30 
Fishery Bulletin 95 ( 1 ), 1997 
and spring chinook salmon. The most numerous in- 
sects from spring chinook salmon stomachs were 
chironomids (31%), book and bark lice (23%), aphids 
(9%), tipulids (crane flies, 8%), and plant bugs (4%). 
Other prey categories that occurred frequently in 
stomachs of both fall and spring chinook salmon were 
barnacle molts (47% and 51%, respectively), algae 
and other plant material (46% and 68%), gammarid 
amphipods (41% and 43%), fishes (40% and 37%), and 
crab larvae (27% and 35%). Isopods, caprellid amphi- 
pods, nonanomuran or nonbrachyuran decapod larvae, 
spiders, unidentified arthropods, and molluscs were less 
common, occurring in 14% or fewer of stomachs 
Gammarid amphipods were a moderately impor- 
tant component of the diet of fall chinook salmon (4% 
by weight), but were less important in the diet of 
spring chinook salmon (only 1% by weight). A vari- 
ety of gammarid species were eaten by fall chinook 
salmon, the most abundant were Jassa spp. uniden- 
tified gammarids, Megalorchestia pugettensis, Ischy- 
rocerus spp., Atylus tridens, and Corophium spp. 
(2.0%, 0.6%, 0.3%, 0.2%, 0.2%, and 0.1% of total food 
weight respectively). 
Dietary overlap between juvenile fall and spring 
chinook salmon, according to the relative weights 
(Eq. 2) of the 14 major food categories (Table 1), was 
high (0.82), owing largely to the predominance of fish 
prey in diets of both groups. Diet overlap based on 
relative weights of prey identified to the lowest pos- 
sible taxonomic level (86 categories of varying taxo- 
nomic level) was lower but still relatively high (0.66). 
Diets by fish length 
Insect prey were relatively more important and fish 
prey were relatively less important in the diet of the 
