FOOD HABITS AND DIETARY OVERLAP OF SOME SHELF ROCKFISHES (GENUS SEBASTES) FROM THE NORTHEASTERN PACIFIC OCEAN

Euphausiids were the major food offive co-occurring species ofrockfishes (Sebastes spp.l along the west coast of North America from Vancouver Island to northern California. Copepods. decapods. cephalopods. amphipods. fishes. and other pelagic prey were also consumed but were less important to the overall diet. Two species. S. {lavidus 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. alutlls but were moderate for most comparisons involving S. {lavidus and S. crameri. The diets of S. {lavidus and S. pinniger were examined in more detail to explain some of the variability 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. {lavidus consumed a wider size range of prey. Size ofprey and dietary composition did not vary much with size offish. There were significant seasonal. geographical, and die\ differences in food composition for both species. which may be a function of varying food availability. Factors that allow coexistence ofa large number of morphologically similar species have been the focus of numerous studies and continued debate in the ecological literature. Competition and resource partitioning have been reviewed in general by Schoener <1974>, and for fishes by Helfman <1978l. 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; McPherson 1981l. Rockfishes (Sebastes spp. >of the family Scorpaenidae are, a priori, interesting subjects for examining the various modes of resource partitioning. 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 1975l. 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 (Jat. 34°-38°N) off central California (Chen 1971). Several of these congeners are morphnlogically 'School of Oceanography. Oregon State University. Marine Science Center, Newport. OR 97365. Manuscript accepted October 1983. FISHERY BULLETIN: VOL. 82. NO.2. 1984. similar and occupy similar habitats, so the potential 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 a1. 1964; Alton 1972: Gabriel and Tyler 1980; Gunderson and Sample 198m. Despite their abundance in the northeastern Pacific, relatively few quantitative studies exist on rockfish 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 a1. 1965: Larson 1972: Hobson and Chess 1976: Love and Ebeling 1978). Descriptions of the diet of offshore species of Sebastes generally either lack taxonomic or quantitative detail (Phillips 1964> or encompass limited geographical area or collection times (Pereyra et a1. 1969; Lorz et a1. 1983>. Skalkin (1964) and Somerton et a1. (1978)2 described food habits of rockfishes from the Bering Sea and GulfofAlaska, far ·Somerton. D. , F. Funk, K. Mesmer. L. J. Bledsoe. and K. Thornburgh. 1978. A comparative study of the diets of Pacific ocean perch ISebastes aiutusl and walleye pollock (Theragra chalcogramma) in the Gulf of Alaska. NORFISH Tech. Rep. NPB8. Wash. Sea Grant. 25 p.