Geomorphic, hydroclimatic, and biotic factors influence juvenile emigration timing from tributaries in a southern coho salmon population

Environmental variation gives rise to species’ life history diversity, which is essential to population resilience. Here, we characterize diversity in juvenile emigration timing in an endangered coho salmon population and examine physical and biotic factors contributing to variation among streams and cohorts. Over 11 years, PIT-tagged, juvenile coho salmon from common family groups were released from a conservation hatchery into four tributaries of the Russian River, California, USA. We tracked the timing of fish leaving those streams (emigration) using PIT detection systems and used multistate emigration models to estimate the probability that individuals would emigrate in fall/winter (early) rather than during the typical spring smolt window (late). The probability of emigrating early was higher in streams with less valley bottom area and increased with streamflow, stocking density, stream temperature, and individual size. Both early and late emigrants contributed to adult returns. The life history plasticity observed suggests that the remnant population responds to environmental heterogeneity and that restoring and reconnecting a diversity of habitats could stimulate and support multiple emigration strategies and increase population resilience.