FISHERY BULLETIN: VOL. 70, NO. 3 



0,01 



001 - 



I 



00001 - 



0.00001 



50 45 40 35 30 25 



TIME RELATIVE TO SUNRISE (min) 



20 



Figure 7. — Some characteristics of certain nocturnal 

 fishes during morning twilight. A. Priacanthiis cruen- 

 tatus: when a certain school returned to the reef on 

 six mornings. B. Myripristis spp. when the last indi- 

 vidual descended to the reef on eight mornings. For 

 explanation of symbols, see legends for Figures 2, 4, and 

 5, except that this figure represents morning events. 



species of Holocentrus do not go under cover 

 at about the same time, as is true for Myripristis 

 spp. Instead, their numbers gradually diminish 

 over a period of about 20 min during early twi- 

 light, approximately 50 to 30 min before sunrise. 

 By the time holocentrids of the genus Myripristis 

 have gone under cover, about 30 min before sun- 

 rise, the only nocturnal fishes consistently in 

 view are scattered individuals of Holocentrus 

 samTYiara, Apogon menesemus, and A. snyderi. 

 At this point the morning interim period has 

 begun. 



THE MORNING INTERIM PERIOD 



As the midwaters are being vacated by the 

 species of Myripristis, it is getting light enough 

 for the human observer to visually distinguish 

 details of the reef around him. It is about 30 

 min before sunrise, and relatively few fishes are 

 seen. Scattered individuals of several nocturnal 

 species, especially Holocentrus sammara, Apo- 

 gon menesemus, and A. snyderi, still hover in 



view close among the coral but are not active. 

 Among diurnal fishes, a number of chaetodon- 

 tids, many paired, move about close among the 

 coral; the nocturnal colorations of many are 

 just now giving way to their diurnal hues. De- 

 spite these active fishes, the overall aspect on 

 the reef is quiet. Thus, the morning interim, 

 like the evening interim, is termed the "quiet 

 period." 



About 5 min into the interim, or quiet, period, 

 when sunrise is still about 25 min away, and 

 when the last Holocentrus sammara is now under 

 shelter, the first strong indications of increasing 

 activity in diurnal fishes is seen. At this time 

 the large and medium-sized surgeonfishes begin 

 to assemble in depressions between the towering 

 heads of the coral Porites. The timing of this 

 event was recorded on six different mornings 

 (Figure 8). These assemblages, which include 



30 25 20 15 10 5 



TIME RELATIVE TO SUNRISE (min) 



Figure 8. — Some characteristics of certain diurnal fishes 

 during morning twilight. A. When species of acan- 

 thurids began to assemble in reef depressions on six 

 mornings. B. When diurnal fishes first surged into the 

 water column on nine mornings. C. When the first 

 scarid appeared away from cover on nine mornings. 

 For explanation of symbols, see legends for Figures 2, 

 4, and 5, except that this figure represents morning 

 events. 



726 



