Scientific Background

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 (Kareiva et al. 2000). 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 (Casillas 1999, Bottom et al. 2000).  Since 1998, we have been evaluating the Columbia River plume as a critical habitat for juvenile salmon.  Here, we request a continuation of this work, with modifications based on findings to date.

The Columbia River is a major source of salmon to the sea, thus first principles dictate that it is logical to focus a major effort on studies of the salmon-plume interactions, particularly during May and June when outmigrating Columbia River salmon are first entering the sea.  . Locally, the Columbia River plume, as a natural extension of the estuary, represents a habitat of less saline marine waters that is hypothesized to be critical to salmon survival when they are making the transition from freshwater to saltwater. This is supported by recent studies assessing the importance and impact of river plumes to salmon survival (Beamish et al. 1994, Casillas 1999). The mechanisms by which the Columbia River estuary and plume may affect juvenile salmon survival have not been determined, but likely include provision of food resources, a refuge from predators due to the turbidity (thus low visibility) in the plume, and a refuge from coastal predators due to rapid transport offshore and away from the coastal zone by the riverine plume (Grimes and Finucane 1991, St John et al. 1992, Fukuwaka and Suzuki 1998, Grimes 2001, Pearcy 1992).

The shape and extent of the Columbia River plume is controlled largely by the amount of freshwater flowing out of the Columbia River. The timing and amount of flow affects the amount of sediment (and turbidity), as well as the amount of nutrients which fuel estuarine and oceanic productivity. Flow regulation, water withdrawal and climate change have reduced the average flow and altered the seasonality of Columbia River flows, changing the estuarine ecosystem (National Research Council 1996; Sherwood et al. 1990; Simenstad et al. 1990, 1992, Weitkamp et al. 1995, Bottom et al. 2001). Annual spring freshet flows through the Columbia River estuary are ~50% of the traditional levels that flushed the estuary and total sediment discharge is ~1/3 of 19th Century levels. Decreased spring flows and sediment discharges have also reduced the extent, speed of movement, thickness, and turbidity of the plume that once extended far out and south into the Pacific Ocean during the spring and summer (Barnes et al. 1972; Cudaback and Jay 1996, Hickey et al. 1998.

An emphasis on survival of salmon in the estuarine and marine environment is warranted because approximately half of all pre-adult (egg through juvenile stage) salmon mortality occurs there (Bradford 1995).  Variability in ocean salmon survival is very high, ranging over three orders of magnitude over the past three decades for coho (PSFMC unpublished data 1995).  Abiotic and biotic ocean conditions are highly variable as well, and undoubtedly account for the large range of juvenile salmon ocean survival.  Long-term regime shifts in climatic processes and El Niño and La Niña events affect oceanic features and can produce abrupt differences in salmon marine survival and returns (Francis and Hare 1994).  The latest recognized regime shift occurred in the late 1970s and may have been a factor in reduced ocean survival of salmon in the Pacific Northwest (PNW) and increased survival in Alaska (Mantua et al. 1997) during the period 1977-1998.  Recent changes in ocean conditions, which began in late 1998 and continue to present, provide evidence of a possible new regime shift that is favorable to salmon survival in the Pacific Northwest (Schwing et al. 2000; Peterson and Mackas 2001; Peterson and Schwing in prep).

Our recent work in the plume and adjacent coastal zone has now shown that the northern California Current may have experienced another regime shift, beginning in late 1998.  Due in large part to increases in the length of the upwelling season in 1999, zooplankton biomass has doubled in the coastal waters off Oregon, community composition has shifted to a dominance of cold water species, and salmon survival has increased five-fold (Peterson and Schwing, in prep).  Therefore, it is important to keep in mind that the plume and plume dynamics are modulated by climate influences at decadal scales as well as inter-annual, seasonal and daily scales depending upon the strength of the upwelling process.   Thus, plume-ocean interactions are a key component of our research program.