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The relationships between fluctuations in oceanographic conditions, forage fishes, predatory fishes, predator food habits, and juvenile salmonid marine survival off the Columbia River

ScholarsArchive at Oregon State University

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Title The relationships between fluctuations in oceanographic conditions, forage fishes, predatory fishes, predator food habits, and juvenile salmonid marine survival off the Columbia River
Names Emmett, Robert L. (creator)
Sampson, David, B. (advisor)
Date Issued 2006-04-26T20:25:31Z (iso8601)
Internet Media Type application/pdf
Note Graduation date: 2006
Abstract Salmonid run sizes are strongly affected by their early marine stage. Fully understanding
the life history of salmonids means understanding how they interact with their marine
environment and with other fishes. Changes in the biological and physical environment
off the Columbia River region affects the distribution and abundance of predatory fishes
and their feeding, forage fishes, and juvenile salmonid marine survival. From 1998-2004,
forage fish and predatory fish distribution and abundance off the Columbia River was
quantified by surface trawling at night during spring/summer. The effect of predation on
salmonids was measured by stomach analysis of predatory fishes. During the study
period (1998-2004), forage fishes increased in abundance by orders of magnitude and
were strongly related to the abundance of cold-water copepods the previous year. Higher
forage fish populations were also linked to cooler ocean conditions and perhaps fewer
predatory Pacific hake (Merluccius productus). Most forage fishes were distributed
nearshore while predators had a more offshore distribution. Pacific hake was most
abundant in 1998, 2003, and 2004; warm ocean years. Jack mackerel (Trachurus
symmetricus) was most abundant during 1999-2002; relatively cool ocean years. Deep
(50-m) ocean temperatures and the date of the spring transition, when nearshore currents
switched from northward to southward, were good predictors of Pacific hake abundance
in the study area. Forage fish or salmonid occurrence in a haul was negatively related to
the occurrence of predators. Pacific hake and jack mackerel ate primarily euphausiids
and small fishes. Salmonids were rarely eaten by either predator. However, because the
Pacific hake population can be very large, hake predation can be a significant source of
juvenile salmon mortality off the Columbia River during some years. A trophic model
showed that marine mortality of Columbia River juvenile salmonids may be related to the
abundance of Pacific hake and forage fishes. A multiple regression using the predictions
from the trophic model of annual numbers of juvenile salmonids eaten by hake and
Columbia River spring flows as independent variables, accounted for much of the
variation observed in the annual marine survival of Columbia River coho (Oncorhynchus
kisutch) and Chinook salmon (O. tshawytscha), the dependent variable. Future research
should identify the physical and biological forces that alter the feeding habits, migration
and movements of Pacific hake and jack mackerel off the Northwest, and how Columbia
River flows affect trophic interactions.
Genre Thesis
Topic Forage fish
Identifier http://hdl.handle.net/1957/1783

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