ACTIVITY OF HAWAIIAN REEF FISHES DURING THE EVENING AND 

 MORNING TRANSITIONS BETWEEN DAYLIGHT AND DARKNESS 



Edmund S. Hobson^ 



ABSTRACT 



Activity during the morning and evening transitions between day and night was studied 

 as part of a broad investigation of ecological relations among reef fishes in Kona, Hawaii, 

 during 1969 and 1970. 



Most of the fishes are either diurnal or nocturnal, the former being mainly inactive 

 at night, the latter mainly inactive in daylight. During active periods their behavior is 

 dominated by feeding, whereas inactive periods are dominated by measures to enhance 

 security. Thus twilight behavior involves primarily a changeover from feeding to shel- 

 tering, and the reverse. Migrations of various types between shelter locations, where 

 they rest, and feeding grounds are a major element of the transition periods. 



The transition events proceed in an established, well-defined sequence. In the evening, 

 after the diurnal fishes have descended to shelter and before the nocturnal fishes have 

 risen into the water column, there is an interim period of about 20 min during which 

 both diurnal and nocturnal fishes are close to the substrate; it is at this time that reef 

 fishes are most, vulnerable to predators, and the proximity of cover offers protection. 

 Similar behavior occurs among related fishes in the Gulf of California. The interim 

 period in Kona corresponds in time relative to sunrise and sunset to the period during 

 which schooling fishes in the Gulf of California are under heaviest attack; however, 

 because schooling fishes and the predators that exploit them are not major elements of 

 the Kona fauna, overt predator-prey interactions are not prominent there. Thus the 

 well-ordered pattern of events that characterizes twilight in Kona also occurs on other 

 reefs widespread in tropical seas, whether or not there exists in each of these areas 

 today a severe threat from large piscivorous predators. The twilight pattern of actions 

 is the result of a long evolution that in any one area transcends the existing situation 

 and species. Because predator-prey interactions among fishes have been influenced 

 throughout time by certain basic and unchanging phenomena, the similarity of twilight 

 activity patterns among fishes on widely separated reefs today reflects the impact of 

 a historic threat from predators. 



Most fishes on tropical reefs are active either 

 by day or by night, with the diurnal species 

 mostly inactive in darkness and the nocturnal 

 species mostly inactive in daylight (Hobson, 

 1965, 1968; Starck and Davis, 1966). The 

 changeover from one situation to the other is a 

 complex process, with events following a well- 

 defined sequence. This report describes the sig- 

 nificant events during the evening and morning 



^ National Marine Fisheries Service, Tiburon Fisher- 

 ies Laboratory Tiburon, Calif., and Scripps Institution 

 of Oceanography, University of California, San Diego, 

 La Jolla, Calif. Mailing address: Southwest Fisheries 

 Center, P.O. Box 271, La Jolla, CA 92037. 



Manuscript accepted January 1972. 



FISHERY BULLETIN: VOL. 70, NO. 3, 1972. 



transition periods in Kona, Hawaii, between 

 June 1969 and August 1970. It is a segment of 

 a broad study of ecological relations among Ha- 

 waiian reef fishes. The present report, which 

 deals with the twilight situation, will be followed 

 by a second report covering the situations 

 throughout day and night, respectively, and in- 

 cluding a detailed analysis of food habits (Hob- 

 son, in preparation). Some highlights of this 

 program were outlined earlier (Hobson, 1970). 



METHODS 



This study is based on observations that 

 spanned evening twilight on 20 occasions and 



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