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Fishery Bulletin 107(4) 
we could detect reliably higher-level interactions. Also, 
this strategy was necessary because the degrees of 
freedom could become exhausted as higher interaction 
terms were added to the model. The model was first 
fitted with the full set of main effects and the most 
insignificant terms were then eliminated one at a time 
until all of the remaining variables were significant 
(P<0.05). The resulting model was then fitted with the 
selected main effects and all possible two-way interac- 
tions of those effects. The model was further refined by 
removing the most insignificant interaction terms one 
at a time until no further terms could be removed at a 
significance level of 0.05. 
Results 
Description of the diets 
The detailed stomach contents data for these species 
were summarized in terms of frequency of occurrence 
and percent by weight over each season and year in 
tables (not presented because of space limita- 
tions, but available from the authors). A general 
description of the diet of each species follows. 
5. flavidus — Quarterly fishery samples Yellowtail 
rockfish preyed upon diverse groups of pelagic 
planktonic organisms. Although important prey 
species or groups varied from season to season, in 
general euphausiids (mainly Euphausia pacifica ) 
were the dominant prey group by both occurrence 
and weight across the seasons (Fig. 2A). Various 
fish species frequently were found in the diets and 
fish comprised an important prey group, although 
the species types varied over the seasons. Juve- 
nile Pacific whiting ( Merluccius productus) were 
found in samples from the 1998 spring and 
summer collections. They were not an important 
prey item during spring (0.1% by weight, 17.2% 
by occurrence), but became the most important 
item by weight in the diets during summer (37.4% 
in quarterly fishery sampling, 32.5% in the 1998 
summer survey). Slender sole (Lyosetta exilis ) was 
the top prey item as a single species by weight 
(42.2%) during the fall of 1998. 
Some prey types dominated the diets during 
certain quarters. Jellyfish species were impor- 
tant in spring and summer of 1998. However, 
because of their soft fragile body structure and 
high digestion rate in the stomach, detailed 
species identification was not possible. Whitish 
mucus-like digested material was often encoun- 
tered in the stomachs and was believed to be 
the digested remains of either Siphonophora or 
Ctenophora. This unidentified gelatinous zoo- 
plankton prey group constituted the number 
one prey item both by weight (71.1%) and by 
occurrence (46.2%) in the diets of <S. flavidus 
in fall 1999. 
Shrimp species were moderately important by 
weight and occurrence in the fall of 1998 ( Pan - 
dalus jordani, 7.5%) and 1999 ( Sergestes similis, 
9.8%). Clione spp. occurred frequently only in the 
summer 1998 samples from the quarterly fishery 
and summer survey collections. This prey group 
was moderately important (10.7% by weight and 
27.8% by occurrence) in the 1998 summer survey 
sample but was less frequent (3.4%) and did not 
make up more than 0.1% by weight in the quar- 
terly samples. Salps were the most important 
^ 100% 
80% 
60% 
40% 
20% 
0% 
Sp/98 Su/98 F/98 W/99 Sp/99 F/99 
Sp/98 Su/98 
F/98 W/99 
Quarter 
Sp/99 F/99 
H 
Misc 
□ 
Deca 
0 
Salps 
□ 
Jelly 
□ 
Fish 
□ 
Eupha 
a 
Misc 
□ 
Deca 
0 
Hetero 
0 
Salps 
□ 
Jelly 
□ 
Fish 
□ 
Eupha 
■ Misc 
□ Deca 
□ Eupha 
Figure 2 
Quarterly variations in stomach contents (percent by weight) 
of the three rockfish species: (A) Sebastes flavidus, (B) S. ent- 
omelas, and (OS. pinniger. Prey categories contributing more 
than 1% to the total prey weight in a given quarter were defined 
as separate prey categories. Trivial prey items, contributing 
less than 1%, were grouped into the miscellaneous group. The 
labels for the x axis denote the quarter and year: Sp (spring), 
Su (summer), F (fall), W (winter), 98 (1998), and 99 (1999). 
Deca=decapods, Eupha=euphausiids, Hetero = heteropods, 
Jelly=jellyfishes, Misc=miscellaneous. 
