25 
Abstract— During the summer of 
1987 in Coos Bay, Oregon, dietary over- 
lap (Schoener index) between juvenile 
fall-run chinook salmon, Oncorhynchus 
tshawytscha, and an introduced stock 
of juvenile hatchery-reared spring-run 
chinook salmon was high (0.82), indi- 
cating the potential for competition for 
food between these two groups in times 
of food scarcity. Both groups consumed 
a variety of prey, including fishes, adult 
insects, algae, barnacle molts, gam- 
marid and caprellid amphipods, and 
juvenile decapods. Diets of both salmon 
groups varied with fish size and cap- 
ture location. Overlap was low (0.25- 
0.55) between the smallest juvenile fall 
chinook salmon (<80 mm FL), for which 
insects were the predominant prey 
(26% by weight), and all other length 
groups of both fall and spring chinook 
salmon, for which fish were the predom- 
inant prey (49%-94% by weight). Dietary 
overlap between both salmon groups was 
high in the lower bay (0.82), where fish 
prey predominated in the diets, and 
was also high in the mid bay (0.75), 
where algae and barnacle molts pre- 
dominated in the diets. Three pieces 
of evidence suggest that the introduced 
hatchery-reared spring chinook salmon 
did not outcompete fall chinook salmon 
for food: 1) both the median stomach 
fullness and the percentage of stomachs 
containing food was higher for fall 
chinook salmon than for spring chinook 
salmon, 2) the median stomach fullness 
of fall chinook salmon was as high in 
the period following releases of spring 
chinook salmon into the bay as in the 
period prior to the releases, and 3 ) food 
of high caloric density (i.e. fish prey) 
formed an equally high proportion of 
the diets of both salmon groups, indi- 
cating that the quality of food eaten by 
both was similar. 
Manuscript accepted 31 July 1996. 
Fishery Bulletin 95:25-38 (1996). 
Dietary overlap of juvenile 
fall- and spring-run chinook salmon, 
Oncorhynchus tshawytscha, 
in Coos Bay, Oregon 
Joseph R Fisher 
William G. Pearcy 
College of Oceanic and Atmospheric Sciences 
Oregon State University, Corvallis, Oregon 97331-5503 
E-mail address: fisherjo@ucs.orst.edu 
Estuaries serve as rich feeding 
grounds and as refuges from preda- 
tion for many juvenile subyearling 
fall-run (hereafter referred to as 
“fall”) chinook salmon, Oncorhyn- 
chus tshawytscha, that reside in 
them for weeks or months before 
entering the ocean (Healey, 1980a, 
1982, 1991; Myers, 1980; Kjelson et 
al., 1982; Myers and Horton, 1982; 
Simenstad et al., 1982). The sur- 
vival of subyearling fall chinook 
salmon may be enhanced by ex- 
tended residence in estuaries. 
Reimers (1973) reported that sur- 
vival was greater among juvenile 
fall chinook salmon that resided in 
the Sixes River estuary from early 
summer through early fall than 
among those that quickly migrated 
through the estuary to the ocean in 
early summer. 
Although there is much concern 
about the interaction between 
hatchery and wild stocks of salmon 
(Hilborn and Winton, 1993; Thomas 
and Mathisen, 1993; Winton and 
Hilborn, 1994), few reports docu- 
ment possible competition between 
groups of salmon for food in estuar- 
ies or the ocean. Peterman (1984) 
and Rogers and Ruggerone (1993) 
found negative correlations between 
size of sockeye salmon at different 
ages and their population and sug- 
gested that growth of sockeye 
salmon in the ocean was density 
dependent. Reimers (1973) and 
Neilson et al. ( 1985) found that the 
average growth rate of juvenile fall 
chinook salmon in the Sixes River 
estuary decreased during mid-sum- 
mer when the population of juvenile 
salmon was high. Reimers (1973) 
attributed this drop in growth rate 
to intraspecific competition for lim- 
ited food resources, leading to den- 
sity-dependent growth, whereas 
Neilson et al. (1985), noting that the 
decrease in growth rate occurred dur- 
ing a period of increased abundance 
of the principal prey (Corophium sp. ), 
suggested that lowered conversion 
efficiencies due to high temperatures 
in the estuary as well as intraspecific 
competition may have contributed to 
the drop in growth rate. 
These studies suggest that re- 
leases of large numbers of hatchery 
salmon smolts into an estuarine 
basin could affect the native salmon 
in the system through competition 
for food in the estuary. The effect of 
competition on growth and survival 
of native fish would depend on sev- 
eral factors, among them the inten- 
sity and duration of the competition 
between the two groups. If the 
hatchery-reared fish eat different 
prey from that eaten by the wild 
fish, or if they move quickly through 
the estuary, their impact on the 
native fish may be relatively small. 
On the other hand, if the two groups 
