Adjusting Dorsal Spine Ages to Reduce Bias in Estimates of Growth and Mortality for Wisconsin Walleyes

Estimating ages is an important step in monitoring and managing walleye fisheries and Wisconsin DNR staff assign ages to hundreds of walleyes each year. While otoliths likely provide the most accurate structure for estimating walleye ages, dorsal spines are routinely used for age assignments because spines can be removed without sacrificing fish. Dorsal spines tend to yield younger ages than otoliths for older walleyes, but the age at which disagreement begins and the severity of these disagreements is likely to vary among populations in relation to growth rate. Discrepancies in ages between structures could result in meaningful differences in important population parameters such as growth rate and mortality. Using otoliths and dorsal spines collected from more than 700 walleyes across the state of Wisconsin, we described relationships between ages estimated from otoliths and dorsal spines for fast-, moderate-, and slow-growing walleye populations in order to develop correction factors that could be used to adjust dorsal spine ages. Our initial results suggest that these correction factors can be used to provide estimates of growth and mortality parameters similar to estimates obtained when using otoliths to estimate walleye ages.