Partial Migration of Steelhead in the Lapwai Creek Basin, Idaho

In steelhead (Oncorhynchus mykiss), individual variation in the costs and benefits associated with migration leads to differing migratory strategies for maximization of individual lifetime fitness. While genetics play a role in determining the occurrence of migration, juvenile steelhead migratory behavior is highly plastic in response to conditions experienced in freshwater rearing habitat. In Lapwai Creek, a hydrologically-altered watershed in northern Idaho, we PIT tagged juvenile steelhead during summer from 2010-2013 and monitored downstream movements during the subsequent outmigration periods using in-stream PIT arrays. We used multi-strata mark-recapture modeling to evaluate biotic and abiotic influences on steelhead migration tendencies. Our results illustrate the spatial and temporal variability in outmigration strategies and suggest that population density, individual body size, and effects of water withdrawal play important roles in life-history expression. Additionally, our results show that while a single variable alone may not explain occurrence of migratory behavior, interactions between multiple variables, often overlooked in salmonid outmigration studies, can be strong predictors of individual life-history expression. Understanding which fish migrate and the factors influencing this behavior is important for improving our quantification of life cycle demographics, our understanding of habitat relationships for juvenile salmon, and our conservation strategies for threatened populations.