Scale Ageing Lessons Learned from Known Repeat Spawning Snake River Steelhead

 Accurate age estimates are important in fisheries research and management. Steelhead trout display complex life histories including variation in freshwater and saltwater residence, as well as the ability to repeat spawn. Validation of spawn checks has not been well described in the literature and significant resorption can happen during spawning, leading to loss of annuli. We used scales collected from tagged known repeat spawning steelhead: 1) to assess detection accuracy and 2) to evaluate probability of annulus loss. Readers identified spawn checks on known repeat spawner scales in over 96.0% of samples (n=108) and did not identify any spawn checks in known maiden spawners (n=156). Preliminary results confirm several characteristics frequently occurred on the scales of repeat spawning steelhead: surface erosion 93.1%, crossing over 93.6%, and dark line 92.6%. In maiden spawners characteristics occurred less frequently: surface erosion 12.2%, crossing over 8.6%, and dark line 23.4%. This research project will also include a comparison of ages and scale measurements from paired maiden and repeat spawners (n=88) to determine annulus and scale material loss. Research results are expected to provide guidelines in assessing the potential of significant resorption to affect interpretation of life history.