LOVE and WESTPHAL: GROWTH AND FOOD HABITS OF OLIVE ROCKFISH 



feeding, though important in all size classes, 

 increased to a peak for fish 30.1-40.0 cm long, then 

 declined somewhat, as larger fish ate more sub- 

 strate-oriented prey. Predators in the 20.1-30.0 cm 

 size class had the smallest range of food items 

 (Table 7), eating mostly fish. Range increased for 

 larger predators, as their diets were supplemented 

 by squid and octopus. 



Table 7. — Food breadths (Bray and Ebeling 1975) based on 

 proportionate item volumes for four size classes (total length) of 

 olive rockfish from Avila, Calif. Maximum vol (%) is of the 

 dominant item (Table 5). 



Table 8. — Seasonal variation in percentage frequency of occur- 

 rence of prey types in stomachs of threesizeclasses(total length) 

 of olive rockfish from .'\vila, Calif Seasonal periods are ex- 

 plained in the text. Probabilitie.s determined by 2 x 2 contin- 

 gency G-test(Sokal and Rohlf 1969) on original frequencies. 



Feeding on substrate-oriented forms, primarily 

 gammarid amphipods and isopods, occurred occa- 

 sionally in fish of the smaller size classes, but was 

 much more frequent in larger fish, where foraging 

 on octopus was important. Octopuses are normally 

 secretive during daylight hours, but are often 

 exposed during the night (D. Behrens^). Olive 

 rockfish may feed at night on octopus — an exam- 

 ple of a fish not adapted to benthic feeding, 

 preying successfully on a bottom-dwelling form. 



Food habits showed some seasonal variation. 

 Nekton feeding significantly increased (in the 

 30.1-40.0 and 40.1-50.0 cm size classes) during the 

 upwelling period (Table 8). Predation on both 

 squid and young-of-the-year rockfish (which first 

 appear in large numbers during April and May) 

 increased during this period. Though juvenile 

 rockfish were an important food item throughout 

 the year, their importance decreased during fall 

 and winter, probably because these growing juve- 

 niles become less vulnerable. Other prey, such as 

 the northern anchovy, were of greater importance 

 during the fall oceanic and winter Davidson Cur- 

 rent periods. Tunicates and euphausids were 

 eaten in far greater amounts during the upwelling 

 season, particularly during April and May (an 

 occurrence also noted off Carmel Bay, Calif., by 

 Roberts 1979). 



Limbaugh (1955) speculated that olive rockfish 

 may replace kelp bass ecologically in central and 

 northern California, where kelp bass decline in 



®D. Behrens, Pacific Gas and Electric, Biological Laboratory. 

 P.O. Box 117. Avila. CA 93424, pers. commun. November 1980. 



abundance. The two species are similar in appear- 

 ance — having elongate, fusiform bodies, reduced 

 or (in kelp bass) absent head spines, large mouths, 

 and brownish bodies with light blotches along 

 their back. In central and northern California, 

 olive rockfish do, to a certain degree, assume 

 the lifestyle of kelp bass. Olive rockfish live over 

 high relief bottom and feed primarily on nekton, 

 as do kelp bass. However, with the exception of 

 octopus, olive rockfish rarely prey on the sub- 

 strate-oriented food items, such as shrimp, algae, 

 and hydroids, favored by kelp bass (Quast 1968b; 

 Love and Ebeling 1978). In central California, 

 olive rockfish and blue rockfish are the major 

 midwater predators over inshore reefs. Though 

 there is considerable overlap, blue rockfish feed 

 primarily on relatively slow moving or drifting 

 prey, such as tunicates, copepods, and chaetog- 

 naths (Gotshall et al. 1965; Hallacher 1977) while 

 olive rockfish concentrate on more motile forms. 

 Beginning in northern California, olive rockfish 

 give way to midwater feeding yellowtail rockfish 

 and black rockfish, S. melanops (Moulton 1977). 



ACKNOWLEDGMENTS 



We thank Alfred Ebeling, Alice Alldredge, 

 Bruce Robison, and Elmer Noble for critically 

 reading the manuscript and offering much needed 

 suggestions. 



Norm Lammer provided technical help with 

 boating operations. Numerous individuals as- 

 sisted in collecting specimens. Those most helpful 

 included Dave Behrens, Richard Bray, Craig 



543 



