Evaluation of Anal Spines, Dorsal Spines, and Scales as Potential Nonlethal Surrogates to Otoliths for Estimating Ages of Largemouth Bass and Smallmouth Bass

Abstract We compared the precision of age estimates from anal spines, dorsal spines, otoliths, and scales and evaluated the agreement of age estimates from nonlethal structures (i.e., anal spines, dorsal spines, and scales) to estimates obtained from otoliths for Largemouth Bass Micropterus salmoides and Smallmouth Bass Micropterus dolomieu collected in northeastern South Dakota glacial lakes. Reader agreement and coefficient of variation (CV) significantly differed across structures for both species. Otoliths had the highest complete and partial (i.e., at least two of three agree) reader agreement rates, while scales had the lowest for both species along with dorsal spines for Smallmouth Bass. Mean CV for otolith age estimates was 2.0% for Largemouth Bass and 2.3% for Smallmouth Bass; CV means for the nonlethal structures ranged from 8.0% to 15.2% for Largemouth Bass and from 10.0% to 10.7% for Smallmouth Bass. Age estimates derived from anal spines and dorsal spines underestimated ages of both species relative to otoliths. Otolith and scale age estimates were similar through age 9 for Largemouth Bass and through age 7 for Smallmouth Bass. Scale ages generally coincided with otolith ages for Largemouth Bass < 35 cm TL and for Smallmouth Bass < 31 cm TL. Neither anal spines nor dorsal spines proved to be a good nonlethal alternative for estimating ages, while scales may have utility for younger cohorts of both species. We recommend that otoliths be the standard structure for estimating Largemouth Bass and Smallmouth Bass ages when sacrificing fish is acceptable. If sacrificing fish is a concern, then scales may be used as a surrogate for estimating ages of Largemouth Bass < 35 cm TL and Smallmouth Bass < 34 cm TL.