Validation of daily growth increment formation in the otoliths of juvenile cyprinid fishes from the Brazos River, Texas

Age data are commonly used by fisheries biologists to assess a number of important population characteristics. For cyprinid fishes, there have been few attempts to assess the validity of age estimates, particularly those based on otolith microstructure. We assessed the periodicity of growth increment formation in the otoliths of three species of cyprinid from the Brazos River, Texas, two of which are of conservation concern. We immersed juvenile sharpnose shiners Notropis oxyrhynchus, smalleye shiners N. buccula, and plains minnow Hybognathus placitus in a 100-mg/L solution of alizarin complexone for 24 h and sampled fish at 5-d intervals for 30 d. Regression models indicated high correspondence between number of days posttreatment and number of growth increments between the alizarin mark and the edge of the otolith for all three species (sharpnose shiner: r² = 0.99; smalleye shiner: r² = 0.98; plains minnow: r² = 0.97). Slopes of regression lines did not differ from 1.0 (t-test: P < 0.05), indicating that growth increments are deposited daily in these three species. Our results provide evidence that daily growth increments are reliable sources of age information for cyprinids.