440 
Fishery Bulletin 1 10(4) 
earlier studies in the Columbia River (e.g., Haertel 
and Osterberg, 1967; McCabe et al., 1983; Fox et al., 
1984; Bottom and Jones, 1990). Many of these species 
are also commonly caught in marine waters near the 
mouth of the Columbia River (Brodeur et al., 2005). All 
species are native to the Columbia River estuary and 
the Pacific Northwest, with the exception of American 
shad. This species was introduced into the Columbia 
basin from the U.S. Atlantic coast (Wydoski and Whit- 
ney, 2003) and is currently the single most abundant 
30 
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30 
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10 
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15 
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0 
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20 
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0 
30 
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40 
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Chinook 
■ subyearling 
□ yearling 
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Threespine stickleback 
Length (mm) 
Figure 5 
Distributions (%) of length (fork or total; in mm) fre- 
quency for selected fish species — Chinook salmon 
( Oncorhynchus tshawytscha), coho salmon (O. kisutch), 
steelhead ( O . mykiss), American shad (Alosa sapidis- 
sima), northern anchovy ( Engraulis mordax), Pacific 
herring ( Clupea pallasii), shiner perch ( Cymatogaster 
aggregata), surf smelt ( Hypomesus pretiosus), and three- 
spine stickleback (Gasterosteus aculeatus ) — captured in 
the Columbia River estuary during 2007-10, all years 
combined. 
anadromous fish species in the Columbia River (Has- 
selman et al., 2012a). 
The fish assemblage as a whole was extremely dy- 
namic with large variation at daily, seasonal, and in- 
terannual time scales. Although there was a general 
pattern of increasing abundance and species diversity 
as the season progressed, we often observed nearly 
as much variation within a single day or cruise as we 
did within the entire 4 years of our study. Our results 
indicate that the fish assemblage was responding to 
environmental forcing at both local (e.g., tide and salin- 
ity) and seasonal (e.g., river temperature) time scales. 
Not surprisingly, this conclusion is similar to the deter- 
minations of earlier studies: the fish assemblage was 
influenced by tides, season, and river flow (Haertel and 
Osterberg, 1967; Bottom and Jones, 1990; Jones et al., 
1990; Simenstad et al., 1990). 
Our results are consistent with (and likely in re- 
sponse to) highly dynamic and complex physical cir- 
culation in the estuary. Columbia River estuarine cir- 
culation is influenced by both riverine processes (e.g., 
seasonal flow cycles) and ocean processes (e.g., wind- 
forced upwelling and daily tidal inundation) and their 
interactions, which result in circulation patterns and 
water column properties that vary at temporal scales 
ranging from minutes to years (Jay and Smith, 1990; 
Chawla et al., 2008; Roegner et al., 2010b). In addi- 
tion, the north and south channels each have distinct 
circulation dynamics: strong river flow and a weak salt 
wedge in the south channel and more salinity intrusion 
and less river flow in the north channel (Jay and Smith, 
1990; Chawla et al., 2008). Given these physical differ- 
ences between the 2 channels (and, therefore, our study 
sites), it is surprising that our catches at the 2 stations 
were not more different than they were. 
In addition to physical forcing, variation in the estua- 
rine fish assemblage also reflects use patterns by differ- 
ent species-and-age classes of fish (Bottom and Jones, 
1990; Simenstad et al., 1990). For example, many of the 
species that we caught frequently are typical euryhaline 
estuarine residents for at least part of their life cycle; 
this list includes surf smelt, threespine stickleback, 
shiner perch, longfin smelt, and juvenile Pacific herring 
and American shad. Primarily marine species (north- 
ern anchovy, whitebait smelt, Pacific sardine, lingcod 
[Ophiodon elongatus], and Pacific tomcod [Microgadus 
proximus ]) were caught less consistently in the estuary, 
likely a reflection of seasonal patterns of estuarine use, 
offshore abundances, and whether conditions in the 
Columbia River estuary were favorable for occupation 
(Bottom and Jones, 1990). Finally, many species use 
the estuary primarily as a migration corridor and were, 
therefore, consistently caught every year at modest 
frequencies; downstream migrants include all juvenile 
salmon and river lamprey {Lampetra ayresii), and up- 
stream migrants were adult age classes of American 
shad, Pacific lamprey {Lampetra tridentata), Pacific 
herring, Chinook salmon, and steelhead. 
We observed changes in the fish assemblage in re- 
sponse to high flow events in 2008 and 2010; these 
