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



Scorpaenidae: Sebastes serranoides 



The olive rockfish progresses through stages 

 characterized by distinctive trophic relationships 

 (as described by Hobson and Chess 1976). These 

 stages parallel those described above for the ser- 

 ranid Paralabrax clathratus, although the noctur- 

 nal element in the subadults is much stronger in 

 S. serranoides . 



Juveniles appear inshore w^hen they are about 

 30 mm SL, and feed by day on zooplankton until 

 they have grown to about 55-65 mm. When about 

 55 mm SL they begin to feed at night, a transition 

 most of them complete by the time they are 65 mm 

 SL. These nocturnal feeders, which are our major 

 concern here, have assumed a pattern of activity 

 that will characterize them until they are about 

 120-150 mm SL. By day they hover in relatively 

 inactive schools, often at the edge of kelp forests, 

 and during evening tvdlight they disperse over the 

 surrounding area, where they forage in the water 

 column. Many of them move away from the shel- 

 tering kelp to hover solitarily in open water. On 

 the five evenings that the event was noted, the first 

 one seen in a nocturnal mode away from cover was 

 noted 16-30, x = 25.8, min after sunset. They re- 

 main 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. 



When these fish shift to nocturnal foraging they 

 also change their diet. From the copepods and 

 other small zooplankton (about 1-2 mm long) they 

 had taken during their earlier diurnal phase, they 

 now capture larger organisms that swim in the 

 nearshore water column only after dark (Hobson 

 and Chess 1976). These nocturnal food habits were 

 defined by study of gut contents. Of 72 individuals 

 (65-157 mm SL, x = 85) collected more than 4 h 

 after sunset, 70 (97%) were full of prey, many of 

 them fresh. Major prey were the gammarid Batea 

 transversa (2-5 mm), the mysid Siriella pacifica 

 (3-9 mm), the cumacean Cyclaspis nubila (4-6 

 mm), the caprellid Caprella pilidigita (5-11 mm), 

 and the isopod Paracercies cor data (3-6 mm). 



Scorpaenidae: Sebastes serriceps 



The treefish rests in rocky crevices at all hours 

 of day and night, and evidence of specific feeding 

 times is limited; nevertheless, it appears to be 

 predominantly nocturnal or crepuscular. Of six 

 individuals (138-227 mm SL, x = 189.5) collected 



during the hour before dawm, all contained rela- 

 tively large crustaceans, including the shrimps 

 Lysmata californica (12-70 mm), Alpheus bel- 

 limanus (60 mm), and Spirontocaris sp. (25 mm). 

 Greatly reduced feeding during the day was evi- 

 denced by 22 individuals (122-330 mm SL, x = 

 276.1) collected during the afternoon from among 

 rocks; of these, the guts of 13 (59%) were empty, 

 and the other 9 contained material that was for the 

 most part extensively digested — seven had each 

 taken a single Chromis punctipinnis (68-135 mm 

 SL), while other prey were the crab Paraxanthias 

 taylori (16-38 mm wide), and the carideans Lys- 

 mata californica (38-45 mm) and Herbstia parvif- 

 rons (10 mm). Most of these prey had been in the 

 guts for some time: only the two crabs appeared 

 recently ingested. We suspect that all but the two 

 crabs were captured during the previous night or 

 morning twilight. We would expect the crusta- 

 ceans to have been more accessible after dark, 

 when we have seen the species in exposed loca- 

 tions. Further, the major prey in these collections, 

 C. punctipinnis , generally swims high in the water 

 column during the day, and would seem ill-suited 

 then as prey of this bottom-dwelling rockfish. 

 Chromis punctipinnis would be more vulnerable 

 at night, when it settles to shelter on the seafloor 

 (see account of C. punctipinnis above). However, 

 the absence of C. punctipinnis in the guts of 

 Sebastes serriceps taken before dawn indicates 

 that the prey in these samples may have been 

 captured during morning twilight, perhaps as 

 they left cover for their ascent into the midwaters. 



Haemulidae: Xenistius californiensis 



The salema assembles in relatively inactive 

 schools during the day, then disperses at nightfall 

 and forages in the water column at night (Hobson 

 and Chess 1976). This diel activity pattern is wide- 

 spread among the many species of haemulids in 

 tropical waters (Starck and Davis 1966, in the 

 tropical Atlantic, and Hobson 1968a, in the Gulf of 

 California). Frequently. Z. californiensis schools 

 in the upper levels of the water column within the 

 kelp forests, and during twilight, moves consider- 

 able distances to nocturnal feeding grounds. One 

 location at Santa Catalina Island, knovm to be a 

 nocturnal feeding ground for this species, was 

 >400 m from the nearest point where the species 

 had been seen during the day (Hobson and Chess 

 1976). On four evenings we noted when the first 

 salema arrived on this feeding ground, and this 



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