Comparing the Use of Dorsal Fin Spines with Scales to Back-Calculate Length-at-Age Estimates in Walleyes

Abstract

Using scales to age and back-calculate previous lengths has long been a management tool in fisheries. However, difficulties with using scales to interpret ages in older fish have led managers to investigate other bony structures. Dorsal fin spines have been used and evaluated for aging fishes, but their utility for back-calculating length estimates is largely unknown. We compared back-calculated estimates along two different scale transects and three dorsal fin spine transects in walleyes Stizostedion vitreum. Back-calculated estimates were obtained by using the standard Fraser–Lee proportional method and a regression equation derived from the relationship between body length and spine radius. Dorsal fin spines were easier to interpret than scales, especially for older walleyes. Back-calculated lengths among transects, both within and between the two structures, agreed quite favorably. Our data suggest that applying the Fraser–Lee proportional method to dorsal fin spines will give lengths that closely approximate the back-calculated lengths obtained from scales. Although we observed differences among individual walleyes in the back-calculated lengths estimated from each structure, estimates of the back-calculated lengths obtained from dorsal fin spines compared reliably with scale-based estimates for walleye populations as a whole. Scales are much easier to prepare, however, and have a longer history of use for back-calculating lengths at age. We recommend using scales for aging and back-calculating length at age in younger fish, but managers may wish to consider the use of spines for obtaining length-at-age estimates from older individuals.