FISHERY BULLETIN: VOL. 82, NO. 2 



the same prey items are equally available to all 

 species. These can be seen most clearly in the 

 divergence of the cumulative curves of the number 

 of prey species (Fig. 2). Three species (S. pinniger, 

 S. alutus, and S. crameri) tend to be steno- 

 phagous, with very few prey items represented in 

 large volumes of prey organisms. Euphausiids 

 appear to be the most sought after or available 

 prey, and other prey taxa occur in low numbers. 

 These three species show similar low food breadth 

 values. 



Sebastes flavidus and S. diploproa , on the other 

 hand, have steadily rising prey curves that con- 

 tinue to rise and approach an asymptote beyond 

 the limits of the figure. These curves are charac- 

 teristic of euryphagous predators which show high 

 overall prey diversity as well as high within- 

 stomach diversity. This high prey diversity can be 

 seen in the greater food breadth values attained 

 by these two species. Although euphausiids pre- 

 dominate in these stomachs, high abundances of 

 other prey, which may be preferred but have lower 

 abundances and availabilities than euphausiids, 

 also occur. 



The diet overlap measurements calculated here 

 may be useful in comparing how similar the food 

 habits of two species are but may be of limited 

 use when interpreted in an ecological sense. The 

 interaction of factors that affect or determine the 

 diet of a particular species is complex and may 

 include such factors as temporal and spatial dis- 

 tribution of prey, behavioral adaptations of pred- 

 ator and prey, prey detection capabilities, and 

 feeding morphologies of predators (Hyatt 1979). 

 Caution should be exercised when inferences are 

 made about possible species interactions based on 

 diet overlap measurements alone. Two species 

 may have broadly overlapping diets in terms of 

 prey composition but segregate with respect to 

 prey sizes selected, time of feeding, or habitat 

 utilization (Schoener 1974; Ross 1977; Werner 

 1979; Macpherson 1981). 



Sebastes pinniger and S. flavidus are two of the 

 most abundant rockfish species within the geo- 

 graphical confines of this study. They inhabit 

 similar depth ranges, latitudinal ranges, and 

 show broadly overlapping areas of peak abun- 

 dances according to trawl survey data (Alverson et 

 al. 1964; Richardson and Laroche 1979; Gunderson 

 and Sample 1980). Adams (1980) found that these 

 two species had the highest positive association in 

 trawl catches using presence-absence data of the 

 seven abundant species he examined. Little is 

 known, however, about their small-scale hori- 



zontal and vertical distribution. Although they 

 may occupy similar bottom habitat, S. flavidus 

 may be more pelagic (Alton 1972). 



Seasonal, geographical, and diel variations in 

 the abundance and availability of the important 

 prey of S. pinniger and S. flavidus could be a 

 major cause of the variations in the diet of these 

 species. These variations may be the result of 

 intrinsic prey population fluctuations with sea- 

 son, behavioral adaptations such as diel and 

 ontogenetic vertical migration, or may stem from 

 the prevailing oceanographic conditions either 

 concentrating, dispersing, or transporting prey 

 so that all prey are not equally available in the 

 limited time and space frame of the individual 

 predator. Current patterns alone are known to 

 vary with season, depth, and geographic area 

 (Huyer et al. 1975; Ingraham and Love 1978) and 

 may affect the availability and concentration 

 of prey. 



Quantitative estimates of the seasonal and 

 areal distributions of the total prey spectrum 

 consumed by these rockfishes are limited. Day 

 (1971) sampled macrozooplankton and micronek- 

 ton from the northern part of the range of this 

 study (lat. 46° 45 '-50° 02' N) using a 0.9 m Isaacs- 

 Kidd midwater trawl in the upper 150 m of the 

 water column during the spring and fall. He found 

 a peak in the biomass of catches at the outer edge 

 of the continental shelf. Euphausiids dominated 

 the catch at most stations, and E. pacifica and T. 

 spinifera together accounted for 90% of the total 

 abundance of all organisms collected, which is 

 similar to the abundances found in the stomachs of 

 several species examined here. Although the pro- 

 portional abundance of E. pacifica varied greatly 

 relative to T. spinifera, E. pacifica dominated the 

 catches and was most concentrated during the 

 spring when it comprised the largest proportion of 

 the stomach weights in our study. Mesopelagic 

 fishes were commonly collected in Day's sampling, 

 but mostly at the offshore stations. 



Pearcy (1972) reviewed the species composition, 

 vertical and horizontal distribution, and varia- 

 tions in abundance of the macrozooplanktonic and 

 nektonic fauna derived from 8 yr of sampling off 

 Oregon. Annual and seasonal changes in the 

 abundance and distribution of many species could 

 be correlated with changes in oceanographic con- 

 ditions. Following the cessation of upwelling in 

 fall, surface waters flow predominantly inshore 

 and northward, transporting shrimps and myc- 

 tophids onto the shelf. We found that shrimp and 

 myctophids became more important in the diets of 



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