Quantifying Environmental and Biological Factors Contributing to Juvenile Salmon Predation Risk during Early Marine Residence

To understand predation impacts on salmon survival, predator-prey relationships during early marine residence need to be understood.  Two seabird species, common murre (Uria aalge) and sooty shearwater (Puffinus griseus), occur in densities that can exceed 50 birds/km² near the mouth of the Columbia River during peak smolt outmigration.  These two predators consume juvenile Chinook (Oncorhynchus tshawytscha), steelhead (O. mykiss) and coho (O. kisutch) salmon, and forage fish including northern anchovy (Engraulis mordax).  It has been hypothesized that increased forage fish densities near the river mouth may reduce predation on juvenile salmon.  We studied correlations among seabirds, prey, salmon, and plume location.  Murres consistently occupied low salinity plume waters (21-28 psu) whereas shearwaters utilized intermediate and mixed plume waters (>28 psu).  Geographic distances between seabirds and individual fish species suggest that subyearling Chinook may be buffered from predation by murres when anchovy occur in the plume.  Shearwaters consistently utilized waters with higher forage fish densities, which were also locations of greater densities of yearling Chinook and coho salmon.  Our results suggest that yearling Chinook and coho may experience greater predation by shearwaters once they exit the plume.  We are incorporating these results into a model of juvenile salmon predation risk under varying conditions.