Climate and conspecific density inform phenotypic forecasting of juvenile Pacific salmon body size

1. Predicting effects of climate on fitness-linked phenotypic traits, such
as body size, is important for the management and conservation of species
in the face of global change. During sensitive life stages, small changes
in mean trait values can have large effects on survival and population
productivity. 2. The transition from freshwater to saltwater by migrating
anadromous fishes such as Pacific salmon (genus Oncorhynchus) is a
critical life history transition, where survival is mediated by the size
of migrating individuals. For salmon that spend extended periods rearing
in freshwater, the size at ocean entry (i.e., smolt length) may be
sensitive to changes in freshwater conditions shaped by biotic and abiotic
factors, yet long-term phenotypic time series for exploring these
responses are rare. 3. We reconstructed a four-decade time series of smolt
length from archives of returning adult scales to quantify
population-specific responses to climate and conspecific density in a
small watershed. 4. Dynamic linear modeling found that the relationship
between a proxy for cohort density and smolt length at ocean entry was
consistently negative, suggestive of competition. In contrast, there was a
positive, yet dynamic, relationship between a proxy for density of
emerging fry during the second summer of growth and smolt length. 5. The
effect of temperature on smolt length was not consistent between two
populations of sockeye salmon O. nerka that likely use distinct habitats
within the watershed. A positive relationship between smolt length and
temperature was only detected for the less abundant, early returning
population. 6. Predictions of smolt length showed variable responses under
scenarios of increasing temperature and high and low densities of
conspecifics. Collectively, these results reveal population-specific
responses to temperature and density, suggesting that local habitat
conditions may filter larger-scale climate drivers.