Fisher and Pearcy: Dietary overlap of juvenile fall- and spring-run Oncorhynchus tshawytscha 
35 
Table 7 
Dietary overlap of fall and spring chinook salmon during two time periods. 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 
3-13 Aug 
29 Jun-17 Jul 3- 
-13 Aug 
Fall chinook salmon 29 Jun-17 Jul 
0.66 
0.67 
0.73 
0.26 
0.44 
0.32 
3-13 Aug 
~ 
0.83 
0.49 
0.73 
0.70 
Spring chinook salmon 29 Jun-17 Jul 
— 
— 
0.75 
0.56 
fore and after spring chinook salmon were released 
into the bay (Fig. 5). In fact, stomach fullness of fall 
chinook salmon was usually higher than that of 
spring chinook salmon throughout the study period 
(Figs. 3 and 5). The low stomach fullness among 
spring chinook salmon following releases from the 
Anadromous, Inc. facility (Fig. 5) may reflect a delay 
in the start of feeding on natural prey by these hatch- 
ery fish. Paszkowski and Olla (1985) suggested that 
the inability of some hatchery fish to adapt to 
the natural environment may contribute to the poor 
survival of some groups of hatchery salmon. We con- 
clude that the high dietary overlap between juvenile 
fall and spring chinook salmon indicates the poten- 
tial for competition for food between these salmon 
groups in Coos Bay, but that in the summer of 1987 
there was little evidence of actual food limitation or 
competition. 
Differences between smaller fall chinook salmon 
and larger hatchery spring chinook salmon in spa- 
tial distribution and duration of residence within 
estuaries may tend to minimize their competition for 
food. Small fish tend to occur in shallow, nearshore 
areas or in salt marshes, whereas large fish tend to 
occur in deeper channel areas (Healey, 1980a, 1991; 
Kjelson et al., 1982; Levings, 1982; Simenstad et al., 
1982; McCabe et ah, 1986; Macdonald et ah, 1987). 
Larger juvenile chinook salmon also tend to spend 
less time in estuaries than do smaller fish (Myers, 
1980; Simenstad and Wissmar, 1984; Fisher and 
Pearcy, 1990). Both these differences may tend to 
decrease competition for food between hatchery- 
reared and wild chinook salmon in estuaries if there 
is a large difference in their size. However, large re- 
leases of hatchery salmon smolts into an estuary may 
affect wild smolts detrimentally by attracting birds 
and other predators that prey on juvenile salmon 
(Emlen et ah, 1990). 
We did not investigate rates of secondary produc- 
tion in the bay, rates of exchange of prey between 
the adjacent ocean and the bay, the rations required 
by juvenile salmon to maintain optimum growth 
rates, or the fractions of available prey in the bay 
eaten by juvenile salmon and other potential com- 
peting species. Without such information it is diffi- 
cult to assess the likelihood that the growth and sur- 
vival of juvenile salmon was limited by food in Coos 
Bay in 1987. The lower half of Coos Bay is strongly 
influenced by the adjacent ocean (Burt and McAlister, 
1959; Fisher and Pearcy, 1990). In a study ofYaquina 
Bay, an Oregon estuary with physical characteris- 
tics similar to Coos Bay, Myers ( 1980) suggested that 
much of the food for juvenile salmon residing in the 
bay was supplied by tidal exchange with the ocean. 
Undoubtedly, the productivity of the adjacent ocean 
has a strong influence on the capacity of Coos Bay to 
support juvenile chinook salmon. 
Upper-bay and Jower-bay gradients in diet 
Between the mid and lower sections of Coos Bay the 
diet of juvenile fall chinook salmon shifted from pre- 
dominantly drift insects, barnacle molts, and drift 
algae to predominantly marine fishes (Table 4). A 
similar increase in piscivory in the lower bay also 
occurred among spring chinook salmon (Table 4). 
Shifts in the diet of juvenile chinook salmon as they 
move from the river, through the estuary, and to the 
ocean appear to be related to the changes in habitat 
and foraging behavior which occur as a consequence 
of growth and development. Macdonald et al. ( 1987) 
observed that large hatchery-reared chinook salmon 
were often found in deeper, more saline waters of 
the salt-wedge of the Campbell River estuary, 
whereas smaller wild chinook salmon were often 
found in the freshwater layer near the surface. Small 
