Weitkamp et al.: Seasonal and interannual variation in juvenile salmonids in the lower Columbia River 
447 
years, major changes to the estuary have included de- 
creased magnitude of the spring freshet, decreased sa- 
linity intrusion, and changes in estuarine bathymetry, 
the latter of which has influenced circulation patterns 
and habitat availability (Sherwood et ah, 1990; Bottom 
et ah, 2005). However, how environmental conditions 
have changed in the last 30 years has received rela- 
tively little attention (Bottom et ah, 2006). It is known 
that spring river flow has been greatly reduced due 
to increased storage capacity of the hydrosystem. For 
example, average river flow in May and June measured 
at The Dalles Dam in 1977-81 was similar to recent 
(2007-10) levels; however, spring river flow in the 1960s 
and early 1970s was considerably greater (Bottom et 
ah, 2006). Unfortunately, long-term salinity data are 
not available to determine how these flow declines may 
have influenced salinity intrusion. Limited tempera- 
ture data from these earlier years suggest spring river 
temperatures were within the range presently observed 
(USGS National Water Information System). 
It has been suggested that fish distributions in the 
Columbia River estuary are determined in part by prey 
availability (Bottom and Jones, 1990; Simenstad et ah, 
1990). If there was a time lag between declines in flow 
rates and a response by the fish assemblage (as might 
be expected if their prey are responding to flow rates), 
it may explain potential changes in the fish assem- 
blage over the last 3 decades. Increasing numbers of 
forage fish also may explain the increasing abundance 
of avian predators in the Columbia River estuary, 
where these predators were largely absent 3 decades 
ago (Roby et al., 2003; Lyons, 2010). At present, we do 
not know whether changes in the fish assemblage have 
been beneficial or detrimental to juvenile salmon in 
the Columbia River estuary. Depressed populations of 
forage fish in Maine river basins are thought to inhibit 
the recovery of Atlantic salmon through a variety of 
ecological processes (Saunders et al., 2006). Whether 
similar ecological interactions between forage fish and 
Pacific salmon are occurring in the Columbia River 
basin is unknown. 
Conclusions 
We studied juvenile salmon and their associated fish 
community in open waters of the lower Columbia River 
estuary during spring in 4 years, 2007-10. During this 
period, we observed predictable seasonal changes in 
the abundance of all species of juvenile salmon, with 
low abundances in mid-April, peak abundances in May, 
and near absence by late June, except for subyearling 
Chinook salmon, which reached peak abundances in late 
June or early July. All caught juvenile salmon originated 
from throughout the Columbia River basin, and the 
vast majority (>90%) were of hatchery origin. Hatchery 
(i.e., clipped) steelhead and coho salmon were larger 
than individuals of the same species of unknown origin 
(unmarked hatchery and wild) but hatchery Chinook 
salmon were not, likely because of size overlap between 
large hatchery subyearling Chinook salmon and small 
wild yearling Chinook salmon. Although there was 
some variability in the abundance, timing, and size 
of juvenile salmon between years, no single year was 
exceptional with respect to these factors, as might be 
expected for fish with strong environmental influence. 
We hypothesize that the limited interannual variability 
we observed would have been higher if the proportion of 
wild fish also had been higher. 
In contrast to highly predictable seasonal abundance 
of juvenile salmon, the abundance and species composi- 
tion of the larger estuarine fish assemblage were ex- 
tremely variable at temporal scales ranging from hours 
to years. This high variability is likely in response to 
extremely dynamic physical environments in the Co- 
lumbia River estuary. Comparisons of our results with 
similar studies conducted over the previous 3 decades 
suggest major changes in the estuarine fish assemblage 
as a result of recent higher contributions of forage fish 
and threespine stickleback. The consequences of this 
change — whether it is beneficial or detrimental — to 
juvenile salmon is currently poorly understood. Al- 
though our research program has greatly increased our 
understanding of estuarine ecology in open waters of 
the Columbia River estuary in general and of juvenile 
salmon in these habitats in particular, there is clearly 
much that we do not know and hope to find out. 
Acknowledgments 
This research was conducted under Oregon Scientific 
Research Permits OR2007-3920, OR2008-3265, 14480, 
and 15374, and NOAA Fisheries Service ESA Permit 
1290-6M. This work was only possible with an excep- 
tional field crew, including C. Johnson, T. Sandel, P. 
Peterson, A. Claiborne, and A. Claxton, and boat opera- 
tors C. Taylor, B. Kelly, and R. Nelson. B. Emmett, S. 
Hinton, and G. McCabe developed the original idea for 
this study and provided considerable insight and guid- 
ance. This study was funded by the Northwest Fisheries 
Science Center and Bonneville Power Administration. 
The manuscript was greatly improved by constructive 
comments provided by R. Gustafson, D. Bottom, M. Ford, 
and J. Kocik. 
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Anderson, C. D., D. D. Roby, and K. Collis. 
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Anderson, S. K., D. D. Roby, D. E. Lyons, and K. Collis. 
2007. Relationship of Caspian tern foraging ecology to 
nesting success in the Columbia River estuary, Oregon, 
USA. Estuar. Coast. Shelf Sci. 73:447-456. 
Bottom, D. L., and K. K. Jones. 
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