Validation of the Dahl–Lea and Fraser–Lee Back-Calculation Models by Using Oxytetracycline-Marked Bluegills and Bluegill × Green Sunfish Hybrids

Abstract

We assessed the accuracy of the Dahl–Lea (direct proportion) and Fraser–Lee (intercept-corrected direct proportion) back-calculation models to estimate past growth from scale measurements of tank-reared bluegills Lepomis macrochirus and hybrids of bluegills × green sunfish L. cyanellus. Bluegills and the hybrid sunfish were fed oxytetracycline (OTC) to produce multiple marks that simulated the annuli used for model validation. Back-calculated lengths (BCLs) from the Dahl–Lea and Fraser–Lee models were compared with the actual lengths measured at the time of OTC marking. The Fraser–Lee model with the 20-mm bluegill standard intercept produced more accurate BCLs than the Dahl–Lea model for both bluegills and hybrid sunfish with errors ranging from −13.1% to 0.8%. However, for all fish and both models, BCLs to the first OTC mark were significantly different from the actual length. At later OTC marks, the Dahl–Lea and Fraser–Lee models produced BCLs not significantly different from each other, indicating the intercept's influence decreased with increasing fish length. For hybrid sunfish, the lowest error (−1.6% to 0.7%) for back-calculations from scales was found by using the 20-mm bluegill standard intercept. We recommend 20-mm as a standard intercept in the Fraser–Lee model for hybrid bluegill × green sunfish back-calculations from scales. Overall, we recommend using the Fraser–Lee model for accuracy in estimated BCLs from scales of Lepomis spp., particularly when younger ages are of concern.