Abstract:
A trophic model that simulates interactions between
a predatory fish (Pacific hake, Merluccius productus), forage
fish, and juvenile salmon off the Columbia River
was constructed to identify if trophic interactions could
account for marine mortality of Columbia River juvenile
salmon. The model estimates the number of juvenile
salmon that are eaten annually by Pacific hake off
the Columbia River for a given hake and forage fish
population. Model results indicate that the presence of
high numbers of Pacific hake could account for high
mortality of some juvenile salmonid species/stocks leaving
the Columbia River, and that this mortality would
be much reduced when forage fish are abundant.
Estimates of hake and forage fish abundance, based on
field data collected from 1998–2005, were used in the
model to derive annual estimates of the number of salmon
possibly eaten by hake. A multiple regression analysis
using the output from the trophic model and average
May/June Columbia River flows accounted for much
of the annual variation in Columbia River fall Chinook
(Oncorhynchus tshawytscha) and coho (O. kisutch) salmon
marine survival (p < 0.05, R2 > 60%), but not spring or
summer Chinook salmon. For these two stocks, average
May/June sea-surface temperature was the best predictor
of marine survival. Results support the hypothesis
that for some Columbia River salmon species/stocks,
marine survival is predation-driven and affected by the
interaction between the abundance of Pacific hake, forage
fish, Columbia River flows, and possibly ocean turbidity.
Future modeling work should include predation
estimates of other large fishes, marine mammals, and
sea birds.
