Predator–prey interactions influenced by a dynamic river plume

Marine predator–prey interactions are often influenced by oceanographic processes that aggregate prey. We examined density distributions of seabirds and prey fish associated with the Columbia River plume to determine whether variation in plume size (i.e., volume or surface area) or location influences predator–prey interactions. Common murre (Uria aalge), sooty shearwater (Ardenna grisea), and forage fish, including northern anchovy (Engraulis mordax) and juvenile salmon (Oncorhynchus spp.), occurred disproportionately in plume waters relative to adjacent marine waters. Water clarity, an indicator of plume-influenced waters, was a significant predictor of seabird and prey densities throughout the survey area. Murres occurred within 20 km of the plume center of gravity, whereas shearwaters occurred ∼100 km north of the plume center of gravity, concurrent with the highest densities of prey fish. Global indices of collocation were relatively low between murres and prey compared with the high values between shearwaters and prey. Seabird densities were negatively correlated with plume size, suggesting that seabirds concentrate in the plume to maximize foraging effort. We conclude that variation in Columbia River plume size and location influences predator distributions, which increases predation pressure on prey, including threatened salmonid species.