Recent evidence suggests that improvement in survival of the estuarine and early ocean life history phase of Columbia River salmon may be critical to recovery of endangered stocks. When coupled with evidence that the estuary and the plume (as an extension of the estuary) may be highly modified habitats due to river modifications (e.g. altered channel morphology) and modified flows as a result of the federal hydropower system, potential for achievable improvement in salmon survival from management actions seems plausible. An understanding of which factors affect survival will require a partitioning of survival (and mortality) among freshwater, estuarine, and early ocean phases. This in turn will require knowledge of the processes that limit and/or enhance salmon survival in these habitats. These processes, though well characterized in the freshwater environment, have not been characterized in the estuarine and plume environment.

Tagging studies examining the ratio of outmigrating smolt to adult returns (SAR) reveal that the timing of ocean entry can have a large effect on adult survival. Muir et al. (2006) found there are both weekly and seasonal differences in ocean entry timing and SAR, but the dates of higher SARs varied among years. A two-component mechanistic explanation for this variation in adult returns has been proposed. First, yearly, seasonal, and weekly changes in the density of fish predators on smolt (e.g. avian predators) have a direct impact on smolt survival. Second, similar variation physical conditions such as river flow, estuary hydrography, and plume/ocean hydrography affect the timing of arrival of forage fishes that serve as prey for potential salmon predators (prey buffering). As an example, if smolt entered the ocean at a time when there were few predators but abundant forage fish, then SAR for that period would be high.

If predation pressure and prey buffering could be measured in the ocean, then those data could be used by managers to inform decisions about hatchery release timing and to improve forecasts of adult returns based on ocean-entry conditions. Acoustic data are necessary to test the hypothesis that salmon survival is positively correlated with predator density and the availability of non-salmon prey to predators. Data collected in this study will allow us to compare the spatial and temporal overlap of juvenile salmon with forage fish and salmon predators such as birds, fish, or mammals. Because acoustic instruments sample the entire water column and data can be collected continuously while the ship is underway, these data will also provide more complete, higher-resolution maps of fish and plankton distributions than can be achieved using net-based sampling alone.

In 2013, efforts will focus on processing and analyzing data collected during the May and June 2012 sampling cruises.