Quantitative Assessment of Scale Resorption in Migrating and Spawning Steelhead of the Snake River Basin

Scales have been used for decades as a tool to interpret life histories in steelhead Oncorhynchus mykiss . Resorption can affect the accuracy of life history interpretations based on scale patterns, depending on the amount of material resorbed. For example, resorption can affect the distinctiveness and characteristics of spawn checks. Spawn checks have been reported in iteroparous salmonids, but no published experimental studies have established the precise relationship between reproduction and scale features. Our objectives were (1) to quantify scale resorption, and (2) to identify contributing factors to the observed resorption in migrating and spawning Snake River steelhead. Prespawn and postspawn scale samples from 72 fish were paired for analysis. We found considerable individual variability in the amount of material resorbed between prespawn and postspawn samples (mean, 26%; SD, 13.7%). Most resorption occurred during the winter as gonads matured and secondary sex characteristics were formed. In over half of the postspawn samples, resorption was sufficient to obscure or eliminate an annulus. In a few cases, resorption was minor enough that the eventual spawn check may be indistinct or absent. We recommend that ancillary marks be investigated as a means to help identify weak spawn checks and an index of resorption developed to determine if resorption was sufficient to cause loss of an annulus. Received March 27, 2014; accepted August 5, 2014