Bollens et al.: Feeding ecology of juvenile Oncorhynchus spp. 
395 
Table 1 
Number of juvenile Chinook (Oncorhynchus tshawytscha), chum (O. keta ), coho (O. kisutch), and pink (O. gorbuscha) salmon 
stomach samples and zooplankton samples collected during 12 visits to each of two stations in Dabob Bay, Washington, 1985-87. 
Sampling dates are grouped by year within season, number of samples for the deep (D, 190 m) station are indicated on the left 
and those for the shallow (S, 60 m) station indicated on the right. Diet samples were pooled from midwater trawl and surface 
trawl collections at each station for each date or dates; duplicate zooplankton samples were collected at each station on each date 
by vertical hauls of plankton nets. Fish samples were not collected in spring of 1985. 
Number of juvenile salmon stomachs 
Number of 
zooplankton 
samples 
Chinook 
Chum 
Coho 
Pink 
Season and sampling dates 
D 
S 
D 
S 
D 
S 
D 
S 
D 
S 
Spring 
1985 
30 Apr and 29 May 1986 
0 
0 
16 
15 
0 
0 
3 
7 
2 
2 
2 
2 
6-7 May 1987 
0 
0 
6 
10 
0 
0 
0 
0 
2 
2 
Subtotal 
0 
0 
22 
25 
0 
0 
3 
7 
6 
6 
Early summer 
19-20 Jun 1985, 24-25 Jun 1985, 
26 Jul 1985 
5 
10 
14 
9 
0 
0 
0 
0 
2 
2 
4-5 Jun 1986 
4 
3 
10 
4 
5 
1 
0 
0 
2 
2 
17-18 Jun 1987, 13 Jul 1987 
3 
5 
0 
14 
1 
2 
0 
0 
2 
2 
Subtotal 
12 
18 
24 
27 
6 
3 
0 
0 
6 
6 
Late summer 
19-20 Aug 1985, 26-27 Aug 1985 
4 
5 
10 
12 
0 
0 
0 
0 
2 
2 
12 Aug 1986, 14 Aug 1986 
5 
7 
7 
3 
0 
0 
0 
0 
2 
2 
19-20 Aug 1987 
11 
6 
7 
0 
1 
0 
0 
0 
2 
2 
Subtotal 
20 
18 
24 
15 
1 
0 
0 
0 
6 
6 
Autumn 
7-9 Oct 1985 
0 
6 
5 
0 
0 
0 
0 
0 
2 
2 
22 Oct 1986 
6 
10 
7 
0 
0 
0 
0 
0 
2 
2 
14-15 Oct 1987 
29 
3 
3 
0 
0 
0 
0 
0 
2 
2 
Subtotal 
35 
19 
15 
0 
0 
0 
0 
0 
6 
6 
Total 
67 
55 
85 
67 
7 
3 
3 
7 
24 
24 
75-99 mm, 100-149 mm, >150 mm) were removed and 
0. 5-3.0 cc of undiluted formaldehyde were injected into 
their gut cavity to halt digestion. Each fish was then 
stored in dilute (5%) formalin-seawater solution. Fish 
from both trawl types were combined to yield a repre- 
sentative sample of fish residing in the upper 50 m of 
the water column at night. Subsequently, each fish was 
weighed (g, wet weight) and measured in the labora- 
tory, and its stomach excised. Stomach contents were 
weighed (wet weight), digestion stage was recorded, and 
then prey were identified to the lowest possible taxon 
(often to species), and enumerated and weighed (wet 
weight) by category. Diet composition was summarized 
as the normalized average percent prey biomass ( [bio- 
mass of taxonomic group]/[total weight of stomach - the 
weight of the unidentified portion of the stomach con- 
tents]). A total of 294 salmon stomachs were examined; 
six were empty and were not considered further. 
For ease of comparison, we pooled the original 122 
prey taxa observed into 13 categories that made up at 
least 10% of any one fish’s total normalized prey bio- 
mass. Data were stratified by salmon species and size 
class, season, and station across the three years pooled. 
In most cases the pooled data were a good representa- 
tion of the three individual years of data, but in some 
cases there were interesting interannual differences, 
as discussed below. 
Collection and processing of zooplankton 
Zooplankton collections were made at the two stations 
within 1-2 hours of the fish trawls by using a 1-m 2 
mouth opening and 333-pm mesh multiple net sampler 
(Frost, 1988) in 1985 and 1986 and a 1-m diameter 
mouth opening and 216 pm mesh Puget Sound net 
(Research Nets Inc., Seattle) (Miller et al., 1977) in 
1987. Duplicate zooplankton samples were collected at 
each station and on each date (Table 1). A subsample 
(collected with a Stempel pipette) of 1-2% of the ani- 
mals was taken and the species were enumerated and 
