FISHERY BULLETIN: VOL. 72, NO. 4 



only a few individuals, often close to the sub- 

 stratum. Most representatives seen inshore are 

 not feeding, but rather move uniformly together 

 closely spaced in schools When they do feed, the 

 schools are abandoned for aggregations in which 

 loosely spaced individuals act independently. 



During evening twilight many individuals 

 move in from deeper water over the shallower 

 parts of the reef. Larger representatives are in the 

 shallows only after dark. On dark nights, the 

 species is scattered close among rocks and corals, 

 relatively inactive, but alert. However, on moonlit 

 nights some swim above the reef in small groups. 



Sixteen individuals (261: 202-392 mm) were 

 speared at various times of night and day. Because 

 larger individuals are less accessible, especially 

 during the day, the sample is biased toward small- 

 er members of the species. All four solitary indi- 

 viduals taken close among rocks or coral at night 

 (later than 4 h after sunset and before daybreak) 

 had empty stomachs, whereas only one of nine 

 others taken from schools above the reef at various 

 times of day had material in its stomach, and this 

 one came from a school that had just appeared over 

 the reef from offshore during midafternoon. Fi- 

 nally, all three that had been observed feeding 

 when speared above inshore reefs (on three after- 

 noons over 2 mo) had full stomachs. Items in the 

 four individuals whose stomachs contained food 

 are listed in Table 61. 



Like Acanthurus thompsoni, this acanthurid 

 feeds mostly on semitransparent, often gelati- 

 nous, prey. Of the four that contained food, the 

 three taken from inshore feeding aggregations 

 were relatively small fish (233-238 mm) whose 

 major food was planktonic fish eggs. Perhaps 

 significantly, thei'e were no fish eggs in the fourth 

 specimen, which had just appeared over the reef 

 from offshore. This individual was larger than the 



others, about 300 mm long, but was collected 

 within 30 min of one of them. The major item in its 

 stomach was filamentous red algae, which did not 

 occur in the smaller three. Only chaetognaths and 

 larvaceans occurred in the stomachs of all four 

 specimens. These limited data suggest there may 

 be distinctive differences in diet and feeding 

 grounds over the size range of individuals sam- 

 pled. 



The high incidence of empty stomachs among 

 individuals over the inshore reefs during the day, 

 as well as at night, suggests that many may feed 

 offshore, and be relatively inactive, or at least not 

 feeding, when they are inshore. 



Jones (1968) included N. hexacanthus with A. 

 thompsoni when reporting the diet of copepods, 

 crab zoea, crab megalops, and mysids noted above. 

 My comments concerning the reported diet of A. 

 thompsoni (see above) apply equally here. 



CONCLUSION. — Naso hexacanthus is a diur- 

 nal planktivore that takes mostly semitranspar- 

 ent, often gelatinous, prey — especially chaeto- 

 gnaths, larvaceans, and fish eggs. Limited data 

 suggest that drifting pieces of filamentous algae 

 may also be important. 



General Remarks on Surgeonfishes 



Surgeonfishes are widespread on tropical reefs, 

 and usually are described in a general way as 

 herbivores (e.g. in the Bahamas by Bbhlke and 

 Chaplin, 1968; and in the West Indies by Randall, 

 1967). Jones (1968) grouped the many Hawaiian 

 surgeonfishes according to their habitats and 

 methods of foraging. In categorizing the bottom- 

 foraging species, not studied by me, he defined 

 three types of habitats, and listed the surgeon- 

 fishes characteristic of each: 1) The turbulent 



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