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
It is critical to resolve the effects of regeneration, false annuli, and resorption on interpretation of scale patterns. We used scales collected from known repeat‐spawning steelhead Oncorhynchus mykiss from the Snake River to (1) determine the accuracy of spawn check identification and (2) investigate errors in assignment of saltwater ages before and after spawn checks. Scale readers (n = 4) accurately identified over 96% of known repeat spawners (n = 107) and did not identify spawn checks in known first‐time spawners (n = 197). Scale resorption associated with the spawn check caused an obscured or lost saltwater annulus in 66% of samples, most frequently in fish that spent 2 years versus 1 year in saltwater. Presence or absence of an annulus after the spawn check was accurately assigned in 86% of samples (n = 100). The potential for scale resorption to affect interpretation of the life history of iteroparous anadromous salmonids is significant. We supply two examples of how circulus counts can provide guidance to scale readers, but this analysis should be customized to the growth characteristics of each stock of interest.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.