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] ¼ 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 ¼ 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. *

A Comparison of Techniques Using Dorsal Spines to Estimate Sauger Age

“…The percent reader agreement that we found for the side illumination method (73.1%, CV ¼ 7.3%) was similar to that in another study of walleyes, namely, Logsdon (2007), in which the agreement between readers was 70.0% for walleyes from Mille Lacs Lake, Minnesota. However, the same study found 95.0% reader agreement for walleyes from Red Lake, Minnesota.…”

“…Logsdon (2007) also suggested that side illumination enhances the appearance of annuli that may appear split or discrete if the spine is viewed as a thin section. In contrast, we found that the outer annuli were difficult to distinguish because they were crowded together.…”

“…These drawbacks were mitigated in the case of whole channel catfish Ictalurus punctatus pectoral spines by cutting them and sanding them on the proximal end and then viewing them under a microscope using side illumination (Buckmeier et al 2002). The same method is effective for estimating walleye age (Logsdon 2007). Faint or merged rings, partial rings, double rings, crowding of distal rings, false annuli, and difficulty distinguishing the first annulus frequently cause problems in age estimation from dorsal spines (Kocovsky and Carline 2000;Bruesewitz et al 2002;Isermann et al 2003;Logsdon 2007).…”

Validation of otolith ages for walleye (Sander vitreus) in the Winnebago System

“…These findings add to the growing body of literature reported earlier demonstrating that otolith-based age estimates are accurate, while scale and-spine based estimates are not accurate, especially for older fish. Age estimates from walleye otoliths were also found to be more precise than those from dorsal spines, complementing other studies reporting similar results for walleye age estimates (Marwitz and Hubert, 1995;Kocovsky and Carline, 2000;Isermann et al, 2003;Logsdon, 2007).…”

“…Like scales, spines or fin rays can be quickly collected without long-term negative impacts to the fish sampled. However, as with scales, spines display crowding of annuli on the edge of the aging structure when decreases in somatic growth occur following maturation, leading to the underestimation of age for larger, older fish (Erickson, 1979(Erickson, , 1983Marwitz and Hubert, 1995;Logsdon, 2007;Koenigs et al, 2013).…”