How to survive a drought: modeling shows evidence of the importance of foraging plasticity to juvenile salmon during a drought

Salmon fisheries managers often use models to determine the growth of individuals and populations for a variety of management objectives. Most models of juvenile salmon growth during their freshwater residency only incorporate activity costs for drift foraging when making growth assessments and predictions. I propose that fish may need to include a searching foraging strategy to meet their energy needs under certain situations, such as when prey are scarce, difficult to find, or if low water velocities do not promote high drift delivery. To investigate this I tested how the growth derived from otoliths collected from a cohort of wild juvenile Chinook Salmon (Onchorhyncus tschawytscha) in the San Joaquin River compared to predictions of growth derived from the individual based model inSTREAM. Model predictions of search foraging events were confirmed by stable isotope analyses, which suggested that a significant portion of total consumed biomass was derived from oligochaetes. The model was then used to simulate a variety of discharge and prey production scenarios to investigate their effects on growth and survival. Results from these scenarios suggest that prey production has the most significant effect on growth and survival, while all discharge levels above Critical Low are beneficial for growth and survival.