Weitkamp et ai.: Seasonal and interannual variation in juvenile salmonids in the lower Columbia River 
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Table 5 
Geographic origins of Chinook ( Oncorhynchus tshawytscha) and coho (O. kisutch ) salmon and steelhead (O. rnykiss) caught 
during spring of 2007-10 in the lower Columbia River estuary, determined on the basis of recovery of coded wire tag (CWT) 
and passive integrated transponder (PIT) tags, shown by major regions of the Columbia River basin. Chinook 0 and Chinook 
1 refer to subyearling and yearling age classes, respectively. No tagged steelhead were released in the Willamette and lower 
Columbia Rivers. 
Species-and-age class 
Chinook 0 
Chinook 1 
Coho 
Steelhead 
PIT 
CWT 
PIT 
CWT 
PIT 
CWT 
PIT 
CWT 
Number of tags recovered 
0 
127 
27 
254 
2 
87 
9 
33 
Release or tagging region^ 
Percentage of tags recovered by region 
Lower Columbia River 
29.9 
3.7 
5.9 
0 
54.1 
0 
0 
Willamette River 
0 
0 
4.3 
0 
1.1 
0 
0 
Mid-Columbia River 
48.8 
14.8 
14.6 
0 
3.4 
11.1 
0 
Upper Columbia River 
0.8 
29.6 
42.1 
100 
39.1 
0 
21.2 
Snake River 
20.5 
51.9 
33.1 
0 
2.3 
88.9 
78.8 
1 Release or tagging regions: lower Columbia River — below river km (rkm) 235 (Bonneville Dam), excluding the Willamette River; Willamette 
River — all locations in the Willamette River; mid-Columbia River — between Bonneville Dam and confluence with the Snake River (rkm 522); 
upper Columbia River — accessible waters above the confluence with the Snake River (rkm 522); Snake River — all accessible waters of the Snake 
River beginning at rkm 522. 
the largest variation (annual means ranged from 104 
to 135 mm), but the larger size likely reflects that some 
fish emigrated as 2-year-old smolts (age 2.0) rather 
than as yearlings (age 1.0). In addition, when mean 
size was examined across all years, in any given year 
at least one species-and-age class of juvenile salmon 
was the largest and one was the smallest, with the 
exception of 2008, when no species-and-age classes were 
the smallest. This result suggests that interannual 
variability in juvenile salmon size was independent for 
each group (Fig. 6). 
We also observed seasonal changes in the size of 
juvenile salmon; the seasonal size data, like the an- 
nual size data, displayed no cross-species patterns. 
Both coho and yearling Chinook salmon declined in 
length as the season progressed, whereas steelhead 
and subyearling Chinook salmon increased in length. 
Taking into account this seasonal change in length, the 
mean size of both juvenile subyearling Chinook salmon 
and steelhead varied by year (ANCOVA test of means, 
variance ratio [F]>8.0, P<0.01), but the size of yearling 
Chinook and coho salmon did not (ANCOVA test of 
means, F<1.8, P>0.10). We examined the length-weight 
relationships of juvenile salmon; common slopes (across 
all years) between transformed length and weight data 
ranged from 0.312 for yearling Chinook salmon (« = 658) 
to 0.329 for subyearling Chinook salmon (n= 776), with 
intermediate slopes for steelhead (0.325, n=250) and 
coho salmon (0.315, rc = 637). Coho salmon and both age 
classes of Chinook salmon had different slopes between 
length and weight among years (ANCOVA test of slopes, 
F> 2.3, P<0.05), but, for steelhead, no difference in slope 
was detected during the 3 years for which data were 
available (2008-10; F<1.0, P>0.10). 
Finally, we compared the size of clipped (hatchery) 
and unclipped (wild and hatchery) fish to determine 
whether clipped hatchery fish were consistently larg- 
er than unclipped fish (Fig. 7), as might be the case 
if most unclipped fish were of wild origin. Examined 
across all cruises and years, clipped individuals were 
statistically larger than unclipped fish for coho salmon 
(mean lengths of 148.7 vs. 140.6 mm, respectively; Two 
sample 7=6. 9, P<0.05) and steelhead (221.6 vs. 198.2 
mm, respectively; two sample 7=11.1, P<0.05), but no 
difference was detected for subyearling or yearling Chi- 
nook salmon (7<1.0, P>0.10). When examined for differ- 
ences among cruises (to account for seasonal changes 
in size) the findings were the same: significant differ- 
ences for coho salmon and steelhead (two-way ANOVA, 
F> 36, P<0.05) but not subyearling or yearling Chinook 
salmon (two-way ANOVA, F<1.0, P>0.10) (Fig. 7). In 
addition, we found interaction effects (F>1.8) for all 
species at a significance level of P<0.10, indicating that 
although clipped individuals were generally larger than 
unclipped individuals by cruise, the patterns were not 
consistent across all cruises. 
Discussion 
The pelagic fish assemblage we observed in open waters 
of the lower Columbia River estuary during spring 
2007-10 was characteristic of other Pacific Northwest 
estuaries including those assemblages examined in 
