Weitkamp et al : Seasonal and interannual variation in juvenile saimonids in the lower Columbia River 
443 
during 2007-10, the estimated in-river run size of adult 
spring Chinook salmon averaged 400,000 fish (PFMC, 
2011), and an average of 21,000 steelhead were counted 
crossing Bonneville Dam between April 15 and June 30 
(FPC database). Although adult salmon generally are 
believed to cease feeding once they enter freshwater 
(Groot and Margolis, 1991; Quinn, 2005), if they con- 
tinue to feed while in the Columbia River estuary, their 
predation effect on juvenile salmon may be considerable. 
Unfortunately, we were unable to examine adult salmon 
diets because of restrictions on ESA-listed species. De- 
spite considerable focus on other piscine predators in 
freshwater regions of the Columbia River (ISAB 1 ), this 
potential predation source has been largely overlooked 
but clearly deserves further attention. 
Several highly visible predators in the Columbia 
River estuary (e.g., harbor seals [Phoca vitulina], Cali- 
fornia [Zalophus californianus] and Stellar [Eumetopias 
jubatus ] sea lions, Caspian Terns [Hydroprogne caspia], 
and Double-crested Cormorants [Phalacrocorax auritus]) 
consume many of the fish species we caught in our 
study (e.g., anchovy, smelt, clupeids, and shiner perch), 
including juvenile salmon (Collis et al., 2001; Browne et 
al., 2002; Anderson et al., 2004, Anderson et al., 2007; 
Lyons et al., 2007). A primary objective of research on 
avian predators in the Columbia River estuary is to 
identify factors influencing predation rates on juvenile 
salmon, including the abundance of alternative prey 
(Anderson et al., 2007; Lyons et al., 2007). Flowever, 
these studies have been hampered by a lack of direct 
measurements of the fish assemblage or information re- 
garding how it varies at seasonal or annual time scales. 
Our results provide this critical information and even 
simple comparisons offer unique insight. For example, 
the size distribution of forage fishes in our study fully 
overlaps the size distribution of juvenile salmon (Fig. 5), 
suggesting that forage fishes are appropriate prey for 
predators focused on fish that are the size of salmon. 
In addition, the proportions of salmon in the diets of 
both Caspian Terns and Double-crested Cormorants 
are consistently highest in early May (Bird Research 
Northwest [BRN], http://www.birdresearchnw.org, ac- 
cessed August 2011), at a time when yearling salmon 
abundance is rapidly increasing (Fig. 4), yet densities of 
forage fish are still relatively low (Fig. 2). Furthermore, 
our findings that river flow influences the estuarine fish 
assemblage were successfully used to relate flow to the 
diets of Caspian Terns (Lyons, 2010). Future analyses 
will include comparisons of the abundance and size of 
fish (both salmon and nonsalmonids) in the environ- 
ment and in predator diets to gain further understand- 
ing of the dynamics of this important predator-prey 
relationship. 
Juvenile salmon in the Columbia River estuary 
Compared with high variation in the overall fish assem- 
blage, the seasonal pattern of juvenile salmon abundance 
was quite consistent between years (Fig. 4, Table 3). 
These seasonal patterns are similar to the patterns 
reported in studies conducted in the Columbia River 
estuary in the late 1970s and early 1980s (Dawley et 
al. 2 ; McCabe et al., 1983; Bottom and Jones, 1990) and 
typical of other Northwest estuaries (Bottom and Jones, 
1990; Groot and Margolis, 1991; Quinn, 2005). We also 
observed consistency between contemporary migra- 
tion patterns and those patterns determined before the 
advent of large-scale hatchery production and other 
anthropogenic changes in the Columbia River basin 
and estuary (Sherwood et al., 1990; NRC, 1996). In 
particular, Burke (2004) analyzed migration patterns 
of juvenile Chinook salmon through the Columbia River 
estuary in 1914-16 using data provided by Rich (1920). 
This early study found that yearling Chinook salmon 
migrated through the estuary at approximately the 
same time that we find (peak in mid-May and low in 
late June), although they (Rich, 1920; Burke, 2004) also 
observed an earlier abundance peak in March. We did 
not sample during March; however, we did not observe 
elevated abundances in mid-April (suggestive of an 
earlier abundance peak), nor was one apparent in the 
daily smolt counts at Bonneville Dam that commence 
each spring in early March (FPC database). The size 
of these historical yearling migrants (80-125 mm FL; 
Burke, 2004) also was much smaller than the size we 
currently observe (Fig. 6), although part of the size dif- 
ference may result from a different sampling location. 
Juvenile salmon collected near shore (where Rich [1920] 
likely collected his fish) tend to be smaller than the ones 
collected in open waters (senior author, unpubl. data). 
Our trends for subyearling salmon most closely mirror 
the “fingerling-estuarine rearing” group described by 
Burke (2004) that was at low abundance in mid-May, 
reached peak abundance in mid-July, and continued to 
be present in the estuary through fall. Like yearling 
migrants, these historical fish (mean size ~85 mm FL) 
were smaller than the fish we currently catch, but they 
also were caught near shore where we expect they would 
have been smaller. 
Similar to our findings, Dawley et al. 7 observed a 
similar composition of juvenile salmon by species-and 
age-class by month during 1978-80, with approximately 
equal abundances of subyearling and yearling Chinook 
and coho salmon and steelhead in May and mainly 
subyearling Chinook salmon in June (Fig. 8B). However, 
in April, we caught more subyearling Chinook salmon 
and chum salmon than the earlier catches, which had 
higher levels of yearling Chinook and coho salmon. The 
higher chum catches that we observed likely reflect a 
modest increase in the Columbia River chum popula- 
tions (Ford, 2011). It is not apparent why we observed 
higher proportions of subyearling Chinook salmon in 
7 Dawley, E. M., R. Ledgerwood, and A. L. Jensen. 1985. Beach 
and purse seine sampling of juvenile saimonids in the Colum- 
bia River estuary and ocean plume, 1977-1983: vol. I: Pro- 
cedures, sampling effort, and catch data. Final report of 
research funded by Bonneville Power Administration. [Avail- 
able from Northwest Fisheries Science Center, 2725 Montlake 
Blvd. E„ Seattle, WA 98112.] 
