Accurate quantitative descriptions of bull trout (Salvelinus confluentus) growth are important for understanding life history and developing reliable stock assessments. In the absence of age validation, important steps are to review the precision of age estimation methods and evaluate whether age estimates yield growth rates consistent with known fish growth based on tag recoveries. We assessed the precision of age estimates using pelvic fin rays and scales for migratory bull trout (297–605 mm total length) from the North Fork Clearwater River, and then compared growth estimates derived from both structures with growth based on tag recoveries. Fin rays produced a lower coefficient of variation (CV = 5.84) than scales (CV = 12.56). Ages estimated from scales were higher for fish aged < 5 with fin rays and lower for fish aged ≥ 5. Comparisons of growth estimates derived from 70 tagged bull trout at large from 0.35 to 3.02 years with age-length equations based on fin ray and scale annuli indicated that ages estimated from fin rays (N = 189, predicted length of an age 3 fish = 310mm) were closely related to the apparent ages estimated from the mark-recapture model (apparent age of a 310mm fish = 2.9) whereas scales (N = 65, predicted length of an age 3 fish = 408mm) were not. This is the first study to assess the precision of structures for modeling growth of larger migratory bull trout. However validation of annuli formation from the recapture of known-age fish is recommended.
Vol. 87 • No. 4
Vol. 87 • No. 4