Variability of Life-History Expression in Oncorhynchus mykiss: Causes and Consequences of Partial Migration

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 tributary of the Clearwater River in northern Idaho, we PIT tagged juvenile steelhead during summer from 2011-2013 and estimated summer survival probabilities using a robust design mark-recapture model. We then monitored downstream movements during the subsequent outmigration periods using in-stream PIT arrays. We used multi-strata mark-recapture models 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 and individual survival probability play important roles in 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.