Weitkamp et at: Seasonal and interannual variation in juvenile salmonids in the lower Columbia River 
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April 15-30 
May 1-15 
* May 16-31 
a June 1-15 
♦ June 16-30 
April 15-30 
May 1-15 
x May 16-31 
a June 1-15 
♦ June 16-30 
Nonmetric multidimensional scaling (MDS) by (A) cruise and (B) 
haul based on the 13 most commonly caught species-and-age classes 
in the Columbia River fish assemblage during spring of 2007-10. 
Sampling years in the top graph are abbreviated: 7 for 2007, 8 for 
2008, 9 for 2009, and 10 for 2010. 
and steelhead (CPUE=2-5/1000 m 2 ), and the highest 
abundances estimated for subyearling Chinook salmon 
(CPUE = 8-10/1000 m 2 ). There was also variation in the 
length of time that salmon were present in the estuary: 
both chum and sockeye salmon were caught typically 
only during a 2-4 week period, yearling Chinook and 
coho salmon and steelhead were caught readily in 
the estuary for 6-8 weeks each year, and subyearling 
Chinook salmon were present for at least 2 months 
(and presumably longer but sampling was terminated). 
We also observed interannual variation in this sea- 
sonal abundance pattern, although no single year was 
associated with exceptional catches for any species- 
and-age class of juvenile salmon (Fig. 4). For example, 
the timing of maximum abundances of yearling Chi- 
nook and coho salmon and steelhead occurred in late 
May in 2007 and early May in 2009 and 2010 and 
subyearling Chinook salmon occurred early in 2009 
and 2010 and late in 2008. Similarly, we observed 
no consistent patterns in the magnitude of maximum 
abundances; peak abundances for most species, except 
sockeye salmon, were low in 2007, yet levels were high 
for chum salmon in 2008, coho salmon in 2009, and 
steelhead and yearling Chinook salmon in 2010. 
Origins of juvenile salmon 
We determined the origins of juvenile salmon by both 
rearing conditions (hatchery vs. wild) and geographic 
location. From the number of clipped adipose fins we 
observed in our catches and the number of fish clipped 
basinwide at hatcheries, we concluded that most juve- 
nile salmon that we collected were of hatchery origin 
(Table 4); this percentage ranged from 91% for steel- 
head to 99% for coho salmon, with estimates exceeding 
100% for both coho and subyearling Chinook salmon 
in some years. 
We also were able to determine geographic origin for 
508 juvenile salmon from which we recovered CWTs 
and for 38 juveniles with PIT tags (Table 5). These 
tagged fish originated from locations throughout the 
Columbia River basin— lower, mid-, and upper Co- 
lumbia River and the Snake and Willamette rivers. 
In a notable exception to this pattern, tagged steel- 
