HOBSON and CHESS: TROPHIC INTERACTIONS 



Figure 6.-A daytime aggregation of large juvenile olive rockfish, Sebastcs serranoides. Many nocturnal fishes spend the day in 



quiet schools. 



by examination of gut contents. Of 42 specimens 

 (65-120 mm, x = 91) collected from aggregations 

 during midafternoon, 28 (67%) had empty guts, 

 and 8 (19%) contained only well-digested frag- 

 ments. Six (14%), however, contained relatively 

 fresh prey probably captured earlier that day: the 

 mysid Acanthomysis sculpta (50: 15.7: 32.5); the 

 caridean shrimps Hippolyte clarki (50: 0.5: 14.2) 

 and Euahis herdmani (17: 0.2: 5.2); the cladoceran 

 Evadne sp. (17: 4.3: 10.0); calanoid copepods (17: 1.5: 

 5.2); euphausid larvae, calyptopis stage (17: 0.5: 

 1.6); and harpacticoid copepod Porcellidium sp. A 

 (17: 0.2: 0.1). Also present were extensively 

 digested fragments of cumaceans, tanaids, eu- 

 phausids, and mysids (33: ?: 20.5) that probably had 

 been taken the night before (a judgment in- 

 fluenced by knowledge of nocturnal food habits, 

 defined below). All of the cladocerans, calanoids, 

 and euphausid larvae among this material consti- 

 tuted the entire gut contents of one 82-mm in- 

 dividual, and the contents suggest a mode of 

 feeding like that of the small juveniles above. 



Beginning about 20 min after sunset, large 

 juveniles began leaving the sites of their daytime 

 aggregations. They moved away from the kelp 

 forest, and dispersed over the adjacent field of 

 Dictyopteris. Many of them rose into the upper 

 part of the water column, but most remained 

 within 5 m of the sea floor. They remained in these 

 positions throughout the night, often assuming a 

 tail-down attitude, now and then darting a few 

 centimeters forward and snapping at objects in 

 the dark water. The few that remained in the kelp 

 forest usually hovered high in the water column 

 beneath sizeable breaks in the kelp canopy. They 

 began returning to the forest at first morning 

 light, and by 30 min before sunrise were back in 

 their daytime aggregations. 



Clearly, olive rockfish of this size feed chiefly at 

 night. This conclusion is supported by study of gut 

 contents from 72 specimens (65-157 mm, x = 85) 

 collected in this area at night-later than 4 h after 

 sunset, and before first morning light. Only two of 

 these (less than 3%) had an empty gut, a contrast 



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