HOBSON: ACTIVITY OF HAWAIIAN REEF FISHES 



TRANSITION FROM NIGHT TO DAY 



As is true of the transition from day to night, 

 it is difficult to determine exactly when the night- 

 time situation begins to give away to that pre- 

 vailing during daylight. In large part, this dif- 

 ficulty reflects the great variation in activity 

 among fishes at night relative to the amount of 

 moonlight. The situation throughout the night, 

 including variations under diflFerent levels of 

 moonlight, is described elsewhere (Hobson, in 

 preparation) . This report is concerned with the 

 more striking transformations that occur during 

 the transition from darkness to daylight, begin- 

 ning about 45 min before sunrise and lasting 

 until about 15 min after sunrise — a total of about 

 1 hr. As is true of the evening transition, de- 

 scribed above, three major periods are readily 

 recognized: 1) the cover-seeking of the noc- 

 turnal fishes; 2) the morning interim period; 

 and 3) the mass emergence of diurnal species. 



COVER-SEEKING OF THE 

 NOCTURNAL FISHES 



Return of the Offshore Feeders 



When there is no moonlight, an underwater 

 observer notices the first trace of sunlight on the 

 waters surface overhead about 1 hr before sun- 

 rise. Nevertheless, not for another 10 to 15 min 

 do the fishes display overt signs that the tran- 

 sition to their daytime behavior is underway. 

 Then, about 40 to 50 min before sunrise, the 

 numbers of Myripristis spp. and Priacanthiia 

 cruentatus increase sharply near the caves where 

 they pass the daylight hours. To some extent 

 this is the assembling here of those individuals 

 that had remained on the reef during the night, 

 but mostly this marks the return to the reef of 

 individuals that had migrated elsewhere. 



The species of Myripristis become simultane- 

 ously so abundant everyw^here that it is uncer- 

 tain to what extent they have arrived together in 

 schools or have converged simultaneously as sol- 

 itary individuals onto the reef. The behavior 

 of P. cruentatus is clearer. Being far less 

 abundant than Myripristis, individuals of P. cru- 

 entatus are seen arriving in discrete schools, 

 swimming high above the reef. These schools 



were especially apparent when an arrival point 

 for the species was discovered at Honaunau, at 

 a location near a coral cave midway down a steep 

 slope. The crest of the slope is in water 12 m 

 deep, the base is about 35 m deep, and the cave 

 is at a depth of about 20 m. On six mornings, 

 I witnessed the arrival here of a large school 

 of P. cruentatus (Figure 7). Although a few 

 early arrivals had already been in the area for 

 as long as 10 min, the sudden appearance of 

 the school, containing 30 to 50 fish, was a readily 

 recognized event. After arriving together, many 

 of these fish remained as a unit, milling close 

 to the substrate just outside the entrance to the 

 cave; however, many others of the group dis- 

 persed along the face of the slope, where much 

 cover is available under overhanging coral 

 growths. After about 10 min, during which 

 these fish hovered close to the coral, all gradu- 

 ally took shelter: the majority entered the cave, 

 whereas others, either as individuals or groups 

 of a few fish, found cover in the varied crevices 

 and smaller caves along the face of the slope. 

 On the two occasions when the time was noted, 

 the last of these fish were seen entering shelter 

 at 29 and 32 min before sunrise, respectively. 

 After the initial upsurge in their numbers on 

 the reef, some Myinpristis go under cover right 

 away, but most continue to swim above the coral 

 for 10 min or so. On eight mornings I noted 

 when the last of these had entered their caves 

 (Figure 7). 



Cover-Seeking of Other Nocturnal Fishes 



About the time that Priacanthus cruentatus 

 and Myripristis spp. are reappearing on the reef 

 in large numbers, members of at least most of 

 the other, less conspicuous nocturnal species are 

 still active close to the substrate. It remains un- 

 certain just when, in response to the approach- 

 ing dawn, many nocturnal fishes cease to range 

 into exposed positions. Holotrachys lima, Den- 

 drochirus brachypterus, Pterois sphex, Scorpae- 

 na coniorta, Brotula multiharhata, and others 

 are among the species that are seen only irreg- 

 ularly during the night, partly because they are 

 not especially numerous. The behavior of the 

 more numerous squirrelfishes of the genus Ho- 

 locentrus is less obscure. Individuals of any one 



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