Fisher and Pearcy. Dietary overlap of juvenile fall- and spring-run Oncorhynchus tshawytscha 
29 
Range in stomach fullness was similar among fish 
of different lengths, and stomach contents weights 
of 8% of body weight or higher occurred in fish from 
69 mm to 145 mm FL (Fig. 4). No significant differ- 
ence in median stomach fullness was found among 
four FL classes (<80 mm, 81-100 mm, 101-120 mm, 
and 121-140 mm) of fall chinook salmon (Kruskal- 
Wallace test, P=0.09). However, a significant differ- 
ence in median stomach fullness was found among 
the three FL classes (101-120 mm, 121-140 mm, and 
>141 mm) of spring chinook salmon (Kruskal-Wallace 
test, P=0.03). Median stomach fullness was lowest 
(0.4%) for the largest spring chinook salmon (>141 
mm FL). 
Median stomach fullness of fall chinook salmon 
was fairly constant during the study period, both 
before and after spring chinook salmon were released 
into the bay. No short-term decreases in stomach full- 
ness of fall chinook salmon were associated with in- 
dividual releases of spring chinook salmon, except 
for the 4 August release (Fig. 5). Conversely, median 
stomach fullness of spring chinook salmon was low 
immediately following releases of large numbers of 
spring chinook salmon from the Anadromous, Inc. 
C 
0) 
Fork length (mm) 
Figure 4 
Weight of stomach contents as a percentage of body weight 
versus fish length for juvenile fall chinook salmon (top 
graph) and spring chinook salmon (bottom graph). 
facility, especially the 4 August and the August 31-3 
September releases (Fig 5). 
Diets of fall and spring chinook salmon 
Percent FO and percent by weight of fourteen major 
prey categories from stomachs of juvenile fall and 
spring chinook salmon are summarized in Table 1. 
By weight, juvenile or larval fish were dominant prey 
of both fall and spring chinook salmon, representing 
64% and 65% of the total weight of stomach contents, 
respectively. The fish prey of fall chinook salmon were 
juvenile smelt, unidentified fish remains, Ammodytes 
hexapterus, juvenile Sebastes sp., and an unidenti- 
fied cottid, representing 41%, 10%, 8%, 6%, and <1% 
of stomach-content weight, respectively. Fish prey of 
spring chinook salmon were similar: juvenile smelt, 
Ammodytes hexapterus , unidentified fish remains, 
and Sebastes sp., accounted for 49%, 13%, 3%, and 
<1% of stomach-content weight, respectively. 
Other prey categories accounted for much smaller 
fractions of stomach-content weights of the two 
groups of juvenile chinook salmon. Of the nonfish 
prey, insects and plants (mainly algae) composed the 
largest fractions by weight in stomachs of fall chinook 
salmon (8% and 7%, respectively), whereas plants 
(mainly the algae Ulva sp. and Enteromorpha sp.) 
and barnacle molts composed the largest fractions 
by weight in stomachs of spring chinook salmon ( 16% 
and 12%, respectively; Table 1). 
The most numerous insects 2 in fall chinook salmon 
stomachs were adults of terrestrial taxa (61% of the 
total) and adults of taxa having aquatic or semi- 
aquatic larvae (36% of the total). Larvae and pupae 
composed only 3% of the total number of individu- 
als. Adults in the orders Diptera, Hemiptera, 
Homoptera, Pscoptera, Hymenoptera, Coleoptera, 
and Trichoptera accounted for 33%, 23%, 15%, 10%, 
7%, 6%, and 2% of the total number of insects in fall 
chinook salmon stomachs, respectively. The most 
numerous taxa in these insect orders (and their per- 
centages of total insect numbers) were midges 
(Chironomidae; 25%), plant bugs (Miridae; 22%), 
aphids (Aphididae; 11%), book and bark lice (10%), 
parisitoid wasps (5%), rove beetles (Staphylinidae; 
4%), and caddis flies (2%), respectively. 
Although insects were a much larger fraction by 
weight of the diet of fall chinook salmon than of the 
diet of spring chinook salmon ( 8% vs. 1%, respectively, 
Table 1 ), they occurred frequently in stomachs of both 
salmon groups (80% and 60%, respectively). Many 
of the same insect taxa were consumed by both fall 
2 The different insect taxa were not weighed separately, but in- 
dividuals of each taxon were counted. 
