The relationships between predatory fish, forage fishes, and juvenile salmonid marine survival off the Columbia River: a simple trophic model analysis

Previously published in California cooperative oceanic fisheries investigations, progress report v.48 (2007); access courtesy of publisher and authors.

A trophic model that simulates interactions betweena predatory fish (Pacific hake, Merluccius productus), foragefish, and juvenile salmon off the Columbia Riverwas constructed to identify if trophic interactions couldaccount for marine mortality of Columbia River juvenilesalmon. The model estimates the number of juvenilesalmon that are eaten annually by Pacific hake offthe Columbia River for a given hake and forage fishpopulation. Model results indicate that the presence ofhigh numbers of Pacific hake could account for highmortality of some juvenile salmonid species/stocks leavingthe Columbia River, and that this mortality wouldbe much reduced when forage fish are abundant.Estimates of hake and forage fish abundance, based onfield data collected from 1998–2005, were used in themodel to derive annual estimates of the number of salmonpossibly eaten by hake. A multiple regression analysisusing the output from the trophic model and averageMay/June Columbia River flows accounted for muchof the annual variation in Columbia River fall Chinook(Oncorhynchus tshawytscha) and coho (O. kisutch) salmonmarine survival (p < 0.05, R2 > 60%), but not spring orsummer Chinook salmon. For these two stocks, averageMay/June sea-surface temperature was the best predictorof marine survival. Results support the hypothesisthat for some Columbia River salmon species/stocks,marine survival is predation-driven and affected by theinteraction between the abundance of Pacific hake, foragefish, Columbia River flows, and possibly ocean turbidity.Future modeling work should include predationestimates of other large fishes, marine mammals, andsea birds.