HOBSON and CHESS: TROPHIC INTERACTIONS 



sea floor, with larger individuals mostly in the 

 lower regions. Far fewer numbers also occurred 

 close above low fields of benthic algae some dis- 

 tance from the kelp forest. It assumed similar 

 attitudes in the same places during both day and 

 night, but after dark there seemed to be more of 

 them in the mid-waters along the outer edge of the 

 kelp. 



Most kelp perch feed by plucking material from 

 the surface of algae, but plankton-feeding is 

 widespread, especially among those aggregated in 

 the mid-waters at the edges of the forests. Lim- 

 baugh (1955) reported that the kelp perch feeds on 

 small crustaceans, particularly those that occur on 

 giant kelp. Quast (1968) also reported a predomi- 

 nantly crustacean diet, with a preponderance of 

 amphipods, but also including mollusks and 

 bryozoans. 



Preliminary assessment of our food-habit data, 

 along with direct observations, showed that in this 

 species it is primarily the smaller individuals that 

 feed on plankton. Consequently, we consider for 

 this paper only those less than 100 mm long, 

 leaving the larger individuals for a later paper. 

 This point is drawn somewhat arbitrarily, al- 

 though plankters generally become noticeably less 

 prevalent in the diet at about this size. With kelp 

 perch more so than with the other species treated 

 in this paper, however, many of the individuals 

 considered had mixed a diet of plankters with 

 organisms plucked from a substrate. Bray and 

 Ebeling (1975) reported that kelp perch feed 

 mainly on tiny plankters, mostly copepods, based 

 on a sample of predominantly small individuals 

 (43-142 mm, ,r = 103). 



All 35 specimens (40-99 mm .f = 81) collected 

 during the afternoon as they swam over various 

 locations in the study area, usually close to kelp, 

 contained food, much of it fresh. On the other 

 hand, of 34 specimens (38-99 mm, .r = 76) collected 

 during the 2 h of night before first morning light 

 25 (74%) were empty. The other nine, however, 

 contained food, including relatively fresh items. 

 Thus, although the kelp perch within this size 

 range clearly fed mostly by day, some apparently 

 fed at night. Individuals evidencing nocturnal 

 feeding ranged from 81 to 99 {x = 95) mm long, 

 and so were among the larger ones in the sample. 

 Recognizing that the contrasting conditions 

 between day and night undoubtedly influenced the 

 composition of the diet, food data from individuals 

 collected during the afternoon (when presumably 



most fresh items in the gut had been taken by day) 

 were considered separately from food data from 

 individuals collected during the last hours of the 

 night (when presumably most fresh items in the 

 gut had been taken after dark). 



In addition to the high incidence of empty guts 

 in kelp perch collected at night, the guts of those 

 that had taken prey after dark averaged 50% full, 

 compared with an average of 72% full for the day 

 feeders. Furthermore, the night feeders contained 

 an average of 38 prey items, compared with an 

 average of 252 for the day feeders (at least in part, 

 however, this difference reflects the larger size of 

 nocturnal prey). These data strengthen our con- 

 clusion that over the size range studied, nocturnal 

 feeding is relatively unimportant to this species. 

 Bray and Ebeling (1975) also noted that kelp perch 

 feed mainly by day. 



Foods taken by individuals that had been feed- 

 ing during the day are ranked below: 



1. CALANOID AND CYCLOPOID COPEPODS (94: 157.7: 49.1) 



calanoids, including Calannn pacijicax, and Rhincalanus 

 nasutus (71: 137.2: 44.2); cyclopoids, including Corycaeus sp. 

 and Oticea sp. (74: 20.5: 4.9). 



2. GAMMARIDEAN AMPHIPODS (63: 57.9: 37.0) 



Microjassa litodes (46: 23.1: 15.1); Ericthonias braziliensis 

 (14: 3.9: 2.6); Gitanopsifi vilordes (11: 3.0: O.l); Ampithoe spp. 

 (3: 0.1: 0.2); Hyale nigra (3: 0.3: 0.2); Batea transversa (3: 0.1: 

 0.1); unidentified (63: 27.4: 18.7). 



3. CLADOCERANS(37:26.1:6.9) 



Evadne sp. 



4. CIRRIPEDIAN LARVAE (31: 1.8: 0.9) 



cvpris stage. 



5. POLYCHAETES, NONSWIMMING (11: 0.4: 1.9) 



Spirorbis sp. (9: 0.4: 1.8); unidentified (3: 0.1: 0.1). 



6. HARPACTICOID COPEPODS (14: 1.6: 0.7) 



Porcellkiium sp. A (11: 1.5: 0.6); Porcellidium sp. B (3: 0.1: 

 0.1). 



7. OSTRACODS (26: 0.6: 0.6) 



Cythereis sp. (17: 0.3: 0.2); Philomedes sp. B (11: 0.2: 0.1); 

 unidentified sp. C (3: 0.1: 0.3). 



8. CAPRELLID AMPHIPODS (9: 0.3: 0.7) 



Caprella pUidigita (6: 0.2: 0.4); C. califurnica (3: <0.1: 0.3). 



9. FISH EGGS (14: 0.4: 0.3) 



unidentified. 



10. PELECYPODS(11:0.4:0.3) 



Hiatella arctica (9: 0.3: 0.3); Halodakra brunnea (3: <0.1: 

 <0.1). 



11. ISOPODS(14:0.7:0.2) 



Paracercies sp. (6: 0.5: 0.1); gnathiid juveniles (6: 0.1: <0.1); 

 unidentified fragments (3: 0.1: 0.1). 



12. BRYOZOAN LARVAE (9: 0.1: 0.2) 



cyphonautes. 



13. CARIDEAN LARVAE (9: 0.2: <0.1) 



unidentified. 



14. FISHES (6: 0.2: <0.1) 



unidentified larvae. 



589 



