34 
Fishery Bulletin 95(1 ), 1997 
Table 6 
Percentage by weight and frequency of occurrence (in parentheses) of fourteen major food categories in stomach of juvenile fall 
and spring chinook salmon caught during two time periods in 1987 in Coos Bay. Numbers in brackets are sample sizes. Mean fork 
lengths (FL) of fish caught during each time are also shown. 
Fall chinook salmon Spring chinook salmon 
Food category 
29 Jun-17 Jul 
85 mm FL 
[89] 
3-13 Aug 
95 mm FL 
[26] 
29 Jun-17 Jul 
117 mm FL 
[26] 
3-13 Aug 
123 mm FL 
[39] 
Cirripedia molts 
6 (54) 
3 (23) 
17(65) 
8 (41) 
Isopods 
<1 (11) 
0 
<1 (4) 
<1 (15) 
Caprellid amphipods 
1 (17) 
<1 (4) 
<1 (12) 
<1 (10) 
Gammarid amphipods 
6 (47) 
<1 (19) 
1 (38) 
1 (46) 
Brachyuran, anomuran larvae 
2 (29) 
<1 (15) 
1 (27) 
2 (41) 
Other decapod larvae 
<1 (10) 
0 
0 
<1 (3) 
Crustacean fragments 
9 (24) 
<1 (8) 
<1 (15) 
2 (31) 
Araneae 
<1 (17) 
<1 (4) 
<1 (8) 
<1 (3) 
Insects 
11 (84) 
2 (65) 
<1 (65) 
1 (56) 
Other arthropods 
<1 (8) 
0 
<1 (4) 
0 
Molluscs 
<1 (3) 
<1 (8) 
0 
<1 (21) 
Teleosts 
51 (36) 
84 (54) 
73 (38) 
58(36) 
Algae, plants 
6 (45) 
7 (50) 
6(62) 
24(72) 
Other material 
5 (47) 
3 (31) 
1 (38) 
3 (67) 
diet overlap based on the lowest identified taxa (86 
categories) was low for all comparisons except that 
between fall and spring chinook salmon caught in 
the period 3-13 August. Although a variety of fish 
prey were eaten by both salmon groups during the 
earlier period, during the later period fish prey were 
nearly all juvenile osmerids. 
Discussion 
Potential for competition 
The high dietary overlap values (Tables 3, 5, 7) be- 
tween juvenile fall chinook salmon >81 mm FL and 
hatchery spring chinook salmon suggest that there 
is the potential for competition for food between these 
two groups in Coos Bay under conditions of food limi- 
tation. However, whether or not the two groups were 
competing for food in 1987 cannot be determined from 
dietary overlap alone. In fact, high dietary overlap 
may sometimes indicate a condition in which abun- 
dant food resources are shared between potential 
competitors rather than a condition in which there 
is competition for a resource in short supply (Zaret 
and Rand, 1971; Myers, 1980). Zaret and Rand 
(1971), in a study of tropical stream fishes, found 
that dietary overlap between species was high dur- 
ing the rainy season, when food resources were abun- 
dant, and low during the dry season, when food re- 
sources were scarce and when the different fish spe- 
cies targeted different prey. 
We found little evidence in this study that the in- 
troduced hatchery-reared spring chinook salmon 
outcompeted native and STEP-reared fall chinook 
salmon for food. One potential result of competition 
for food between groups is a shift to less desirable 
prey in the diet of the weaker competitors (Hanson 
and Leggett, 1986). However, during the period when 
both fall and spring chinook salmon were in Coos 
Bay, calorically dense (high-quality) fish prey made 
up an equally large fraction by weight of the diets of 
both salmon groups (Table 1); i.e. fall chinook salmon 
were eating just as nutritious prey as that eaten by 
spring chinook salmon. Another potential result of 
competition is a decrease in growth rate (or average 
stomach fullness) of one or all of the competing groups 
(Reimers, 1973; Nielson et al., 1985; Hanson and 
Leggett, 1986). If spring chinook salmon outcompeted 
fall chinook salmon for food, the average stomach 
fullness of fall chinook salmon might be expected to 
drop following releases of the spring chinook salmon; 
this, however, did not occur. Stomach fullness of fall 
chinook salmon was equally high in the periods be- 
