FOOD HABITS AND DIETARY OVERLAP OF 



SOME SHELF ROCKFISHES (GENUS SEBASTES) FROM 



THE NORTHEASTERN PACIFIC OCEAN 



Richard D. Brodeur and William G. Pearcy 1 



ABSTRACT 



Euphausiids were the major food of five co-occurring species of rockfishes I Sebastes spp. ) along the west 

 coast of North America from Vancouver Island to northern California. Copepods, decapods, cephalo- 

 pods, amphipods, fishes, and other pelagic prey were also consumed but were less important to the 

 overall diet. Two species, S. flavidus and S. diploproa, were relatively euryphagous, utilizing a high 

 number of prey taxa. The other species, S. pinniger, S. alutus, and S. crameri, had a more restricted 

 diet comprised mostly of euphausiids. The numerical composition of prey in the diet of all species was 

 similar due to the preponderance of the two dominant euphausiid species. Diet overlaps based on 

 weight composition were high for S. pinniger, S. diploproa, and S. alutus but were moderate for most 

 comparisons involving S. flavidus and S. crameri. 



The diets of S. flavidus and S. pinniger were examined in more detail to explain some of the vari- 

 ability associated with their food habits. Both species exhibited peak feeding periods at the same time 

 during the day. They consumed about the same mean size of prey, although S. flavidus consumed a 

 wider size range of prey. Size of prey and dietary composition did not vary much with size offish. There 

 were significant seasonal, geographical, and diel differences in food composition for both species, which 

 may be a function of varying food availability. 



Factors that allow coexistence of a large number of 

 morphologically similar species have been the 

 focus of numerous studies and continued debate 

 in the ecological literature. Competition and re- 

 source partitioning have been reviewed in general 

 by Schoener (1974), and for fishes by Helfman 

 (1978). Potential competition for resources is 

 thought to be most common in three aspects of the 

 ecological niche in fish communities: habitat, 

 food, and time of activity (Tyler 1972; Bray and 

 Ebeling 1975; Ross 1977; Werner 1979; Larson 

 1980; McPhersonl981). 



Rockfishes (Sebastes spp.) of the family Scor- 

 paenidae are, a priori, interesting subjects for 

 examining the various modes of resource parti- 

 tioning. This genus is extremely speciose, with 

 about 100 species reported from the North Pacific 

 Ocean. At least 69 of these species are known to 

 occur in the eastern North Pacific (Chen 1975). In 

 addition to the large number of species, rockfishes 

 also exhibit a high degree of overlap in their 

 geographical distributions, with as many as 50 

 species occurring in a narrow latitudinal band 

 (lat. 34°-38°N) off central California (Chen 1971). 

 Several of these congeners are morphologically 



'School of Oceanography, Oregon State University, Marine 

 Science Center, Newport, OR 97365. 



similar and occupy similar habitats, so the poten- 

 tial for resource overlap and competition is high 

 (Larson 1980). 



Many of these species are abundant enough and 

 of sufficient size to contribute substantially to 

 commercial trawl landings in the northeastern 

 Pacific (Alverson et al. 1964; Alton 1972; Gabriel 

 and Tyler 1980; Gunderson and Sample 1980). De- 

 spite their abundance in the northeastern Pacific, 

 relatively few quantitative studies exist on rock- 

 fish feeding habits. Most of the studies to date 

 have dealt with shallow-water, neritic species 

 often taken in recreational fisheries or accessible 

 to in situ observations and sampling by scuba 

 divers (Gotshall et al. 1965; Larson 1972; Hobson 

 and Chess 1976; Love and Ebeling 1978). Descrip- 

 tions of the diet of offshore species of Sebastes 

 generally either lack taxonomic or quantitative 

 detail (Phillips 1964) or encompass limited geo- 

 graphical area or collection times (Pereyra et al. 

 1969; Lorz et al. 1983). Skalkin (1964) and Somer- 

 ton et al. (1978) 2 described food habits of rock- 

 fishes from the Bering Sea and Gulf of Alaska, far 



Manuscript accepted October 1983. 

 FISHERY BULLETIN: VOL. 82, NO. 2, 1984. 



2 Somerton, D., F. Funk, K. Mesmer, L. J. Bledsoe, and K. 

 Thornburgh. 1978. A comparative study of the diets of Pacific 

 ocean perch [Sebastes alutus) and walleye pollock [Theragra 

 chalcogramma ) in the Gulf of Alaska. NORFISH Tech. Rep. 

 NPB8, Wash. Sea Grant, 25 p. 



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