Fisher and Pearcy: Dietary overlap of juvenile fall- and spring-run Oncorhynchus tshawytscha 
27 
Fall Chinook salmon 
Stomachs 225 
Prey identified from 116 
Spring Chinook salmon 
Stomachs 155 
Prey identified from 65 
90 100 110 120 130 140 150 160 170 180 
Fork length (mm) 
+ Stomachs sampled 
Stomach contents identified 
Fin-marked (fall chmook) or adipose-clipped (spring chmook) 
Figure 2 
Length-frequency distributions of juvenile fish classified as fall 
and spring chinook salmon from which stomach samples were 
taken. Dark shading represents those stomach samples in which 
prey species were examined and identified. White bars in the up- 
per and lower graphs represent numbers of fin-clipped STEP- 
reared fall chinook salmon and adipose clipped Anadromous, Inc.- 
reared spring chinook salmon, respectively. 
Methods 
Juvenile chinook salmon were caught by beach 
seine (60 m x 2.5 m with 19- and 13-mm mesh 
in the wings and bunt, respectively) at five 
locations on the margins of channels in the 
lower half of Coos Bay, Oregon, between late 
May and early October 1987 (Fig. 1). The sub- 
stratum was sand at all but station 5, where 
it was a mixture of gravel fill and mud. At sta- 
tions 2, 3, and 4, portions of eel grass beds 
were sampled during low tide. The area of 
Coos Bay we sampled was influenced strongly 
by the ocean and was highly marine in char- 
acter with high salinities at all sampling sites, 
usually greater than 29 psu after mid-June. 
Water temperature (at 0.3 m depth) was fairly 
constant between May and October but in- 
creased with distance from the mouth, averag- 
ing 12.3°C at station 1 and 16.8°C at station 5 
(Fisher and Pearcy, 1990). 
Subsamples of juvenile chinook salmon 
caught in beach-seine sets were preserved in 
approximately 4% formaldehyde solution. 
Later, these were measured to the nearest mm 
FL and weighed to the nearest 0.01 g after 
excess moisture was removed by blotting. 
Stomachs were removed from 380 juvenile 
chinook salmon caught between 31 May and 
4 September 1987 (Fig. 2). Stomach-content 
boluses were weighed to the nearest milligram 
after removing excess moisture by blotting. 
After weighing, they were preserved in 50% 
ethanol, then transferred to 75% ethanol. 
At the time the stomach samples were ob- 
tained, the fish were examined for fin marks 
or for external parasites that could help to 
determine their origin. Most fish with clipped 
adipose fins also contained coded wire tags 
(CWT’s) that identified them as spring chinook 
salmon produced at Anadromous, Inc. Fish with other 
fin clips were mainly STEP-reared fall chinook 
salmon released in freshwater tributaries of Coos Bay 
(Fisher and Pearcy, 1990). 
The encysted metacercarial stage of a strigeoid 
trematode parasite is common in the skin of juvenile 
salmonids found in freshwater tributaries of Coos 
Bay. These cysts are surrounded by a black pigment 
that can be seen easily without magnification 
(Amandi 1 ). The presence of metacercarial cysts on 
the skin or fins of juvenile chinook salmon caught in 
1 Amandi, T. 1995. Oregon Dep. Fish and Wildl., 516 Nash 
Hall, Oregon State Univ., Corvallis, OR 97331. Personal 
commun. 
Coos Bay appeared to be a reliable indicator that the 
fish originated in the freshwater tributaries of the 
bay. Cysts were present on 43% of known fall chinook 
salmon (fin-marked STEP fish or fish caught before 
the first release of spring chinook salmon) and on 
71% of small fish <101 mm FL (>2SD below the mean 
FL of most release groups of spring chinook salmon 
by Anadromous, Inc.). Conversely, cysts were absent 
on adipose-clipped spring chinook salmon and found 
on only 13% of fish in the size range of the spring 
chinook salmon released by Anadromous, Inc. (>101 
mm FL). Fish >100 mm FL with cysts were probably 
native salmon or STEP-reared fall chinook salmon 
that attained these greater lengths through growth. 
On the basis of this evidence, we classified fish caught 
in Coos Bay as fall chinook salmon if they met any of 
