Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 8707
Title: Predator-prey interactions influenced by a dynamic river plume
Author: Elizabeth M. Phillips, John K. Horne, J. E. Zamon
Publication Year: 2017
Journal: Canadian Journal of Fisheries and Aquatic Sciences
Volume: preview online
Keywords: predator-prey interactions, Columbia River plume, juvenile salmon, seabird, forage fish,

Marine predators often rely on physical processes to locate aggregated prey in a relatively featureless environment.  Freshwater flow into the ocean forms buoyant plumes and density fronts that aggregate zooplankton and larval fish.  Observations of piscivorous seabirds and pelagic prey fish (i.e., forage fish) near the mouth of the Columbia River suggest that river plumes may be important areas for predator-prey interactions.  We examined density distributions of seabirds and prey fish near the Columbia River plume to determine if variation in plume volume, area or location influences predator-prey interactions.  Common murre Uria aalge, sooty shearwater Puffinus griseus, and fish including northern anchovy Engraulis mordax and juvenile salmon Oncorhynchus spp. occurred in plume waters (defined by surface salinity ≤28 practical salinity units) disproportionate to area surveyed.  Murres occurred within 20 km of the plume center of gravity (CG) and were associated with increased density and diversity of non-prey organisms near the surface.  Shearwaters occurred ~100 km to the north of the plume CG, where highest densities of prey fish were observed.  Overall global indices of collocation (GIC) were <0.7 between murres and prey, whereas GICs were ≥0.7 between shearwaters and prey including juvenile salmon, suggesting increased predation near the northern edge of the river plume.  Although prey densities did not vary with plume volume, seabird densities were negatively correlated with both plume volume and surface area, suggesting that seabirds utilize the Columbia River plume to maximize prey encounter rates, which are available under varying plume conditions.  We conclude that variation in Columbia River plume size influences predation pressure on prey including threatened salmonid species. 

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Theme: Ecosystem approach to improve management of marine resources
Foci: Characterize the interaction between marine, freshwater and terrestrial ecosystem components.
Characterize ecological interactions (e.g. predation, competition, parasitism, disease, etc.) within and among species.