HOBSONr ACTIVITY OF HAWAIIAN REEF FISHES 



100 



01 



0.01 



0.001 - 



0.0001 



0.00001 



i INTERIM 



§ "QUIET" PERU 



COVER-SEEKING 



OF NOCTURNAL 



FISHES 



40 30 20 10 



TIME RELATIVE TO SUNRISE (mm) 



10 



Figure 13. — Summary of events during the morning 

 transition period. For explanation of symbols, see leg- 

 ends for Figures 2, 4, and 5, except that this figure rep- 

 resents morning events; in addition numbered points 

 represent mean values for events. 1) When a particular 

 school of Priacanthus cruentatus returned to the reef on 

 six mornings. 2) When last Myripristis spp. took cover 

 on reef; the beginning of the interim, or "quiet," period : 

 eight mornings. 3) When acanthurids began assembling 

 in depressions in the reef on six mornings. 4) When an 

 aggregation of Dascyllus albisella in a particular lo- 

 cation had moved away from cover, above the reef, on 

 five mornings. 5) When diurnal fishes first surged 1 to 

 2 m into the water column ; end of the interim, or "quiet," 

 period: nine mornings. 6) When the first scarid ap- 

 peared away from cover on nine mornings. 7) When 

 the aggregation of Dascyllus albisella (see #4), had 

 risen 3 m above the reef on five mornings. 8) When 

 Chromis leucurus first left cover on nine mornings. 

 9) When Labroides phthirophagus first left cover on 

 nine mornings. 10) When Thalassoma duperrey first 

 left cover on 16 mornings. 11) When Acanthurus ni- 

 grofuscus first appeared in school above the reef on three 

 mornings. 12) When the aggregation of Dascyllus al- 

 bisella (see #4 and #7) was 5 m above the reef on three 

 mornings. 



to similarly school during its nocturnal inactive 

 period. Fishes that form large schools in ex- 

 posed locations when inactive are not well rep- 



resented on Kona reefs; possible reasons are 

 given below. The other means of attaining se- 

 curity while inactive — seeking shelter under 

 rocks or coral — is characteristic of many diurnal 

 as well as nocturnal fishes, including labrids, 

 scarids, balistids, holocentrids, and apogonids. 

 This second tactic is employed by most reef fishes 

 in Kona. The advantage of cover in reducing 

 predation is obvious. Some of the nocturnal 

 fishes, for example, species of Myripristis and 

 Holocentrus, as well as Priacanthus cruentatus, 

 characteristically aggregate under cover on the 

 reef in daylight; in contrast, none of the diurnal 

 species were noted to aggregate when sheltered 

 in their nocturnal resting spots. 



Each of the wide variety of fishes on the Kona 

 reef has its own specific feeding habits (Hobson, 

 in preparation), and these relate in large part 

 to where each is active. A suitable feeding lo- 

 cation for any given species may or may not 

 be near areas that offer it suitable security dur- 

 ing its inactive period. Consequently, the major 

 actions of these fishes characteristic of twilight 

 relate to moving between feeding locations and 

 shelter locations. 



TWILIGHT REDISTRIBUTION 



Most of the readily observed movements of 

 Kona reef fishes during twilight fit within a 

 framework of three broad, overlapping categor- 

 ies: 1) individuals of some species migrate ex- 

 tensively between oflFshore feeding grounds and 

 shelter locations on the reef; 2) other species 

 migrate from one part of the reef to another; 

 and 3) still others make short but well-defined 

 vertical migrations between plankton-feeding lo- 

 cations in the water column and shelter locations 

 on the reef below. These movements are all 

 performed by fishes swimming in groups. The 

 movements of most reef fishes, which remain 

 poorly known, do not seem referable to any of 

 these categories; nevertheless, limited evidence 

 indicates that at least many of them follow well- 

 defined patterns of some sort. There is much 

 overlap between the different categories and the 

 activity of many species comprise elements of 

 more than one type. It is for convenience in 



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