Estimating behavior in a black box: how coastal oceanographic dynamics influence yearling Chinook salmon marine growth and migration behaviors

Ocean currents or temperature may substantially influence migration behavior in many marine species. However, high-resolution data on animal movement in the marine environment are scarce; therefore, analysts and managers must typically rely on unvalidated assumptions regarding movement, behavior, and habitat use. We used a spatially explicit, individual-based model of early marine migration with two stocks of yearling Chinook salmon to quantify the influence of external forces on estimates of swim speed, consumption, and growth. Model results suggest that salmon behaviorally compensate for changes in the strength and direction of ocean currents. These compensations can result in salmon swimming several times farther than their net movement (straight-line distance) would indicate. However, the magnitude of discrepancy between compensated and straight-line distances varied between oceanographic models. Nevertheless, estimates of relative swim speed among fish groups were less sensitive to the choice of model than estimates of absolute individual swim speed. By comparing groups of fish, this tool can be applied to management questions, such as how experiences and behavior may differ between groups of hatchery fish released early vs. later in the season. By taking into account the experiences and behavior of individual fish, as well as the influence of physical ocean processes, our approach helps illuminate the “black box” of juvenile salmon behavior in the early marine phase of the life cycle.