HOBSON: ACTIVITY OF HAWAIIAN REEF FISHES 



the subtle behavioral distinctions that character- 

 ize their initial moves away from shelter in the 

 evening. 



The interim is aptly termed the "quiet period," 

 because so little activity is visible to an observer. 

 The small fishes, both diurnal and nocturnal, are 

 close to the sheltering substrate at this time. The 

 term "quiet," as used here, refers to an absence 

 of observable activity, not to an absence of sound. 

 In fact, vocalizing of many animals increases 

 throughout the transition period (see, for ex- 

 ample, Cummings, Brahy, andHerrnkind, 1964) . 



During the latter part of the interim, or quiet, 

 period, a few individuals of Myripristis spp. 

 move out of their caves but stay close to the sub- 

 strate. When the members of these species sud- 



denly boil out of their caves 30 to 35 min after 

 sunset, this striking event marks an abrupt end 

 to the evening interim period. 



THE MASS EMERGENCE OF 

 NOCTURNAL FISHES 



Reoccupation of the Water Column 



The darkening water column suddenly be- 

 comes reoccupied by a horde of fishes when 

 squirrelfishes of the genus Myripristis come 

 streaming out of their diurnal retreats. The 

 timing of this spectacular event was recorded on 

 eight different evenings (Figure 5). Three spe- 

 cies prominent on the study reefs, Myripristis 

 argyromus (176: 116-210 mm), M.multiradiatus 



5 ID 15 20 25 



TIME RELATIVE TO SUNSET (min) 



Figure 4. — Some characteristics of certain apogonids 

 during evening twilight. A. Apogon snyderi: when the 

 first individual left cover on four evenings. B. Apogon 

 menesemus : when the first individual left cover on five 

 evenings. For explanation of symbols, see legend for 

 Figure 2. In addition, the broken diagonal lines rep- 

 resent estimated light values. Because of instrument 

 limitations, light readings later than about 20 min after 

 sunset were often unreliable, and therefore values be- 

 yond this point were estimated by projecting the esta- 

 blished lines. 



15 



20 25 30 35 40 



TIME RELATIVE TO SUNSET (min) 



45 



Figure 5. — Some characteristics of certain holocentrids 

 during evening twilight. A. Holocentrus sammara: 

 when the first individual left cover on four eve- 

 nings. B. Myripristis spp: when large numbers of 

 these fishes abruptly appeared in the water column 

 on eight evenings. For explanation of symbols, see 

 legends for Figures 2 and 4. In addition, broken 

 lines representing range of light values in species sym- 

 bols indicate that these were estimated for each obser- 

 vation by projecting the established line beyond 20 min 

 after sunset. 



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