Bollens et al.: Feeding ecology of juvenile Oncorhynchus spp. 
399 
At the shallow station in early summer, decapod 
larvae dominated the diet of Chinook salmon >75 mm 
(Fig. 3B). During both late summer and autumn, in- 
sects and arachnids dominated the diets of all three 
size classes of Chinook salmon, but in autumn, those 
Chinook salmon >150 mm also consumed hyperiid 
amphipods. 
Diet of other salmon species 
Fewer diet samples were available for juvenile pink and 
coho salmon than for juvenile chum and Chinook salmon. 
Diet samples of pink salmon were available only for 
small fish (<49 mm) in spring. At the deep station these 
fish contained about 50% euphausiids, and gammarids, 
copepods other than C. pacificus, insects, and arachnids 
made up the other 50%. At the shallow station, their diet 
included a variety of prey consisting mainly of teleosts, 
pteropods, copepods other than C. pacificus, and fewer 
insects, decapod larvae, gammarids, and “others.” The 
“others” in this case were mostly bivalves, whereas the 
“others” for the co-occurring juvenile chum salmon were 
mostly chaetognaths. 
A few juvenile coho salmon were also caught at the 
two sample stations. At the deep station, diet of coho 
salmon 100-149 mm consisted primarily of decapod 
larvae and euphausiids, and a single larger (>150 mm) 
coho salmon consumed mostly decapod larvae in early 
summer and another large coho, only gammarid am- 
phipods in late summer. At the shallow station, three 
coho salmon were caught in June: one (75-99 mm) con- 
sumed about 75% “other” taxa (mostly the ostracod 
Euphilomedes), and the two other fish consumed almost 
exclusively decapod larvae (like the five co-occurring 
Chinook salmon). 
Juvenile salmon gut fullness in relation 
to zooplankton abundance 
Juvenile salmon gut fullness (% body weight) was not 
related to zooplankton abundance (dry weight) (Fig. 
4; Spearman’s rank correlation, P>0.05). For all four 
species of salmon, gut fullness was generally greater 
in spring and early summer and declined somewhat 
during late summer and autumn (chum salmon at the 
deep station in 1985 was an exception to this). In con- 
trast, zooplankton dry weight at the deep station peaked 
in the fall, and minima occurred in the spring. At the 
shallow station, peaks in zooplankton dry weight tended 
to occur in the summer, and less pronounced minima 
occurred in spring and autumn. Zooplankton dry weight 
was substantially greater at the deep station than at 
the shallow station (P<0.001, Mann-Whitney U test; 
Zar, 1999), generally by one order of magnitude. Some 
of this difference may have been due to the three-fold 
greater water column depth at the deep station, whereas 
a generally larger part of this difference was due to 
greater abundances of large-body zooplankton in the 
deep station samples. In addition, no significant cor- 
relation was found for individual salmon species’ gut 
1985 1986 1987 
• Chum O Chinook ▼ Pink a Coho 
<=) Zooplankton dry weight 
Figure 4 
Zooplankton (>216 pm) dry weights (g/m 2 ) and fish 
stomach fullness (mean % body weight) for four spe- 
cies of Pacific salmon — chum ( Oncorhynchus keta), Chi- 
nook (O. tshawytscha), pink (O. gorbuscha), and coho 
(O. kisutch ) — sampled between April 1985 and October 
1987 at a deep station (A) and a shallow station (B) in 
Dabob Bay, Washington. Fish collections were made 
with a midwater trawl and a surface tow net, and zoo- 
plankton collections were made with vertical hauls of 
a plankton net at each station on each date. Dashed 
vertical lines separate years. Note the different scales 
for zooplankton dry weights on the two y axes. 
fullness at the two different stations (Spearman’s rank 
correlation, P>0.05; Zar, 1999). 
Community composition of zooplankton 
Zooplankton communities (upper 50 m at night) were 
numerically dominated by copepods at both stations (Fig. 
5). The most striking contrast between our zooplankton 
composition and our juvenile salmonid diet composi- 
tion was the far greater abundance of copepods in the 
zooplankton. Substantially different seasonal patterns 
in zooplankton community composition were observed 
between stations during 1985 and 1986, but not in 1987. 
At the deep station, the greatest species richness was 
typically observed during early spring, whereas two spe- 
cies — Metridia lucens and Calanus pacficus — dominated 
