HOBSON: ACTIVITY OF HAWAIIAN REEF FISHES 



urnal herbivores and plankton feeders, including 

 certain acanthurids, scarids, and pomacentrids. 

 C) Many small plankton-feeding species make 

 short, but well-defined vertical migrations be- 

 tween shelter locations on the reef and feeding 

 locations in the water column directly above; 

 diurnal species in this category include certain 

 pomacentrids and chaetodontids, and nocturnal 

 species include certain apogonids and holocen- 

 trids. 



4. Activity associated with the transitions 

 from day to night, and from night to day, pro- 

 ceeds in an established, well-defined sequence. 

 Both evening and morning periods of changeover 

 comprise three distinct segments. In the eve- 

 ning, these are: 1) the shelter-seeking of the 

 diurnal fishes (from about 30 min before sunset 

 to about 13 min after sunset) , which ends when 

 the last group of diurnal fishes still milling above 

 the reef, mostly acanthurids, abruptly descend 

 to cover; 2) the evening interim, or "quiet," 

 period (beginning about 13 min after sunset 

 and ending about 20 min later) , which is char- 

 acterized by the water column being essentially 

 deserted of fishes; and 3) the emergence of the 

 nocturnal fishes (from about 33 to 50 min after 

 sunset) , which begins when the surge of noc- 

 turnal fishes, mostly Myripristis spp., rise into 

 the water column. In the morning, the sequence 

 is reversed, being: 1) the shelter-seeking of the 

 nocturnal fishes (from about 50 to 30 min before 

 sunrise), which ends when the last of the noc- 

 turnal fishes in the water column, usually My- 

 ripristis spp., have descended to cover; 2) the 

 morning interim, or "quiet," period (beginning 

 about 33 min before sunrise and ending about 

 20 min later) , when the water column is essen- 

 tially deserted; and 3) the emergence of the di- 

 urnal fishes (from about 13 min before sunrise 

 to a point soon after sunrise) , which begins when 

 the surge of diurnal fishes, mostly acanthurids, 

 rise into the water column. 



5. In the evening, diurnal fishes, both with- 

 in and between species, seek cover in an order 

 that corresponds roughly to increasing size. 

 That is, the larger fishes seek cover last. In 

 the morning, the same fishes emerge from cover 

 in an order that corresponds roughly to de- 



creasing size. That is, the larger fishes emerge 

 from cover first. 



6. There is some territorial aggression among 

 diurnal fishes during evening twilight that ex- 

 presses conflict over resting spots. However, 

 the aggression is minimal considering how many 

 fishes settle into resting positions on the reef 

 at this time. Limited evidence suggests that 

 overt conflict is reduced because at least some 

 individuals have established resting spots. 



7. Reef fishes are most vulnerable to preda- 

 tors during twilight because mechanisms that 

 reduce predation during the day and night are 

 less eflfective during the transition between these 

 two major segments of the diel cycle. The in- 

 terim, or "quiet," period in Kona is the time of 

 greatest potential danger and corresponds in 

 time relative to sunrise and sunset to the period 

 when schooling fishes in the Gulf of California 

 are under heaviest attack. Similar overt pred- 

 ator-prey interactions are relatively infrequent 

 at this time in Kona because the schooling prey 

 and the predators that exploit them are not 

 major elements of the Kona fauna. 



8. The well-ordered pattern of twilight acti- 

 vity among Kona reef fishes, which is essentially 

 the same among tropical reef fishes in other seas, 

 has been shaped by a historic threat from cre- 

 puscular predators. In areas where predators 

 are only a relatively mild threat, as in Kona, twi- 

 light activity follows the same pattern as it does 

 in areas where predators are a severe threat, as 

 in the Gulf of California. Because predator- 

 prey relations among fishes have responded 

 throughout time to certain basic, unchanging 

 phenomena, the similarity of twilight activity 

 patterns among fishes on widely separated reefs 

 today is the result of a long evolution that in 

 any one area transcends the existing situation 

 and species. 



ACKNOWLEDGMENTS 



Lloyd D. Richards assisted in collecting the 

 data for this study. I thank Carl L. Hubbs and 

 Richard H. Rosenblatt, Scripps Institution of 

 Oceanography, and John R. Hunter, National 

 Marine Fisheries Service, for helpful comments 



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