on weekdays than weekends, and trends during the angling season differed between day types. The average number of walleyes harvested per hour was similar on weekends and weekdays, but trends differed between day types during the angling season. Average harvest per acre and trends during the angling season were similar on weekends and weekdays. Effort per acre, walleye harvested per hour, and walleye harvested per acre were similar between day types, and did not change significantly during 1991-2002. Angler use and success in northern Wisconsin lakes changed systematically within days and among seasons, but not among years during 1991-2002, which suggests that the walleye fishery was stable during that period in northern Wisconsin lakes. To achieve my second objective, I compared estimates of effort, harvest rate, harvest, RIC ratio, and recaptures from four reductions in sampling effort (one week per month, two weeks per month, odd numbered weeks, even numbered weeks) in each month of the angling season to estimates from full creel surveys during 1991-2002. Estimates and variances of effort, harvest rate, harvest, RIC ratio, and recaptures from the four reductions in sampling effort were significantly different than those from the full sampling effort in several months for nearly all reductions in sampling effort. I conclude that creel survey efficiency could only be improved if losses in accuracy and precision are acceptable to those who use creel survey estimates for making policy decisions. To achieve my third objective, I estimated the recapture rate, RIC ratio, of walleyes previously marked for each month during the angling year for five length categories of walleye: all lengths combined,< 12 inches, 12-15 inches, 15-20 inches, and~ 20 inches. The RIC ratio declined significantly for all length categories except for fish 12-15 inches long. I conclude that fin regeneration or increased mortality due to marking may have reduced the number of marks in the RIC ratio, and therefore may have biased estimates of walleye angling exploitation rate.