Age Validation of Walleye in the Winnebago System and Impacts of Aging Error on Walleye Management

Sport fishing on the Lake Winnebago System contributes $234 million to the local economy and walleye, Sander vitreus, are one of the most important sport fish in the system.  It is essential to use accurate age data to estimate rates of somatic growth and mortality, which are required to effectively manage the walleye fishery.  We removed otoliths and dorsal spines from196 known-age walleye (ages 0-3) that were assigned ages via progression of discrete length modes.  Otoliths and dorsal spines were also removed from an additional 75 known-age walleye that were recaptured fish initially tagged at small enough lengths (<368mm) for age assignment to ±1 year.  We compared the age estimates derived from each structure to the true age of the fish to determine the accuracy of both aging structures.  Otoliths and dorsal spines were also removed from fish collected during spring spawning assessments, tournament monitoring, and weekly electro-fishing and trawling samples to better determine the relationship between age estimates from the two structures.  Age estimates were in agreement for fish 0-6 years old, while spines underestimated age of fish 7 years of age and older (otolith age estimates were .598 to 5.00 years greater for females and .415 to 7.40 years greater for males).  Mean estimated rates of total mortality based on dorsal spine age estimates were higher (36.2% for males and 38.7% for females) than those based on otoliths (23.7% for males and 26.6% for females).  Age distributions within the mortality curve correlated directly with year class strength, suggesting that otoliths, unlike dorsal spines, are accurate for all ages of walleye.  We recommend that otolith age estimates be used to accurately estimate rates of somatic growth and mortality.