BRODEUR and PEARCY: FOOD HABITS AND DIETARY OVERLAP OF SOME SEBASTES 



used for testing differences among the classes 

 within each factor. Any comparisons which ex- 

 ceeded the tabulated 0.05 \ 2 percentage caused a 

 rejection of the null hypothesis of similar diets. 



RESULTS 



General Food Habits 



The results of the stomach content analysis are 

 presented for both surveys and all five species in 

 Tables 4 through 8. Each species will be discussed 

 in detail in this section. 



Sebastes flavidus preyed on a diverse assem- 

 blage of planktonic and micronektonic prey (Table 

 4). Dominating the diet in terms of frequency of 

 occurrence (F.O.), percent by number, and, to a 

 lesser extent, percent by weight were euphausiids, 

 principally Euphausia pacifica and Thysanoessa 

 spimfera. Many species of hyperiid amphipods 

 were represented in the diet, but these were not 

 numerous and did not comprise a major portion of 

 the food on a weight basis. Decapods and cephalo- 

 pods were moderately important in stomachs 

 examined from both surveys. Copepods and larval 

 decapods occurred only in the stomachs from the 

 summer survey, while gelatinous zooplankton 

 were found only in the seasonal study, and were 

 common during late fall and winter. Fish were an 

 important component on a weight basis; they were 

 mainly mesopelagic species and juvenile stages of 

 predominantly benthic species, although many 

 adult Pacific herring, Clupea harengus pallasi, 

 and some smelts were also found. The mean 

 number of taxa and mean number of myctophids 

 per stomach were higher in fish from the seasonal 

 than those from the summer survey. 



Sebastes pinniger had a much more limited diet 

 both in number of prey species and major prey 

 categories consumed than S. flavidus (Table 5). 

 Euphausiids were again the dominant prey con- 

 sumed with proportional abundances and weights 

 exceeding 90% of the total in both surveys. Many 

 stomachs were distended with adult euphausiids 

 (>1,000 individuals). Hyperiid and gammarid 

 amphipods were common but did not appear to be 

 important components of the diet. Mesopelagic 

 fishes, including myctophids and stomiatoids, con- 

 tributed to the biomass consumed during the fall 

 and winter months of the seasonal survey. There 

 was a low number of taxa represented in each 

 stomach, especially in the summer survey. 



Because of the advanced stage of digestion of 

 most of the stomach contents (mean digestion 



score = 1.05), many taxa were not identified to 

 species in the stomachs of S. alutus, although 

 many major prey categories were represented 

 (Table 6). Euphausiids were the principal prey 

 by weight and number. Of the remaining prey 

 species, amphipods were relatively common and 

 numerous. The oceanic shrimp, Sergestes similis, 

 appeared in a significant number of stomachs and 

 may constitute an important prey item. Remains 

 of fishes were found in only a few stomachs, a 

 noteworthy difference compared with the other 

 four species examined. 



Sebastes diploproa utilized a spectrum of prey 

 items as wide as that of S. flavidus, but the 

 smaller mean size of this species is reflected in 

 generally smaller prey taken (Table 7). Euphau- 

 siids were less important, and amphipods, cope- 

 pods, and decapods were more important on a 

 numerical and percentage occurrence basis than 

 for the other species. Sergestes similis contributed 

 heavily in all respects and was found in almost 

 half the stomachs examined. The small hyperiid 

 amphipod, Vibilia propinqua, was common and 

 numerous but contributed little to the bulk of the 

 diet. The mean number of prey found per stomach 

 was second only to the seasonal number of S. 

 flavidus. 



The diet of S. crameri was characterized by very 

 few prey taxa, perhaps because only 30 stomachs 

 were examined (Table 8). Of these, one-third of the 

 stomachs were empty and only about one-third of 

 the total biomass found in these stomachs was 

 identifiable, resulting in very low mean fullness 

 and digestion scores (1.03 and 1.05, respectively). 

 This identifiable fraction was composed of equal 

 numbers of euphausiids, amphipods, and cope- 

 pods. Euphausiids contributed a greater share to 

 the total biomass, however, and completely domi- 

 nated the identifiable contents. Few prey taxa 

 were found, overall, in the stomachs of S. crameri. 



Diet Breadth and Overlap 



In order to quantify the relative food resource 

 used by the various species, niche breadth mea- 

 sures were calculated for all species. The principal 

 prey types (proportional biomasses exceeding 

 1.07c of the total biomass), and niche breadth 

 values (overall and normalized) are given in Table 

 9 for all species analyzed from the summer sur- 

 veys and for S. pinniger and S. flavidus collected 

 during the seasonal surveys. 



Sebastes flavidus utilized the greatest number 

 of prey types (R), had the widest niche breadth 



275 



