HOBSON: ACTIVITY OF HAWAIIAN REEF FISHES 



Acanthurus achilles and A. leucopareiiis, contin- 

 ue to develop for about 5 min. Zanclus canescens 

 soon joins in, swimming in groups of four to six, 

 as do various chaetodontids, most notably the 

 many species of Chaetodon. These become in- 

 creasingly active, while at the same time staying 

 close to the substrate. Pomacentrids, including 

 Pomacentrus jenkinsi and Plectroglyphidodon 

 johnstonianus, have become noticeably active 

 close to the coral and rocks. The chaetodontid 

 Centropyge potteri becomes active in the same 

 area, and joins the pomacentrids in feeding on 

 benthic organisms at this time. In this benthic 

 activity, these fishes swim among the stragglers 

 of Apogon snyderi and A. menesemus, which are 

 now gradually going- under cover. 



At about 20 min before sunrise the assembla- 

 ges that have been developing in depressions 

 among the towering corals begin to rise out of 

 the depressions and range over the reef. At 

 first they stay close to the substrate, but then, 

 in an action comparable in abruptness to their 

 vacating the water column in the evening, these 

 diurnal fishes rise 1 to 2 m above the reef. This 

 event marks the end of the morning interim 

 period. 



THE MASS EMERGENCE OF 

 DIURNAL FISHES 



Twilight Upsurge in Activity and Reoccupation 

 of the Water Column 



The sudden surge of diurnal fishes into the 

 water column was noted on nine mornings (Fig- 

 ure 8). Surgeonfishes, and to a lesser extent 

 parrotfishes, are the predominant forms here, 

 just as they are during the increased activity 

 during evening twilight. As in the evening, they 

 swim in aggregations that move back and forth 

 1 to 3 m over the reef, or in schools that move 

 with seeming purpose from one location on the 

 reef to another. 



The milling assemblages. — The assemblages 

 that develop above the reef in the morning are 

 more segregated by species than are those that 

 occur in these same locations during the evening. 

 The initial surge of activity into the water col- 



umn 10 to 15 min before sunrise involves mostly 

 medium to larger herbivorous species, including 

 surgeonfishes Acanthunis achilles, A. leucopar- 

 eiiis, and Naso lituratus. These do not show 

 any purposeful direction; rather, they mill about 

 over a defined part of the reef. Also swimming 

 in and around these assemblages are a number 

 of larger diurnal fishes that occur as individuals, 

 or groups of two or three fish: these include 

 the filefish Cantherines dumerili, the triggerfish- 

 es Sufflamen bursa and Melkhthys niger, and 

 the parrotfishes Scarv^ perspicillatv^ and S. ru- 

 broviolaceus. 



The time that the first parrotfish was seen 

 was noted on nine different mornings (Figure 

 8). This observation included all species, yet 

 S. perspicillatu^ or S. rubroviolaceu^, the two 

 largest species, were seen first in five of the nine 

 instances, despite their being by far the least 

 numerous species of the genus Scams. The 

 smaller, but much more numerous Scarus spe- 

 cies, S. sordidus, S. dubius, and S. taeniurus, gen- 

 erally appeared later. During the time that the 

 smaller parrotfishes are appearing, the remnants 

 of numerous mucous envelopes, in which many 

 of them had spent the night (see, for example, 

 Winn, 1955; Casimir, 1971), drift about close 

 to the substrate. 



The twilight migrations. — Just as they do 

 during evening twilight, many species migrate 

 from one location on the reef to another at this 

 time. As in the evening, they travel in schools 

 that frequently are drawn out as long proces- 

 sions. Again, specific routes are followed morn- 

 ing after morning, indicating that well-defined 

 patterns exist. Although these patterns remain 

 obscure for most participants, some individuals 

 clearly reverse their evening direction. Thus, 

 having spent the night sheltered on the shallower 

 parts of the reef, many individuals of the plank- 

 ton-feeding surgeonfish Naso hexacanthus, along 

 with many of the damselfish Chromis verater, 

 return to their deeper water feeding grounds 

 high above the oflFshore edge of the reef. As 

 mentioned above, this pattern is obscured by the 

 many other individuals of these species that do 

 not migrate inshore at nightfall, but instead 

 spend the night sheltered on the deeper parts of 



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