Fisher and Pearcy. Dietary overlap of juvenile fall- and spring-run Oncorhynchus tshawytscha 
33 
Diets by sampling period 
Between two sampling periods (29 June-17 July and 
3-13 August) moderate changes occurred in the pro- 
portions of the 14 major food categories in stomachs 
of both fall and spring chinook salmon. In stomachs 
of fall chinook salmon, the percentage by weight of 
insects, gammarid amphipods, and crab larvae was 
higher in the earlier than in the later period, whereas 
the percentage by weight of fish prey was higher in 
the later than in the earlier period (Table 6). In spring 
chinook salmon stomachs, barnacle molts and fish 
were more abundant in the earlier period than in 
the later period, whereas algae and crab larvae were 
more abundant in the later period than in the ear- 
lier period. 
Despite these shifts in prey composition, diet over- 
lap based on the 14 major prey categories was high 
for all comparisons of fall and spring chinook salmon 
caught in the two time periods (Table 7). However, 
Table 4 
Percentage by weight and frequency of occurrence (in parentheses) of fourteen major food categories in stomachs of fall and 
spring chinook salmon caught in 1987 in the lower (stations 1-3) and mid (stations 4-5) sections of Coos Bay. Numbers in 
brackets are sample sizes. Mean fork lengths (FL) of fish in each area are also shown. 
Fall chinook salmon Spring chinook salmon 
Food category 
Sta. 1-3 
87 mm FL 
[90] 
Sta. 4-5 
88 mm FL 
[261 
Sta. 1-3 
123 mm FL 
[39] 
Sta. 4-5 
118 mm FL 
[26] 
Cirripedia molts 
2 (39) 
22 (77) 
4 (33) 
35 (77) 
Isopods 
<1 (4) 
<1 (23) 
<1 (8) 
<1 (15) 
Caprellid amphipods 
1 (14) 
1 (12) 
<1 (15) 
<1 (4) 
Gammarid amphipods 
4 (42) 
2 (35) 
<1 (33) 
2 (58) 
Brachyuran, anomuran larvae 
2 (28) 
6 (23) 
2 (44) 
1 (23) 
Other decapod larvae 
<1 (10) 
0 
<1 (3) 
0 
Crustacean fragments 
6 (22) 
3 (12) 
1 (26) 
1 (23) 
Araneae 
<1 (12) 
<1 (19) 
<1 (3) 
<1 (8) 
Insects 
6 (76) 
17 (96) 
<1 (49) 
1 (77) 
Other arthropods 
<1 (6) 
1 (8) 
0 
<1 (4) 
Molluscs 
<1 (2) 
1 (12) 
<1(13) 
<1 (12) 
Teleosts 
71 (47) 
14 (15) 
79(54) 
24(12) 
Algae, plants 
3 (37) 
31 (77) 
11 (56) 
32 (84) 
Other material 
4 (44) 
3 (38) 
1 (54) 
3 (58) 
Table 5 
Dietary overlap of fall and spring chinook salmon caught in the lower (stations 1- 3) and mid (stations 4- 5) sections of Coos Bay. 
Overlap values based on 14 major food categories are in normal type and those based on 86 lower taxonomic categories are in 
italics. High overlap values (>0.60) are in bold type. 
Fall chinook salmon Spring chinook salmon 
Sta. 4-5 
Sta. 1-3 
Sta. 4-5 
Fall chinook salmon Sta. 1-3 
0.37 
0.82 
0.38 
0.28 
0.68 
0.35 
Sta. 4-5 
— 
0.35 
0.75 
0.22 
0.47 
Spring chinook salmon Sta. 1-3 


0.43 
0.41 
