EBELING and BRAY: ACTIVITY OF REEF FISHES 



the number of individuals appears reduced by 

 more than half, most fishes occupy the bottom and 

 shelter zones. 



Thus, like that of tropical reefs, the vertical 

 distribution of fishes changes markedly between 

 day and night. Planktivores that pack the mid- 

 water zone during the day virtually abandon the 

 area at night to rest on the bottom or seek shelter 

 in reef holes. The vacated mid-water space is only 

 partly reoccupied by a relatively sparse population 

 of nocturnal planktivores and a few remaining 

 generalized carnivores. The largest relative in- 

 crease of individuals occurs in the shelter zone, 

 where superabundant daytime planktivores, such 

 as the blacksmith, hide at night. With so many 

 fishes commuting extensively between the mid- 

 water and shelter zones, it is understandable that 

 the intervening suprabenthic zone shows the 

 greatest species similarity between day and night. 

 Many ambusher-type foragers are always orient- 

 ed to the bottom and change their positions rela- 

 tively little for the night shift. 



It seems likely, therefore, that at night feeding 

 on plankton decreases and most of the foraging by 

 fishes takes place over the bottom. The large- 

 mouthed demersal ambushers— various rockfishes, 

 the cabezon, and others-probably feed almost any 

 time that suitable prey are available. The rubber- 

 lip seaperch may actually feed more actively at 

 night. Nonetheless, many of the fishes that 

 wander over the bottom at night may stop feeding 

 at dusk. Most demersal surfperches remain ex- 

 posed at night, although their foreguts soon 

 empty, and the fish appear more lethargic than 

 they do during the day when they are actively 

 foraging. 



Focal points of daytime fish activity, such as the 

 productive crest of the reef and other prominent 

 landmarks, appear to lose their attractiveness at 

 night. Most aggregations disappear at dusk as 

 fishes generally disperse out over the reef bottom. 

 But unlike many tropical-reef fishes, kelp-bed 

 species do not normally move off the reef to forage 

 over the adjacent sand flats. 



Kelp-bed fishes often show considerable 

 intraspecific variation in vertical distribution and 

 feeding activity. During the day, e.g., noticeable 

 numbers of typically mid-water species invariably 

 seek shelter, while at night some individuals 

 remain in the water column. And fishes differ in 

 the intensity at which they feed during any given 

 period during the day. All this suggests that 

 certain individuals may assume and even main- 



tain distinctive habits that differ from the species 

 "norm," i.e., the behavior of a particular fish is not 

 always predictable from the general characteris- 

 tics of its species. 



Thus, in comparing the diel behavior of kelp-bed 

 fishes as a group with that of their tropical coun- 

 terparts, it becomes apparent that even though 

 both groups follow the same basic patterns, the 

 kelp-bed community is the more loosely 

 "programmed." In the kelp-bed system, for exam- 

 ple, there is less large-scale replacement of fishes 

 between discrete areas or vertical zones at dusk. 

 Here, the night shift offers no real substitute for 

 the dense aggregations of daytime planktivores or 

 demersal microcarnivores, even though these 

 fishes' invertebrate prey are active and exposed at 

 night. Perhaps the better lighted tropical waters 

 allow more specialized activities because here the 

 visually oriented fishes can better see what they 

 are doing, even by moonlight. In the kelp forest, 

 the level of fish activity decreases even during the 

 day when the water becomes very turbid, as often 

 happens with the onset of dense blooms of phyto- 

 plankton during the spring and summer. 



The kelp-bed species that belong primarily to 

 tropical families tend to show the same specialized 

 pattern of nocturnal shelter seeking as do their 

 close tropical relatives, even though the general 

 program of diel activity in the kelp forest appears 

 to be comparatively unstructured. Perhaps the 

 specialized refuge-seeking procedures of kelp-bed 

 pomacentrids and labrids are simply 

 "evolutionary holdovers" that contribute relative- 

 ly little to the present fitness of these fishes. But 

 alternatively, the "tropical derivatives" may ac- 

 tually compete more successfully against primari- 

 ly temperate species such as surfperches for 

 shelter on the reef. Even though the intensity of 

 predation at twilight and perhaps at dark may be 

 somewhat less in our temperate system than in the 

 tropics, a few ingenious and effective predators, 

 such as the Pacific electric ray, patrol the Santa 

 Barbara kelp forests throughout the night. 



ACKNOWLEDGMENTS 



We thank Edmund Hobson, Ralph Larson, and 

 Robert Warner for critically reading the manu- 

 script and offering helpful suggestions. James 

 Cook and several students, especially Larry Asa- 

 kawa, Craig Fusaro, David Laur, Gary Morris, 

 Paul Reilly, Michael Rode, and Dale Sarver, helped 

 with the diving operations. Steve Edwards and M. 



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