The influence of the Columbia River plume on predator-prey interactions
Phillips, Elizabeth Mary
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Oceanographic processes that aggregate prey and facilitate the transfer of energy to higher trophic levels often influence marine predator-prey interactions. Buoyant discharge from the Columbia River forms a large freshwater plume bounded by convergent fronts in the northern California Current. These oceanographic features aggregate zooplankton and attract coastal pelagic fish species (CPS) including northern anchovy (Engraulis mordax). Juvenile salmon (Oncorhynchus spp.) use the Columbia River plume as they migrate to sea and experience elevated mortality during early marine residence. Seabirds including sooty shearwaters (Ardenna grisea) and common murres (Uria aalge) use the plume to forage, consuming CPS and juvenile salmon, and the Columbia River plume may influence predator-prey interactions and juvenile salmon survival. This dissertation examined the influence of the Columbia River plume on distributions of seabirds and fish prey, and characterized conditions that influence juvenile salmon predation risk. Chapter 1 provides a general introduction to the Columbia River plume ecosystem. Chapter 2 demonstrates the disproportionate occurrence of murres, shearwaters, CPS, and juvenile salmon in plume waters, the positive relationship between turbid plume waters and seabird densities, and the aggregation of seabirds in the plume when surface area is low. Chapter 3 used satellite telemetry and a high resolution hydrodynamic model of plume circulation to demonstrate the ability of murres and shearwaters to track the north-south movements of the plume, and the movement of seabirds towards biophysically active plume boundaries when surface areas exceed a threshold of ~1,500–4,000 km2. Chapter 4 compared murre distributions observed from ship, plane, and satellite telemetry data perspectives, and identified similarities and differences in seabird spatiotemporal distributions that can inform species distribution models. Chapter 5 documented a relationship between low plume surface areas and increased juvenile coho and Chinook salmon predation risk from seabirds, and the mediating role of CPS in predator-prey interactions near the plume. Taken together, the results of this dissertation demonstrate that variation in Columbia River plume surface area and river discharge influences predator-prey interactions and juvenile salmonid predation risk. Chapter 6 offers a synthesis of the results and recommendations for future research aimed at informing Pacific salmon ocean ecology, management, and conservation.
- Fisheries