Resample, Recapture, and Re-Analyze: Using the 3 R's to Resurrect the Scale Method of Estimating Fish Age

As imperilment of freshwater fishes increases, non-lethal yet reliable age and growth data is becoming more important. Otolith-based age estimation is the current standard, but may not be practical for species or populations of conservation concern. We marked and resampled three populations of Shoal Bass (Micropterus cataractae), a fluvial specialist considered vulnerable to extinction, in Georgia over a three-year period. For each of the three populations, we obtained scales at initial capture, implanted a PIT tag, and resampled for two subsequent years, taking scale samples for age estimation from all fish. This process allowed us to compare age estimates against known times-at-large, providing an estimate of accuracy. Additionally, two readers independently estimated age twice for each fish, in conjunction with a concert age-estimation, to provide estimates of bias and precision. Precision was high for fish >3 years old, but variable for individuals ≤2 years old. Concert age-estimation reduced between-reader bias across all age classes. Scales ultimately provided accurate age estimates through 10 years of age, but variance increased for individuals ≥6 years old due to small sample sizes. Our results show promise for this method using scales to obtain accurate, non-lethal age estimates of stream-dwelling centrarchid fishes.