438 
Fishery Bulletin 1 10(4) 
Yearling Chinook 
Subyearling Chinook 
Steelhead 
Q. Q. CL 
< < < 
< 0 ( 0(0 
to in m 
Sockeye 
( 0 ( 0(0 
ill 
( 0 ( 0(0 
(0 10(0 
Date 
Figure 4 
Catch per unit of effort (CPUE) of juvenile salmonids — yearling 
and subyearling Chinook salmon (Oncorhynchus tshawytscha), 
steelhead (O. mykiss), coho salmon ( O . kisutch), chum salmon 
(O. keta), and sockeye salmon (O. nerka ) — by date and year in the 
Columbia River estuary during spring of 2007-10; CPUE values 
are given as number/1000 m 2 ±1 standard error of the mean 
(SE). 
head originated only from the Snake or mid- 
Columbia rivers (PIT tag) or Snake and upper 
Columbia rivers (CWTs). 
Fish size 
Relative length The length of commonly caught 
fishes in our study varied considerably, from 
large (>280 mm) American shad and juvenile 
steelhead to consistently small (<70 mm) three- 
spine stickleback (Fig. 5). Because juvenile 
steelhead were the largest juvenile salmon, only 
large American shad were of similar size. In 
contrast, the length range of coho and Chinook 
salmon overlapped the size of many commonly 
caught species, including northern anchovy, 
Pacific herring, shiner perch and surf smelt. 
The size range for many fishes (e.g., American 
shad, surf smelt, and Pacific herring) was quite 
broad because of mixtures of both juvenile and 
adult ages in our catches. 
Juvenile salmon size The size of juvenile 
salmon generally varied by year and season, 
although no clear cross-species (i.e., year- 
specific) patterns were observed in any of the 
comparisons. For mean length by year, statisti- 
cally significant differences among years were 
observed for all species and age classes (H> 11, 
P<0.05), except for coho salmon (P>0.10; Fig. 
6). However, these interannual differences were 
generally small, with absolute mean size differ- 
ences ranging from 4.7 mm (yearling Chinook) 
to 11.6 mm (steelhead), which translated to a 
3-14% difference in length between years. The 
mean size of juvenile sockeye salmon showed 
Table 4 
Estimated percentage of juvenile Chinook (Oncorhynchus tshawytscha) and coho (O. kisutch ) salmon and steelhead (O. mykiss) 
of hatchery origin caught in open waters of the lower Columbia River estuary. Hatchery percentages were estimated from 
the percentage of clipped adipose fins observed in the estuary catches divided by the percentage of clipped fish released from 
hatcheries, averaged over the 4 years of our study, 2007-10. Also included are the mean number of juvenile salmon released 
annually and the percentage of fish clipped at hatcheries. Hatchery release numbers and information on clipped fish are from 
the Fish Passage Center (data available at http://fpc.org/, accessed July 2011). Chinook 0 and Chinook 1 refer to subyearling 
and yearling age classes, respectively. 
Species-and-age class 
Chinook salmon 0° 
Chinook salmon 1 
Coho salmon 
Steelhead 
Hatchery release (millions) 
49.5 
32.4 
22.4 
15.0 
Percentage clipped at hatchery 
75.7 
91.9 
67.8 
85.4 
Percentage clipped (observed in estuary) 
76.0 
86.9 
76.9 
77.6 
Estimated percent hatchery 
95.7 b 
94.6 
99.0 6 
90.9 
a Includes only hatchery fish released at least 10 days before the final cruise of the season: before June 19 in 2007 and 2008 and June 14 in 2009 
and 2010. 
b Capped at 100% in some years; therefore, estimated percent hatchery does not equal the percent clipped in estuary divided by percent clipped 
at hatchery. 
