Scale Resorption in Migrating and Spawning Steelhead

Iteroparity (repeat spawning) is an important aspect of steelhead life history. It has been documented but not well described in steelhead of the Snake River basin. Loss of scale material occurs during migration, spawning, and overwintering, which leads to formation of a spawn check. Variability of resorption leads to differences in the strength of spawn checks, thus complicating the identification of repeat spawners.  We quantified resorption seen in scales as fish transition from a pre- to post- spawn state. We obtained 72 paired scale samples from fish collected at Lower Granite Dam during the upriver migration and as post-spawn kelts moving downriver. In our preliminary work (n=26), we observed a wide range of material loss between individuals. Loss was distributed around the scale in all samples, however, regions showing the most loss varied between fish. Total area loss ranged from 6-55%, with a median of 33.5%. Roughly equivalent losses in anterior and posterior fields were seen in 38% of fish, while 42% had more loss in the posterior field and 20% in the anterior field. The outer annulus was completely lost in 19% of fish, while 73% had partial loss of the outer annulus. Time spent above Lower Granite Dam, and date of upriver migration did not strongly correlate to mean area lost from scales.These results show that there is variation in the amount of material loss among individuals, which leads to differences in strength and characteristics of spawn checks. This level of variation can lead to mis-identification of repeat spawners and assignment of incorrect ages to steelhead in the Snake River basin. Results also indicate that resorption is caused by migration and overwintering behavior, rather than mechanical wear from the act of spawning. We expect further research will aide in the interpretation of scale characteristics seen in repeat spawning steelhead.