HOBSON: ACTIVITY OF HAWAIIAN REEF FISHES 



water column at the outer edge of the reef. Many 

 of these fishes show a migratory pattern up into 

 the shallower parts of the reef at nightfall, but 

 the pattern is obscured by the circumstance that 

 other individuals of these same species do not 

 move inshore, but instead remain throughout the 

 night at the outer edge of the reef, nestled among 

 the coral on the sea floor below their midwater 

 feeding areas. 



Many of the smaller herbivores, including the 

 surgeonfishes Zehrasoma flavescens and Cte- 

 nochaetus strigosus, as well as the parrotfish 

 Scarus sordidus, exhibit similar movements. 

 These patterns are obscured because on any giv- 

 en night relatively few individuals of these spe- 

 cies join such migrations; many others of the 

 same species do not, including some that are 

 prominent' in the milling assemblages noted 

 above. It remains unknown just what sort of 

 redistribution pattern is achieved by these move- 

 ments. 



Descent of the Plankton-Feeding Damselfishes 



Individuals of the smallest plankton-feeding 

 damselfish Chromis vanderbilti have scattered 

 among the rocks by about 15 min before sunset, 

 as noted above. During the few minutes imme- 

 diately following sunset, members of the next 

 smallest species, C. leucurus, settle individually 

 in coral crevices. On three evenings the time 

 and light levels were recorded as the last indi- 

 vidual of C. leucurus in my surroundings took 

 shelter (Figure 3). 



The larger species, including Abudefduf ab- 

 dominalis, Chromis' ov alls, C. verater, and Da- 

 scylhis albisella, continue to drop lower in the 

 water column throughout this period. Precise 

 times for the different stages of the descent are 

 not available, but all drop at a similar rate, a 

 fact probably related to their similar sizes. The 

 descent begins well before sunset, when indi- 

 viduals in 8 m of water, for example, are about 

 5 m over the sea floor. At 8 to 10 min after 

 sunset they are about 1 to 2 m over the coral, 

 and here contribute to the general activity, 

 where the milling aggregations and migrating 

 schools, described above, dominate the scene. 



Vacating the Water Column 



Throughout the time that the milling aggre- 

 gations and migrating schools swim over the 

 reef, individuals from these assemblages are de- 

 scending to cover below. Nevertheless, a sub- 

 stantial number still swim in the water above 

 the reef, along with the small groups of larger 

 plankton-feeding damselfishes, at about 10 to 

 15 min after sunset. Then, abruptly, the vast 

 majority of these fishes suddenly drop to the reef 

 below, leaving the water column essentially de- 

 serted. This is a well-defined phenomenon, and 

 on 12 evenings the time and light were recorded 

 when it occurred (Figure 3). 



Cover-Seeking of Other Diurnal Fishes 



Also going under cover at about the time that 

 the water column is vacated are the larger par- 

 rotfishes Scarus rubroviolaceus (366: 267-475 

 mm) and S. perspicillatus (348: 269-463 mm). 



100 



10 5 5 10 15 



TIME RELATIVE TO SUNSET (min) 



Figure 3. — Some characteristics of certain diurnal fishes 

 during evening twilight. A. Chromis leucurus: when 

 the last individual took cover on three evenings. B. When 

 the water column was abruptly vacated by diurnal fishes 

 on 12 evenings. For explanation of symbols, see legend 

 for Figure 2. 



721 



