Evidence of Iteroparity in Wild Snake River Summer-Run Steelhead from Scale Patterns

Iteroparity has been documented for steelhead populations in the Pacific Northwest, but the phenomenon has not been well-described for Snake River steelhead (Oncorhynchus mykiss).   To our knowledge, the only study to investigate iteroparity in Snake River steelhead was conducted in the early 1950’s, and was specific to the Clearwater River. In that study, the author used scale pattern analysis but did not elaborate on the criteria used to identify repeat spawners.  We investigated patterns from scale samples collected at Lower Granite Dam (N=1,897) in order to develop criteria for identifying repeat spawners. We focused on two patterns commonly reported in the literature and a third displayed only in published photographs: scar-like marks which are known indications of previous spawning, continuous strong dark lines visible in the anterior and posterior regions of the scales, and folding of circuli.  Of the samples viewed, scar-like marks were rare (n=21).  Strong dark lines (n=208) and folding (n=167) occurred frequently. Of the samples demonstrating scar-like marks 71.4% additionally displayed a dark line and 9.5% displayed folding. The three patterns may not be obvious in all scales from an individual sample. In the literature, the appearance of scar-like marks are considered diagnostic; we speculate that dark lines and folding of circuli may be indicative of weak spawn checks and help flag a sample for further examination.  We intend to validate all three patterns with samples from PIT tagged repeat spawners.