Precision and Accuracy of Age and Growth Estimates Based on Fin Rays, Scales, and Mark-Recapture Information for Migratory Bull Trout

Accurate quantitative descriptions of bull trout Salvelinus confluentus growth are important for understanding life history and developing reliable stock assessments. We assessed the precision of age estimates using pelvic fin rays and scales for migratory bull trout 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) and less between-reader bias 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 migratory bull trout with age-length equations based on fin ray and scale annuli indicated that ages estimated from fin rays (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 (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-aged fish is recommended.