FISHERY BULLETIN; VOL. 76, NO. 1 



Resource Breadth and Overlap 



Olive rockfish from Naples Reef had the smal- 

 lest food breadth, less than half as large as 

 breadths of the others (Table 9). The Naples Reef 

 fish, which occurred at relatively low density (Ta- 

 bles 7, 10), ate mostly fish. Blue rockfish and kelp 

 bass, whose diets were much more varied (Table 

 9), supplemented their fare with plankton and 

 substrate-oriented prey. Olive rockfish from 

 One-Mile Reef extended their diet with plankton. 



The kelp bass was the most widespread species 

 both at Naples Reef and at Santa Cruz Island 

 (Table 10). Kelp bass tended to aggregate more at 

 Naples Reef, as indicated by a larger Hill's ratio 

 and smaller spatial breadth. Blue rockfish were 

 also more clumped at Naples Reef. Olive rockfish, 

 which were relatively rare at Naples, were more 

 evenly distributed there. 



In diet, the kelp bass overlapped the two 

 rockfishes more broadly than either rockfish over- 

 lapped the other (Table 11). The kelp bass and 



Table 9. — Food breadths of the three species of kelp-bed fishes 

 in the 151- to 300-inm size group (Table 1, Figure 1) from Naples 

 Reef or One-Mile Reef (olive rockfish only) off Santa Barbara, 

 Calif The text defines the breadth measure B, which is based on 

 proportionate item volumes. Sample size is the number of fish 

 examined that had food in their stomachs; S is the number of food 

 items eaten; and maximum % volume is of the dominant item 

 (Table 2). 



Table 10. — Spatial breadths of the three species of kelp-bed 

 fishes from Naples Reef or Santa Cruz Island study sites off 

 Santa Barbara, Calif The text defines the breadth measure B, 

 which is based on proportionate abundances of the species in 297 

 Naples Reef or 331 Santa Cruz Island movie strips (cinetran- 

 sects). Sample size is the total fish counted (cf. Table 7); S is the 

 number of cinetransects in which the species was observed; and 

 HR is a measure of concentration ( larger values indicate that 

 more individuals are concentrated in fewer of the S cine- 

 transects — see text). 



olive rockfish overlapped most in diet and over- 

 lapped least in space both at Naples Reef and at 

 Santa Cruz Island. 



The concordance of food and spatial breadths 

 (Tables 9, 10) indicates that the arithmetic mean 

 of food and spatial overlaps may be a realistic 

 measure of total overlap in resource use (Cody 

 1974; Pianka 1974; Bray and Ebeling 1975). This 

 is because concordance in breadths suggests that 

 diet and spatial distribution may not vary inde- 

 pendently; i.e., certain areas may be best for 

 gathering one prey type, while other areas may be 

 best for another. Total overlap does not vary 

 markedly among the three species pairs because 

 food and spatial overlaps are nearly complemen- 

 tary (Table 11). Even so, total overlap between 

 rockfishes is clearly less than that of either 

 rockfish with the kelp bass. 



Table ll. — Overlap in food and space between members of all 

 pairs of the three species of kelp-bed fishes from Naples Reef or 

 Santa Cruz Island (spatial overlap only) study sites off Santa 

 Barbara, Calif Thus food overlap, determined from dietary item 

 volumes, and total overlap pertain only to the fish from Naples 

 Reef Spatial overlap, determined from cinetransect fish counts, 

 is measured separately for Naples and Santa Cruz Island fish. 



Paired species 



Blue rockfish x 

 Kelp bass 



Blue rockfish « 

 Olive rockfish 



Kelp bass x 

 Olive rockfish 



DISCUSSION 



We first examine possible sources of sampling 

 bias and how they were minimized. Then we argue 

 that within the size range of individuals studied, 

 the three species are indeed able to switch from 

 one prey type to another, and that this ability is 

 not a universal trait of fishes in general. We dis- 

 cuss the circumstances under which the three 

 species may change their diets and why their diets 

 may vary from one place to another. Finally, we 

 discuss coexistence of the three species from an 

 evolutionary viewpoint. 



Sampling Bias 



Sport fishing activities may bias samples. Fish 

 collected from partyboats often contain anchovies 

 used as chum (Quast 1968d), and the mere pre- 



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