FISHERY BULLETIN: VOL. 76, NO. 1 



prey type, fish may, e.g., switch to plankton when 

 it is particularly dense. This implies that all 

 switch feeders may eat mostly plankton on certain 

 occasions and eat alternative prey on others. 

 There did seem to be a tendency for species to eat 

 mostly plankton during winter-spring when 

 plankton volumes are characteristically large in 

 this area (Smith 1971, 1974) or when other food 

 may be relatively scarce. Yet a fish may spend 

 more energy ingesting many plankters or tiny 

 substrate-oriented prey than a few large prey. 

 Quast (1968c:92) found it ". . . difficult to under- 

 stand how the effort required to pick caprellids 

 from kelp fronds may be rewarding to a fish as 

 large as 200 mm SL." 



Reasons for Switching 



In the simple proximate sense, a fish should 

 switch from a dwindling or less accessible type of 

 prey to an increasing or more accessible type (e.g., 

 Murdoch et al. 1975). Yet the factors that ulti- 

 mately condition fish behavior and control food 

 availability may be many and complex. Quast 

 (1968b) listed predators, hunger, breeding condi- 

 tion, water turbidity, temperature, and neighbor- 

 ing species or conspecific individuals as such 

 factors. Lowe-McConnell (1975) reviewed con- 

 siderable evidence that generalized predators in 

 tropical freshwaters eat different prey as their 

 environment changes with time, as they occupy 

 different geographic areas and habitats, or simply 

 as they become able to choose among equally 

 abundant food items in a plentiful array. There- 

 fore, we discuss dietary variation with 1) season, 

 2) geographic areas and faunal mix, 3) habitat, 

 and 4) the presence of large predators. 



Unlike wide-ranging, migratory fishes, the 

 three species are limited to the food in their im- 

 mediate environment. Tagging studies show that 

 even adults have small home ranges. Off central 

 ^California, juvenile blue rockfish move less than 

 90 m from their place of settlement unless dis- 

 turbed by severe winter storms; adults either re- 

 main as kelp-bed residents or migrate to deeper 

 water and disperse more widely (Miller and Geibel 

 1973). Similarly, some 80% of thousands of adult 

 kelp bass tagged off southern California were re- 

 covered at or near the release site (Limbaugh 

 1955; Collyer and Young 1953; Young 1963), and 

 but a small percentage had ventured as far as 8 km 

 (Young 1963). Displaced individuals of Sebastes 

 flavidus, a sibling of the olive rockfish, show re- 



268 



markable homing capabilities (Carlson and 

 Haight 1972). 



Feeding habits of kelp-bed residents vary sea- 

 sonally. All three species eat relatively more 

 plankton on emptier stomachs during the cool- 

 water seasons. Similarly, blue rockfish off central 

 California feed less during winter and more dur- 

 ing summer (Gotshall et al. 1965). Unlike Santa 

 Barbara fish, however, their feeding increases 

 during the spring upwelling season when they 

 grow rapidly eating abundant plankton, and de- 

 creases during the fall when they grow more 

 slowly eating relatively more substrate-oriented 

 prey and nekton (Miller and Geibel 1973). Like 

 Santa Barbara fish, kelp bass off San Diego feed 

 less during winter, when they are difficult to catch 

 (Limbaugh 1955; Quast 1968c). Quast (1968c) 

 concluded that feeding peaks during fall and late 

 spring may relate to reproductive cycles. Yet in 

 the present study, olive rockfish, which were 

 mostly prereproductive, show the same seasonal 

 feeding cycle as the others. Perhaps here, the sea- 

 sonal cycle of switching among prey types simply 

 reflects greater availability of larger or more eas- 

 ily accessible prey when fish are most active dur- 

 ing warmwater seasons. 



Seasonal variation in food overlap corroborates 

 this. Overlap is greatest when stomachs are fullest 

 during summer-fall, and least when stomachs are 

 least full during winter. Zaret and Rand (1971) 

 found that food overlap among sympatric Central 

 American stream fishes was greatest during the 

 food-rich wet season and least during the im- 

 poverished dry season when intraspecific competi- 

 tion was presumably greatest. Also, Lowe- 

 McConnell ( 1975) summarized evidence that diets 

 of species in large African lakes overlap most 

 when food is abundant. Yet we have no direct 

 evidence that smaller overlaps reflect greater 

 competition, because we do not know when, if ever, 

 food is limiting. 



Feeding habits vary geographically. Blue rock- 

 fish seem to differ markedly in diet, distribution, 

 and behavior between Santa Barbara and San 

 Diego. Quast ( 1968d) noted that the few blue rock- 

 fish sampled from a relatively sparse, marginally 

 distributed population off San Diego (ca. 300 

 km southeast of Santa Barbara) had eaten little. 

 This prompted him to suggest (1968d:132), "The 

 blue rockfish may be poorly adapted to the envi- 

 ronment of this region and the schools may com- 

 prise expatriate populations." Off Santa Barbara, 

 a denser population contains a larger size range of 



