Photoperiod at emergence regulates early life history plasticity in fall Chinook salmon

Early life history pathways for Chinook salmon are directly related to environmental variation, which can shift phenotypic traits expressed by a population. Fall Chinook salmon in the upper Columbia River Basin historically timed migration downstream (smolted) as subyearlings; however, some wild and hatchery-reared fall Chinook salmon also express a successful yearling smolt life history strategy. Previous research suggests that this shift to yearling smoltification represents evolution—a genetic change due to an increase in fitness and improved downstream survival. Alternatively, the shift in smolt timing could be a phenotypically plastic response that optimizes life history traits for current environmental conditions. We raised hatchery-origin fall Chinook salmon from the Umatilla River (Oregon, USA) in a controlled laboratory experiment to explore environmental drivers of early life history. We found smolt timing to be age/size dependent for subyearlings, and photoperiod dependent for yearlings. We found the propensity for precocious male maturation at age-1 (microjack) to be primarily photoperiod dependent and for age-2 (minijack) to be primarily growth dependent. The resulting adaptable life history portfolio may provide this population with some baseline capacity to cope with future environmental change.