HOBSON: ACTIVITY OF HAWAIIAN REEF FISHES 



by diurnal fishes during morning twilight, or 

 by nocturnal fishes during evening twilight, un- 

 til after the time that predation in the Gulf has 

 subsided. Thus, the time of maximum preda- 

 tion in the Gulf is equivalent to what is aptly 

 called the "quiet" period in Kona. Obviously the 

 term "quiet" would be a misnomer if applied to 

 this part of the transition period in certain parts 

 of the Gulf, it being the time when the schooling 

 fishes are under heaviest attack. These school- 

 ing species, for example the herring Harengiila, 

 cannot take shelter under rocks or coral on the 

 reef, as have the other smaller fishes, and thus 

 are exposed to predators during a time when 

 they seem to be especially vulnerable. The dis- 

 advantage of schooling fishes at this time, as 

 compared to other potential prey of the pisci- 

 vores, was pointed out earlier (Hobson, 1968). 

 Thus, if there is an advantage in being close to 

 shelter when predators are most eflfective, the 

 actions of Kona reef fishes during twilight are 

 adaptive to any increased threat from predators 

 that might exist at that time. 



Other behavioral characteristics of Kona reef 

 fishes during the day-night transition period can 



 KONA QUIET PERIOD ^ 



— 

 40 



Z=L 



20 10 



TIME RELATIVE TO SUNRISE (min) 



Figure 15. — Time of peak predation (Mycteroperca ro- 

 sacea when preying on schooling Harengula thrissina) 

 in the Gulf of California relative to the morning interim, 

 or "quiet," period in Kona. For explanation of symbols, 

 see legend for Figure 14. The mean and ranges of both 

 the beginning (A) and end (B) of the Kona quiet pe- 

 riod, defined by events described at length in this report, 

 are plotted. (For further information regarding this 

 figure, see text footnote 4.) 



be interpreted as mechanisms to reduce preda- 

 tion. If one accepts the premise that smaller 

 fishes are more vulnerable to predation than 

 larger ones, it is significant that smaller individ- 

 uals seek cover earlier in the evening, when dan- 

 ger from predators apparently is progressively 

 intensifying. The same significance can be at- 

 tributed to their leaving cover later in the morn- 

 ing when danger from predators apparently is 

 progressively diminishing. Consider too the in- 

 creased tendency to aggregate among so many 

 fishes that just mill over a given section of the 

 reef. Although this behavior probably has other 

 functions as well, it may also contribute some 

 measure of security from predators. It was 

 pointed out earlier (Hobson, 1968) that fishes 

 in such aggregations attain the same protection 

 from predators as do those in schools (if one 

 wishes to distinguish between such assem- 

 blages). 



Clearly the overall pattern of activity in Kona 

 reef fishes during twilight can be interpreted as 

 being strongly influenced by crepuscular preda- 

 tors. Yet, critics of this hypothesis can point 

 out that an obvious piscivorous threat is absent, 

 at least relatively so when the Kona habitats are 

 compared to those of the Gulf of California. 



Hawaiian reefs have been known to be lacking 

 in some of the major predatory fish-groups that 

 are common on most other tropical Pacific reefs. 

 Most notable among these are the shallow water 

 groupers (Serranidae) and snappers (Lutjani- 

 dae) — Gosline and Brock, 1960; Randall and 

 Brock, 1960. There are several large carangid 

 species in Hawaii, but the only one that is nu- 

 merous in Kona, Caranx melampygus, is repre- 

 sented mostly by small individuals. Possibly 

 heavy pressure from fishermen along Hawaiian 

 shores keeps the numbers of large carangids at 

 a low level. Because the major piscivorous fish- 

 es on the reef at Kona (aside from some of the 

 moray eels, which probably do not prey signifi- 

 cantly on free-swimming fishes) are relatively 

 small, they do not seriously threaten the adults 

 of the species being considered here. These 

 include such predators as Fistularia petimba, 

 Aulostomns chineTisis, Paracirrhites for'steri, 

 Parupeneus chryserydros, and Bothus mancus. 

 The predatory activity of these and other species 



737 



