Impacts of Aging Error on Walleye Management in the Winnebago System

The age data used to manage Walleye Sander vitreus fisheries are not always accurate, as otoliths typically provide more accurate age estimates for larger, older Walleyes than dorsal spines. We assessed the impacts that the aging error associated with the use of dorsal spines has on the estimated age distribution, growth and mortality rates, and yield per recruit for Walleyes in the Winnebago system, Wisconsin. Age distributions derived from otolith age estimates more accurately portrayed variable recruitment than those derived from dorsal spine age estimates. The mean estimates of instantaneous total annual mortality developed from sex-specific catch curves were greater when dorsal spine age estimates were used (0.515 for males, 0.493 for females) than when otolith age estimates were (0.349 for males, 0.396 for females), with most of the differences being observed in natural mortality estimates. The von Bertalanffy growth models were not significantly different, but the yield-per-recruit models (which rely heavily on von Bertalanffy model parameters) incorporating spine age data produced average yield estimates 50-70% lower than the models developed from otolith ages. The yield-per-recruit models derived from spine-determined ages also resulted in lower recommendations as to the age and size of fish for maximum yield at all exploitation levels than the models developed from otolith age data. The mean dorsal spine and otolith age estimates were similar for male Walleyes <457mm and female Walleyes <508mm, but for larger fish otoliths yielded significantly older ages than spines. Given the economic and social impacts of the Winnebago system Walleye fishery, we recommend exclusive use of otoliths to obtain the most accurate estimates of fish age, growth, mortality, and yield. In populations in which the sacrifice of large quantities of fish is not acceptable, we recommend using spines to estimate the ages of smaller fish (males <457mm, females <508mm) and otoliths to estimate the ages of fish larger than these critical lengths. Received December 26, 2012; accepted June 10, 2013