Use of Unsectioned Dorsal Spines for Estimating Walleye Ages

Abstract

Dorsal spines may be a suitable structure for aging walleyes Sander vitreus when the otoliths are unavailable due to live release of the catch, but the specialized equipment and additional time necessary to remove and mount a readable cross section can discourage their use. I evaluated a simple method of obtaining age estimates from unsectioned dorsal spines. The basal end of each dorsal spine was sanded smooth and viewed under a dissecting microscope with side illumination. For Red Lake, Minnesota, walleyes, where consensus otolith ages by two experienced readers indicated that most fish were younger than age 7, reader agreement rates (95%) and precision (mean coefficient of variation (CV) = 0.95%) of dorsal spine estimates were relatively high. Age estimates by individual readers inspecting spines generally agreed with consensus otolith ages (≥95%). Reader agreement rates (70%) and precision (mean CV = 2.71%) were much lower for a sample of walleyes collected from Mille Lacs Lake, Minnesota, where 94% of consensus otolith ages were greater than age 6. Furthermore, spine-based age estimates for Mille Lacs Lake walleyes exhibited relatively poor agreement with consensus otolith ages (≤40%); however, most of the discrepancies were noted for individuals of consensus otolith age 7 or more, and consistent underestimation of ages did not occur until age 13. Based on these findings, unsectioned dorsal spines offered a reasonably precise, nonlethal, and simple approach for replicating consensus otolith ages for walleyes younger than age 7. Although otoliths may still be necessary to estimate the ages of older individuals within a sample, use of unsectioned dorsal spines could allow biologists to sacrifice fewer walleyes while closely replicating otolith-based estimates of population age structure.