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Fishery Bulletin 1 10(4) 
Yearling Chinook 
Subyearling Chinook 
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Date 
Figure 7 
Mean fork length (FL, in millimeters ±1 standard error [ SE ] of the mean) of clipped 
(hatchery) and unclipped (wild and hatchery) juvenile salmonids — yearling and sub- 
yearling Chinook salmon ( Oncorhynchus tshawytscha ), coho salmon (O. kisutch), and 
steelhead (O. mykiss)— arranged by sampling date (all years combined). 
June onwards) and again on 8 June 2010 (cruise 10-5) 
during the brief high flow event. Northern anchovy are 
primarily a marine species but move into estuaries 
from marine waters in the spring and summer (Fox 
et ah, 1984; Bottom and Jones, 1990). We observed 
daily movement into the estuary with the flood tide; 
our largest catches (CPUE>300/1000 m 2 ) of this spe- 
cies occurred an average of 5 h after low tide when 
both 1-and 7-m salinities were relatively high (5 and 21 
psu, respectively). Given the largely marine nature of 
northern anchovy, we believe the high flows inhibited 
their entry into the estuary. In contrast, low abundance 
of northern anchovy in the estuary in 2009, a year with 
“normal” flow, was likely a result of their low abundance 
in marine waters off the mouth of the Columbia River 
during the spring of 2009 (M. Litz, unpubl. data). 
We also suspect that high flows may displace juve- 
nile salmon downstream and transport them rapidly 
through the Columbia River estuary. For example, in 
the years with normal flow (2007 and 2009), juvenile 
Chinook and coho salmon and steelhead reached peak 
abundances on different dates, and they were at mod- 
est abundances (CPUE>2/1000 m 2 ) for 6-8 weeks. In 
contrast, in 2008 all yearling fish reached peak abun- 
dances on 20 May (cruise 8-4) as the flow was rapidly 
increasing, with much lower catch rates on the cruise 
after (but not before) 20 May. This finding suggests 
that the fish were being flushed downstream. Other 
research efforts in the Columbia estuary have shown 
that juvenile salmon initiate migration in response to 
increasing flows (Coutant and Whitney, 2006) and that 
migration rates are highest during high flows (Giorgi et 
ah, 1997; Friesen et al., 2007). It currently is unknown 
whether juvenile salmon might also respond to high 
flows by moving laterally into shallower water. Because 
both migration timing and flow rates influence juvenile 
salmon behavior and survival (Scheuerell et ah, 2009; 
Petrosky and Schaller, 2010), better understanding of 
these fine-scale processes is essential for conservation 
and recovery of at-risk salmon populations. 
Predation is a likely source of mortality for juvenile 
salmon in the Columbia River estuary (ISAB 1 ). There- 
fore, knowing who the predators are is essential. We 
caught several fishes that can be piscivorous as adults 
(e.g., lingcod and Pacific tomcod; Love, 2011); however, 
all individuals were juveniles (and often smaller than 
juvenile salmon) and therefore unlikely predators of 
juvenile salmon in the estuary. Three potential piscine 
predators were cutthroat trout and adults of both spring 
Chinook salmon and steelhead, all of which are highly 
piscivorous (Quinn, 2005; Duffy and Beauchamp 2008). 
Whereas cutthroat trout have relatively small popula- 
tions in the Columbia River (ISAB 1 ), both spring Chi- 
nook salmon and steelhead have large populations: 
