FISHERY BULLETIN: VOL 76, NO. 1 



that for complex communities in stable environ- 

 ments, predators may crop prey populations below 

 their environmental carrying capacity. Hence, 

 only top predators must partition resources to 

 avoid competitive exclusion. Thus if adult switch 

 feeders are heavily exploited by sharks, marine 

 mammals, man, etc., or young are decimated by 

 smaller predators, the three species may have lit- 

 tle, if any, competitive effect on one another. 



Evolutionary Viewpoint 



Ultimately, the tendency to choose different 

 prey may be an evolutionary response to coexis- 

 tence with a close relative. The two rockfishes, 

 which cooccur throughout much of their ranges 

 (Phillips 1957; Quast 1968a), may have coevolved 

 their divergent food habits. Most species of 

 rockfish are spiny types that sit on the bottom 

 and/or live in deep water (Phillips 1957). How- 

 ever, the blue and olive rockfishes are members of 

 a derived group of related species that have 

 smoother, more streamlined bodies and inhabit 

 the entire water column. Extending its distribu- 

 tion from bottom to surface, the common ancestor 

 of this species group could eat plankton and sur- 

 face nekton as well as benthic prey. Such an ances- 

 tor would have the ability to hunt in open water 

 and exploit all three prey types by evolving a more 

 streamlined morphotype. Then, during the pro- 

 cess of speciation within the group, the blue and 

 olive rockfishes may have themselves diverged in 

 food habits as might be expected of two cooccuring 

 congeners (e.g., Mayr 1963; MacArthur 1972). 



Thus even if their numbers are not limited by 

 predators or other disturbances, the three super- 

 ficially similar species may coexist by partitioning 

 resources. As a more distantly related serranid, 

 the kelp bass broadly shares the food spectrum 

 with both scorpaeniform rockfishes: the plank- 

 ton-eating and browsing blue rockfish and the 

 fish-eating olive rockfish. Yet the kelp bass and 

 olive rockfish have the greater dietary overlap and 

 so tend to stay out of each others' way where both 

 are common off Santa Barbara. And if conditions 

 warrant it, kelp bass and olive rockfish can switch 

 to plankton and other tiny prey although they are 

 apparently less well adapted than blue rockfish to 

 do so. 



ACKNOWLEDGMENTS 



We thank Richard Bray, Mark Hixon, Ralph 

 270 



Larson, Robert Warner, and two anonymous re- 

 viewers for critically reading the manuscript and 

 offering a large number of helpful suggestions. 

 Norm Lammer provided invaluable technical help 

 with equipment and boating operations. Mary 

 Ankeny typed several tables. This work is a result 

 of research sponsored by NOAA, Office of Sea 

 Grant, U.S. Department of Commerce, under 

 grants no. 2-35208-6, 04-3-158-22 (Project R-FA- 

 14), and 04-6-158-44021 (R/NP-II); and by NSF 

 Grant GA 38588 and Sea Grants GH 43 and 95. 

 Supplementary funding was provided by a 

 U.C.S.B. Faculty Research Committee grant (No. 

 369) for Computer Center user services, and by the 

 Marine Science Institute, through the courtesy of 

 Director Henry Offen, for interim project support. 



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