Weitkamp et al.: Seasonal and interannual variation in juvenile salmonids in the lower Columbia River 
429 
fishes (e.g., larval smelt [Osmeridae] and small Pacific 
sand lance [Amrnodytes hexapterus]). We restricted sam- 
pling to depths of 8-10 m; therefore, the net fished the 
entire water column. We set the net in 1 of 2 configura- 
tions (round hauls or towed), depending on our specific 
objectives (quantitative catches or maximization of the 
catch of fish); 2 boats were used in both configurations 
to deploy and retrieve the net. Quantitative round hauls 
(n- 210) involved setting the net in a circle (area=1913 
m 2 ). We define catch per of unit effort (CPUE) as the 
abundance of fish per 1000 m 2 (round hauls only), and 
we tried to complete at least 3 round hauls per station 
per cruise. When densities of juvenile salmon were low 
(CPUE<2/1000 m 2 ), we increased the number of fish 
caught by towing the net for 8-10 min upstream before 
closing and pursing the net (« = 81 sets). We did not at- 
tempt to estimate abundances from these nonquantita- 
tive sets but did use the salmon collected during these 
sets to estimate origin and length-weight relationships, 
and we used length data from all fishes caught. 
Regardless of the set configuration, once the net was 
pursed and fish had been crowded into the knotless 
bunt, they were transferred to large (190-L) buckets 
with running river water. All nonsalmonid fishes were 
identified to species (Eschmeyer et al., 1983; Hart, 
1973) and enumerated, and all but 30 of each species 
were released. The retained fishes were anaesthetized 
with tricaine methane sulfonate (MS-222), measured 
(fork length [FL| or total length [TL] to the nearest 1 
mm as appropriate), allowed to fully recover, and then 
released. Fewer individuals (up to 30) of nonsalmonid 
species were measured from each set than of salmonids 
(up to 100) because the focus of our study was on juve- 
nile salmon. The measurement of 30 individuals of each 
nonsalmonid species provided adequate sample sizes 
(722-3674 length measurements for commonly caught 
species across the 4 years) without being overly time 
consuming. We noted the presence of invertebrates in 
our catch (e.g., California bay shrimp [ Crangon francis- 
corum], Dungeness crab [Cancer magister], and jelly- 
fishes [ Aequorea spp., Aurelia sp., Chrysaora fuscescens, 
and Eutonina indicans]) but did not attempt to quantify 
their abundances. All invertebrates that we encoun- 
tered are believed to be native to the Pacific Northwest, 
with the possible exception of Aurelia sp. (Kozloff, 1987; 
Wrobel and Mills, 1998). 
In the case of extremely large catches (>5000 indi- 
viduals), the total volume of fish in the net was visually 
estimated: 3 subsamples of known volume (5500 cm 3 ) 
were collected and transferred to separate buckets, and 
the remaining fish were released directly from the net 
without having been taken on board the vessel. Fish in 
each subsample were then identified to species, enumer- 
ated, and measured as described above. We estimated 
the total abundance of each species in the haul by tak- 
ing the average density of each species across the 3 
subsamples and multiplying it by the estimated total 
volume of fish. 
Juvenile salmon were anaesthetized, identified to spe- 
cies, checked for the presence of tags (passive integrated 
transponder [PIT], visible implant elastomer, coded 
wire tags [CWTs]), or clipped adipose fins, and mea- 
sured (FL to the nearest 1 mm). Juvenile salmon that 
were not needed for laboratory analyses were allowed 
to fully recover and released; tag codes of individuals 
tagged with PIT tags were “read” electronically before 
release. Salmon that were retained were given a lethal 
dose of MS-222, checked for tags and clips, and mea- 
sured as above; then, they were individually tagged, 
bagged, and immediately placed on ice. Once on shore, 
these fish were transferred to a -80°C freezer for later 
laboratory analysis. In the laboratory, thawed juvenile 
salmon were remeasured (FL to the nearest 1 mm), 
weighed (total wet weight in grams), and rechecked for 
tags and clip marks. Snouts were removed from fish 
with CWTs for extraction (see the next section Origins 
of juvenile salmon). 
Juvenile Chinook salmon were segregated into 2 age 
categories: subyearling (age 0.0) or yearling (age 1.0) 
on the basis of fish length (Dawley et al. 2 ). The length 
dividing subyearling from yearling Chinook salmon 
ranged from 115 mm FL in April to 140 mm on July 
1; it was developed from 1) seasonally adjusted length- 
frequency histograms (Hinton 3 ), 2) known ages based 
on scale analysis (Fisher 4 ), and 3) known ages deter- 
mined from PIT tags or CWTs. 
Origins of juvenile salmon 
We estimated the hatchery or wild origins and geo- 
graphic sources of juvenile salmon. Most of the -140 
million hatchery-origin Pacific salmon released in the 
Columbia River basin annually during the period of 
2007-10 were externally marked by clipping (i.e., ampu- 
tating) the adipose fin before release. These basinwide 
percentages of hatchery marking ranged from 67.8% 
(coho salmon) to 91.9% (yearling Chinook salmon) (FPC 
database). However, because of the large number of 
hatchery-produced fish that were unmarked (e.g., 16.7 
million subyearling Chinook, 7.2 million coho salmon), 
individuals with intact adipose fins could either be wild or 
unmarked hatchery fish. We used annual species-specific 
mark rates from the FPC online database to estimate 
the percentage of hatchery fish in our catch (%//), as 
mark rate in catch 
% H = , 
mark rate at hatcheiy 
where the mark rate in catch = the percentage of marked 
fish that we observed in the estuary for each species- 
and-age class in each year. 
In cases where mark rates in our catch exceeded those 
levels reported at hatcheries (i.e., %H> 100%), we capped 
%H at 100% for that year. Because some subyearling 
3 Hinton, S. 2010. Unpubl. data. Northwest Fisheries Sci- 
ence Center, Hammond, OR 97121. 
4 Fisher, J. 2010. Unpubl. data. Oregon State Univ., Cor- 
vallis, OR 97331. 
