Validation of scales and otoliths for estimating age of redband trout in high desert streams of Idaho

Studies validating aging structures for rainbow trout are sparse and none have been conducted for redband trout, a common western U.S. sub-species. Oxytetracycline mark-recapture methods (MR), marginal incremental analysis (MIA), and comparisons across multiple populations were used to evaluate the utility of two structures for aging redband trout in high desert streams. We assessed periodicity of annulus formation on scale and otolith samples from all age classes of trout residing in two streams, identified the location of the first annulus on otoliths, and compared age estimates and between-reader coefficient of variation on nine additional streams. The use of MIA successfully validated opaque zone periodicity for fish transitioning from age-0 to age-1, and from age 1 to age 2 in two streams. For fish at liberty 13 and 28 months in the same two streams, MR-derived age estimates from whole and sectioned otoliths were 94–100% accurate for fish from 2–9 years old. Scales were only 77% and 38% accurate for fish at liberty 13 and 28 months, respectively. Between-reader coefficient of variation (CV) for scales was high (11.5%), while CV for sectioned otoliths using whole otoliths as corroboratory structures averaged 2.3%. Scales were thus, an unacceptable aging structure for desert redband trout. Given the confusion in the literature, we suggest that more rigorous research should be conducted to define and explain otolith zone formation.