520 
Fishery Bulletin 107(4) 
ent feeding guilds and they differ in their trophic 
adaptability (Dill, 1983). Sebastes flavidus and S. 
entomelas seem to be more opportunistic in their 
feeding habits and have a greater feeding plasticity 
in response to changing prey environments, where- 
as Sebastes pinniger is a specialist and has very 
limited feeding plasticity. The diverse rockfish spe- 
cies complex off Oregon seems to share some com- 
mon physical features for feeding, but each species 
still holds distinctive features. Sebastes flavidus 
and S. entomelas are characterized as having small 
mouths with low protrusibility and a relatively long 
intestine, whereas S. pinniger has a high number of 
long and slender gill rakers that may facilitate its 
specialized feeding on euphausiids (Pequeno, 1984; 
York, 2005). Although it is not easy to pinpoint 
exactly which morphological characteristics are 
responsible for differences in the diets of the ex- 
amined rockfish species, the subtle but distinctive 
dissimilarity in features, which would have been 
developed through evolutionary adaptations, could 
in part explain the differences in their diets. 
It is challenging to infer which feeding habits 
are more advantageous to these three rockfish spe- 
cies. But in an environment where food resources 
are scarce, fish species with specialized feeding 
habits would generally have a harder time find- 
ing adequate prey. It is not known whether eu- 
phausiids species are a limiting food resource for 
these rockfish species or for other fish species in 
the region. Tanasichuk (1998a, 1998b) noted that 
euphausiid populations ( E . pacifica and T. spi- 
nifera) have decreased fivefold in abundance since 
the early 1990s near Barkley Sound, a southwest 
Vancouver Island coastal embayment. It is not clear 
whether there has been a similar decrease in abun- 
dance of the euphausiid populations off Oregon during 
that time period, but it could be hypothesized that a 
similar decrease may have caused S. flavidus and S. 
entomelas to shift to other available prey resources, 
whereas S. pinniger suffered from the decreased avail- 
ability of euphausiids. These hypotheses, however, are 
difficult to test given the lack of information on the 
trends in macrozooplankton abundance in the area 
and on rockfish food habits during that time period. 
Mackas et al. (2001; 2004) reported observing sea- 
sonal and interannual changes in zooplankton biomass 
and community composition off Oregon and southern 
Vancouver Island, and reasoned that the changes were 
responses of the zooplankton community to ocean cli- 
mate fluctuations and changing current patterns. The 
waters off California, Oregon, and Washington are 
subject to broad disturbances by El Nino events, which 
result in increased surface and near-surface water 
temperature, elevated coastal sea level, a deeper ther- 
mocline, anomalous coastal currents, reduced coastal 
upwelling, reduced nutrient concentrations and abun- 
dance of phytoplankton and zooplankton (Huyer et 
al., 2002). Keister and Peterson (2003) sampled the 
zooplankton community off the central Oregon coast 
A 
Figure 8 
Predicted values from the general linear model (GLM) fitted 
to the principle component analysis (PCA) axes scores from 
the S. flavidus diet composition data of the NMFS summer 
survey collections. (A) PCA axis 1 predicted values by year 
for each latitude zone and (B) PCA axis-2 predicted values 
by time of day for each latitude zone. 
over the time period of 1998-2000 and found that there 
were zonal and seasonal variations in the zooplankton 
community and that the 1997-98 El Nino played a sig- 
nificant role in structuring the zooplankton community. 
The effect of the 1997-98 El Nino on zooplankton was 
noted to have lasted far longer than its physical effects. 
It is clear from these studies that organisms at lower 
trophic levels in the coastal ecosystem are strongly 
influenced by long-term and short-term environmen- 
tal perturbations. Many rockfish species are bottom 
oriented but feed on pelagic macrozooplankton spe- 
cies. One would expect these rockfish to be influenced 
by changes in zooplankton populations or community 
structure because they feed heavily upon species that 
are sensitive to environmental changes. However, find- 
ings regarding the diets of the three rockfish species 
in our study indicate that rockfish species do not all 
respond in the same way to such changes. A given spe- 
cies’ response will largely depend upon its evolutionary 
traits, which govern how it can adapt to changing food 
environments. It is not known whether strong seasonal 
variations and the frequent dominance of gelatinous 
zooplanktons in the diets of S. flavidus and S. entome- 
las were just a short-term response to changes in the 
zooplankton community caused by anomalous oceanic 
events, El Nino and La Nina, or if these reflect a more 
