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.