Lee and Sampson: Dietary variations for three rockfish species off the Pacific Northwest 
515 
prey species both by weight (61.5%) and occurrence 
(80%) in the spring 1999 samples. The numerous plank- 
tonic amphipod species were frequently observed in all 
seasonal quarters except during the winter of 1999, but 
there were of minor importance in terms of weight. 
S. entomelas — Quarterly fishery samples The prey 
groups eaten by widow rockfish were as varied as those 
eaten by S. flavidus. However, the importance of the fish 
prey species group in the overall diet was less than it 
was for S. flavidus (Fig. 2B). Juvenile Pacific whiting 
were found in the diets during the spring and summer 
of 1998, but were not an important prey in either quar- 
ter (<0.7% in weight, <5.4% occurrence). Euphausiids 
(mainly E. pacifica and Thysanoessa spinifera) and 
jellyfishes were major prey items by both weight and 
occurrence in most quarters. Salps were prey in all 
seasons and became the most important prey in 1999. 
In particular, salps were found in more than 95% of 
the stomach samples collected during the spring of 
1999 and were an exceptional prey source in terms of 
weight (92.6%). This may account for the absence of 
euphausiid species in the diets during that season. The 
heteropods was the highest prey group both in terms 
of weight (45.3%) and frequency of occurrence (45.8%) 
in the fall of 1998. 
S. pinniger — Quarterly fishery samples The number of 
prey taxa identified in the stomachs of S. pinniger was 
substantially fewer than those in the stomachs of S. 
flavidus and S. entomelas: 22, compared to 45 and 44, 
respectively. Euphausiid species (mainly E. pacifica and 
T. spinifera ) were the most dominant (>95%) prey group 
by weight in the diets over all quarters (Fig. 2C). In 
some quarters, decapods (shrimps) occurred frequently 
(around 20%) but with little contribution in weight. 
Salps did not appear in the diets even during quarters 
when salps were a major prey item for S. flavidus and 
S. entomelas. Nonempty stomachs were not present in 
the fall quarter samples of 1999, when few stomachs of 
S. pinniger were collected (n=8). 
There were considerable quarterly variations in the 
prey groups by weight in the diets of S. flavidus (Fig. 
2A) and S. entomelas (Fig. 2B). The dominance and 
the degree of contribution of the prey groups changed 
from quarter to quarter for these two fish species. In 
contrast, over all the quarters studied S. pinniger (Fig. 
2C) maintained a diet consisting almost completely of 
euphausiids (>96.1%) but the sample sizes for this spe- 
cies were quite small. 
S. flavidus — NMFS summer surveys in 1980 and 1998 
The major differences between the diets of S. flavidus 
in 1980 and 1998 were the occurrence of unusual 
prey species of southern origin and the dominance of 
gelatinous zooplankton species in the 1998 samples. 
The southern prey species were frequently found in 
1998 samples, Nyctiphanes simplex (euphausiid) and 
juvenile M. productus, with occurrences of 2.2% and 
36.8%. These prey species, which are thought to be a 
major signature of El Nino in the diets of S. flavidus, 
were not found at all in the stomach samples from 1980. 
Merluccius productus was the single most important 
prey species by weight (32.5%) in 1998, whereas E. 
pacifica was most important (26.4%) in 1980. Fishes 
were important prey items in 1980. The major species 
present was herring ( Clupea harengus pallasi, 18.4%), 
which was not observed in the 1998 stomach samples. 
Various gelatinous zooplankton species frequently 
occurred in the stomach samples, composing the second 
most important prey group by weight (33.3%) after the 
fish group (41.4%). The gelatinous zooplankton species 
found in the 1980 stomach samples were Sagitta elegans 
and Limacina helicina, and they made a minimal 
contribution with less than 0.1% by weight. 
Diet comparison based on major prey groups 
The weight proportions (average ratio) of the seven prey 
groups, calculated across the six quarters, demonstrated 
overall differences in the diets among the three rockfish 
species during the study period (Fig. 3). Euphausiids 
were important for all three species, but especially for 
S. pinniger (98.1%). Salps were the most important item 
for both S. flavidus (35.3 %) and S. entomelas (49.7%). 
S. entomelas had a tendency to prey more on jellyfishes 
(25 %), while fishes were a major item for S. flavidus 
(30.2 %). 
The major difference between the NMFS summer 
survey samples for S. flavidus in 1998 and 1980 was 
the role of jellyfishes (22.1%) in 1998 as a major prey 
group. It was the second most important prey group 
by weight after the fish group (41.4%) (Fig. 4A). In 
1980, jellyfishes did not even represent a major prey 
group (Fig. 4B). When the 1998 NMFS summer survey 
samples for the diets of S. flavidus are compared with 
the 1998 summer samples from the quarterly fishery 
sampling, the more geographically restricted quarterly 
fishery samples show less feeding on jellyfish (7.3%) 
(Fig. 2A). The importance of euphausiids by weight 
was less distinctive in 1998 (20.2%) than in 1980 
(47%). It appears that S. flavidus depended more on 
euphausiids in 1980 than in 1998. The importance of 
the fish prey group in 1998 (41.1 %) was more than 
the fish prey group in 1980 (32.8%) because of the 
major contribution of M. productus in 1998. 
Diet variability in relation to extrinsic factors 
The PCA for the quarterly fishery samples was success- 
ful in accounting for 73.6% of the total variability in 
the prey composition data, with the first two PCA axes 
explaining 43.5% and 30.1%, respectively. Similarly, the 
percentage of the total variability in the summer survey 
data accounted for by the first two PCA axes was 66.2%: 
41.4% by axis 1 and 24.8% by axis 2. The amounts of 
variability explained by those PCA axes were sufficient 
to assure that the PCA components would adequately 
represent the food habits of the fish species based on 
individual stomach content information. 
