Lee and Sampson: Dietary variations for three rockfish species off the Pacific Northwest 
519 
Table 3 
Percentages by weight of the major prey groups for the rockfish species ( Sebastes 
spp.) that collected in the same trawling hauls during the quarterly collections 
from April 1998 to September 1999. There were 15 hauls out of a total of 49 
hauls in which all three rockfish species were collected concurrently. 
Prey groups 
S. flavidus 
iS. entomelas 
S. pinniger 
Euphausiids 
25.8 
7.8 
99.4 
Fishes 
32.1 
0.4 
0.1 
Jellyfishes 
1.4 
24.8 
0.3 
Salps 
39.5 
63.6 
— 
Heteropods 
<0.1 
1.7 
— 
Decapods 
0.6 
0.5 
0.2 
Miscellaneous 
0.6 
1.2 
<0.1 
No. of nonempty stomachs 
87 
104 
42 
Figure 7 
The principle component analysis (PCA) plot of the NMFS 
summer survey collections: 1980 samples (closed triangle) and 
1998 samples (open circles). Each point represents an individual 
stomach sample. In parentheses on the axis labels are the per- 
centages of the total variance in the data explained by each 
axis. The prey species groups (large closed circles) are overlaid 
in the same ordination space according to a weighted average of 
the individual PCA scores: Deca=decapods, Eupha=euphausiids, 
Hetero=heteropods, Jelly=jellyfishes, Misc=miscellaneous. 
and turnover times of jellyfishes, 
it may have been possible for them 
to become a dominant component 
of the ecosystem during the 
period of our study, when it was 
disturbed first by El Nino and then 
again by a transition to La Nina 
(Lavaniegos and Ohman, 2003). In 
general, quantitative measures of 
jellyfish in fish diets are likely to be 
underestimates because jellyfish are 
rapidly digested and are difficult to 
identify due to the damage they 
suffer when consumed and digested 
(Arai et ah, 2003). 
Sebastes flavidus and S. entome- 
las exhibited substantial seasonal 
variations in their diets, as has 
also been reported in other stud- 
ies (Brodeur and Pearcy, 1984; Ad- 
ams, 1987). Interestingly, although the diets 
of these two species varied from season to 
season, possibly responding to changes in the 
prey field, S. pinniger continued to prey al- 
most exclusively on euphausiids during all 
six quarters of our study. This extreme and 
seasonally constant dominance of euphausiids 
agrees with the Brodeur and Pearcy (1984) 
study. Compared with S. flavidus , S. entome- 
las seems to have a greater preference for ge- 
latinous prey organisms, such as jellyfishes 
and salps, which is intriguing because these 
rockfish species tend to co-occur in the fishery 
and are considered to occupy the same habitat. 
The difference between the species does not 
appear to be an artifact of sampling. For the 
quarterly fishery collections in 1998 and 1999 
all three species were caught concurrently 
in 15 out of 49 trawl hauls. The proportions 
by weight of the major prey groups (based on 
only the data from these 15 hauls) were very 
similar to the proportions by weight (based on 
all 49 stations) (Table 3). 
The similarity between the results from the 
subset and overall data set is important in 
terms of regurgitation. Although fishes were 
thoroughly examined for signs of regurgitation, 
it was still possible that partially regurgitated 
samples were included as valid intact samples. 
If that is the case, the data in our study would 
not provide an unbiased view of the feeding 
habits of fishes. However, the similarity 
between the subset and overall set would 
suggest that the regurgitation issue, even if 
it existed, did not affect the results of stomach content 
analyses that were based on proportions. As long as 
equal proportions of prey items by species and size 
were lost through regurgitation, then it would not 
influence the resultant stomach contents analyses. 
The assumption of loss of equal proportion through 
regurgitation, however, needs to be validated by 
alternate methods, possibly by using an underwater 
trapping device to capture the fish. 
The diet analyses in our study indicate that although 
these three rockfish species may occupy similar geo- 
graphical habitat, they have evolved to occupy differ- 
