HOBSON ET AL.: CREPUSCULAR AND NOCTURNAL ACTIVITIES OF CALIFORNIA FISHES 



Serranidae: Paralabrax dathratus 



The kelp bass progresses through three major 

 ontogenetic phases based on trophic relationships. 

 The first phase includes juveniles up to about 65 

 mm SL that feed primarily on zooplankton during 

 the day and pass the night sheltered amid vegeta- 

 tion. The third phase includes the largest 

 individuals — those exceeding about 165 mm 

 SL — which are increasingly piscivorous with 

 growth and may be primarily crepuscular (al- 

 though limitations in our data leave the feeding 

 chronology at this larger size in question). 



In this paper we consider individuals represent- 

 ing the second phase — subadult fish between 65 

 and 165 mm SL. Individuals of this size feed mostly 

 on crustaceans that live on or close to a substrate 

 by day and swim in the water column after dark, 

 including certain gammarid and caprellid am- 

 phipods, isopods, cumaceans, mysids, and cari- 

 deans. The subadult P. dathratus capture these 

 crustaceans mainly by day close to benthic cover. 

 Most subadult P. dathratus do not feed at night. 

 Only 42% (13 of 31, 72-163 mm SL, x = 110.3) of 

 those collected during the hour before sunrise con- 

 tained prey, whereas there was prey in 96% (51 of 

 53, 68-153 mm SL, x = 107.9) of those collected 

 during the afternoon. 



Whether or not the subadult P. dathratus feeds 

 at night, however, seems related to their location. 

 Most of them are amid rocks and vegetation at 

 night, just as during the day, and here they seem to 

 feed little, if at all. Of 16 (95-146 mm SL,x = 108.6) 

 collected in these surroundings during the hour 

 before dawn, only 1 (6%) had food in its stomach (a 

 moderately digested caridean). Nocturnal feeding 

 in subadult P. dathratus seems to occur mostly in 

 those individuals that move after dark out over 

 open sand (Figure 7) — a habitat only infrequently 

 occupied by them during the day. Of the 16 (72-148 

 mm SL, X = 97.4) collected in such places during 

 the hour before dawn 12 (75%) contained food, 

 much of it fresh. Major prey were the cumacean 

 Cyclaspis nuhila (4 mm), the gammarids Am- 

 pelisca cristata (3-4 mm) and Amphideutopus 

 oculatus (2-3 mm), and the caprellid Caprella 

 californica (6-8 mm) — all species that are active 

 on or close above the sand at night. Clearly, the 

 nocturnal move over the sand is a well established 

 feeding pattern in subadult P. dathratus. 

 Nevertheless, even under these special cir- 

 cumstances predatory success after dark seems 

 limited. Among specimens from open sand at 

 night the stomachs containing food averaged only 

 30% full, compared with 66% full for specimens 

 from a wide range of diurnal circumstances. 



FIGURE 7. — A subadult Paralabrax clathmtus , about 150 mm SL, alert on open sand at night. Some individuals of this size feed under 

 these circumstances even while larger and smaller conspecifics are inactive close to rocks and algae. Apparently the sand reflects 

 enough moonlight and starlight to permit some predominantly diurnal fishes to feed in these surroundings at night. 



