Size-selective predation effects on juvenile Chinook salmon cohort survival off Central California evaluated with an individual-based model

Variation in the recruitment of salmon is often found to be correlated with marine climate indices, but mechanisms behind environment–recruitment relationships remain unclear and correlations often break down over time. We used an ecosystem modeling approach to explore bottom-up and top-down mechanisms linking a variable environment to salmon recruitment variations. Our ecosystem model incorporates a regional ocean circulation submodel for hydrodynamics, a nutrient-phytoplankton-zooplankton submodel for producing planktonic prey fields, and an individual-based model (IBM) representing juvenile Chinook salmon (Oncorhynchus tshawytscha), combined with observations of foraging distributions and diet of a seabird predator. The salmon IBM consists of modules, including a juvenile salmon growth module based on temperature and salmon–prey availability, a behavior-based movement module, and a juvenile salmon predation mortality module based on juvenile salmon size distribution and predator–prey interaction probability. Seabird–salmon interactions depend on spatial overlap and juvenile salmon size, whereby salmon that grow past the size range of the prey distribution of the predator will escape predation. We used a 21-year historical simulation to explore interannual variability in juvenile Chinook salmon growth and predation-mediated survival under a range of ocean conditions for sized-based mortality scenarios. We based a series of increasingly complex predation scenarios on seabird observational data to explore variability in predation mortality on juvenile Chinook salmon. We initially included information about the predator spatial distribution, then added population size, and finally the predator's diet percentage made up of juvenile salmon. Model agreement improves with added predator complexity, especially during periods when predator abundance is high. Overall, our model found that when the fraction of juvenile salmon in seabird diet increased relative to alternate prey (e.g., Northern anchovy Engraulis mordax, and juvenile rockfish Sebastes spp.), there was a concomitant decrease in salmon cohort survival during their first year at sea.