Environmental Conditions Impacting Juvenile Chinook Salmon Growth Upon Early Ocean Entry Along The California Coast

Chinook salmon exhibit lower survival during less productive ocean conditions (e.g., El Niño). The probability of survival is positively related to early growth. However, robust relationships between environmental variability and salmon growth have been difficult to establish. We elucidate some of these linkages by combining statistical environmental responses based on empirical observations of growth and occupancy with ecosystem model simulations to generate growth potential distributions. Our work describes a novel approach to identify the effects of environmental conditions and upwelling variability on the growth of juvenile Chinook salmon in central California coastal waters. The method uses a fully coupled ecosystem model framework comprised of an ocean circulation component, a biogeochemical component, and an individual-based model for salmon. We used sub-yearling Chinook salmon observations and oceanographic model output from California Current Large Marine Ecosystem to examine the hypothesis that dynamic environmental variables affecting the water column are important drivers of oceanic distribution of sub-yearling Chinook salmon. Further, we hypothesized that variability in appropriate salmon habitat will relate to survival of salmon populations. We identified interannual shifts in optimal foraging conditions for juvenile Chinook salmon in California coastal waters. Results from the simulations indicate that years favorable for juvenile salmon growth off central California are characterized by particularly intense early season upwelling (i.e., March through May), leading to enhanced krill concentrations during summer near the point of emigration. Seasonal growth rates and body conditions in the model are generally consistent with observed values, and suggest that juvenile salmon entering the ocean later in the season achieve higher weight gains during the critical first period at sea.