HOBSON: ACTIVITY OF HAWAIIAN REEF FISHES 

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TIME RELATIVE TO SUNRISE (min) 



Figure 10. — Some characteristics of certain diurnal fish- 

 es during morning twilight. A. Chromis lericums: when 

 the first individual left cover on nine mornings. B. 

 Acanthtirus nigrofiiscus: when individuals first ap- 

 peared in school above the reef on three mornings. For 

 explanation of symbols, see legend for Figure 2, except 

 that this figure represents morning events. 



Late Risers 



Observations on Acanthurus nigrofuscus. — 

 The assemblages of larger and medium-sized 

 surgeonfishes that have been swimming above 

 the reef since the initial upsurge in activity, be- 

 gin to break up about the time of sunrise. Ini- 

 tially these aggregations become less active, then 

 dissolve into smaller, discrete groups, now closer 

 to the sea floor, that increasingly range out over 

 the reef. Soon they assume activity typical of 

 daylight. At about the time that these aggre- 

 gations are breaking up, the small surgeonfish 

 Acanthurus nigrofuscus makes its first appear- 

 ance. Soon after, individuals of this species as- 

 semble in large schools over the reef and proceed 

 to follow a course of action much like that taken 

 earlier by their larger relatives. The initial 

 appearance of A. nig7'ofuscus in a school above 

 the reef was noted on three occasions (Figure 

 10). They maintain these large, active schools, 

 as did the large species, for about 20 min, when 



finally these schools too dissolve into smaller dis- 

 crete groups that range out over the reef. 



Observations on Thalassoma duperrey. — As 

 is true in the Gulf of California (Hobson, 1965, 

 1968), the smaller labrids are among the last 

 diurnal fishes to become active in the morning. 

 The initial appearance of the ubiquitous Thalas- 

 soma duperrey in the morning is representative 

 of this group, just as is its cover-seeking in the 

 evening. It is much easier to recognize the first 

 individual to appear in the morning than the last 

 individual to take cover in the evening, and on 

 16 mornings I noted the first T. duperrey to be- 

 come active in my immediate surroundings (Fig- 

 ure 11) . These fish are often unmistakable when 

 they are just resuming activity; for several sec- 

 onds after emerging from their resting places 

 many "yawn," opening the mouth wide, and 

 "stretch," drawing their body into various con- 

 tortions. 



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TIME RELATIVE TO SUNRISE (min) 



Figure 11. — Some characteristics of certain labrids dur- 

 ing morning twilight. A. Thalassoma duperrey: when 

 the first individual left cover on 16 mornings. B. La- 

 broides phthirophagus: when the first individual left 

 cover on nine mornings. For explanation of symbols, 

 see legend for Figure 2, except that this figure repre- 

 sents morning events. 



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