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Fishery Bulletin 1 10(4) 
Figure 1 
Map of the 2 sampling stations, North Channel and Trestle Bay, used in our study of the estuarine fish assemblage in 
open-water habitats of the lower Columbia River estuary. Shoreline, 0-m, and 7-m depth contours are indicated, as are 
major towns. The insert map indicates the location of the study area (black rectangle) and 5 geographic regions within the 
Columbia River basin from which juvenile salmon originated: lower Columbia River (LCR), Willamette River (WR), mid- 
Columbia River (MCR), upper Columbia River (UCR), and Snake River (SR). State or province abbreviations: WA=Washington; 
OR = Oregon; ID = Idaho; MT=Montana; BC = British Columbia. 
Materials and methods 
Collection of fish 
Our sampling was directed toward the spring outmi- 
gration of juvenile salmon as they passed through the 
Columbia River estuary. For each sampling trip or 
cruise, we had the following objectives: 1) characterize 
the estuarine fish assemblage by documenting the abun- 
dance, size, and condition of fishes (including juvenile 
salmon) present in the sampling area, and 2) collect 50 
individuals of each of 4 juvenile salmon species and age 
classes (yearling Chinook, subyearling Chinook, and 
coho salmon, and steelhead) for laboratory analyses; 
this number reflects a balance between restrictions on 
lethal sampling of ESA-listed species and the need for 
sufficient samples for statistical rigor. 
To achieve these objectives, fish were sampled dur- 
ing daylight hours every 2 weeks from mid-April to 
late June or early July during 2007-10 at 2 stations, 
North Channel (46°14.2'N, 123°54.2'W, river km [rkm 
17) and Trestle Bay (46°12.9'N, 123°57.7'W, rkm 13) 
(Fig. 1). These stations are located in the lower estu- 
ary adjacent to the 2 deep channels that bisect the 
north and south portions of the estuary, respectively, 
in the “estuarine mixing” region (Bottom and Jones, 
1990; Jones et al., 1990). These stations also have 
historical significance because they have been used 
as study sites since the late 1960s (Johnsen and Sims, 
1973; Dawley et al. 2 ). 
Sampling was restricted to days with early morn- 
ing low tides, which typically occur during extreme 
(minus) tides during springtime; the first set of the 
net was made at approximately low slack water and 
sampling continued for the duration of the flood tide. 
This timetable was adopted because 1) fish are dif- 
ficult to sample during ebb (outgoing) tides owing to 
high currents (which often exceed 11 km/h) and 2) 
strong thermally driven afternoon winds in excess of 
48 km/h limited boat maneuverability and, therefore, 
sampling efficiency. Our timetable left ebb and neap 
tides and nighttime as times when fish were largely 
unsampled, but extremely strong downstream currents 
and close proximity to the hazardous Columbia River 
bar provide challenging conditions for vessels of any 
size (Haertel and Osterberg, 1967; Johnsen and Sims, 
1973). Nevertheless, in studies of the Columbia River 
estuary, abundances of juvenile salmon were found to 
be either greatest during daylight hours (Ledgerwood 
et al., 1991) or were similar between day and night 
(Friesen et al., 2007), and therefore we were unlikely to 
miss large numbers of fish with our sampling schedule. 
Fish were sampled with a fine-mesh purse seine (10.6 
m deep and 155 m long, with stretched mesh opening 
1.7 cm; knotless bunt mesh 1.5 cm); this mesh was suf- 
ficiently fine to effectively catch all but a few elongate 
