HOBSON: ACTIVITY OF HAWAIIAN REEF FISHES 



across the reef in schools while grazing on ben- 

 thic algae during the day. The intrareef migra- 

 tions during evening twilight of such fishes may 

 be a return from such excursions to established 

 resting areas at day's end. 



Vertical Migrations 



The movements discussed so far involve fishes 

 that traverse considerable distances. Many 

 other fishes, some diurnal, others nocturnal, 

 move between feeding grounds and shelter lo- 

 cations in a well-defined pattern that covers no 

 more than a few meters. These are the many 

 plankton feeders that forage in the water col- 

 umn, and find shelter on the reef directly below. 

 Some species, like the diurnal Naso hexacanthus 

 and the nocturnal Myripristis argyromus, both 

 discussed above, undertake such vertical move- 

 ments but also make extensive lateral excursions 

 across the reef and beyond. Nevertheless, many 

 plankton feeders, especially smaller species like 

 Dascylltis albisella, restrict their activity to a 

 limited area on the reef. Restricted though 

 these movements may be, they follow patterns 

 as well defined as any of those discussed above. 



These plankton feeders are either diurnal or 

 nocturnal. Without overlap, those feeding in 

 daylight are inactive after dark, and those feed- 

 ing at night are inactive in daylight. The re- 

 spective feeding periods of the two groups are 

 separated by the 15 to 20 min interim, or quiet, 

 period. I noted a similar absence of overlap be- 

 tween diurnal and nocturnal plankton feeders 

 in the Gulf of California (Hobson, 1968), as did 

 Starck and Davis (1966) in the tropical Atlantic. 

 Obviously conditions of plankton feeding differ 

 between day and night. Emery (1968) reported 

 that the composition of plankton over Florida 

 reefs diff'ers between day and night, and I found 

 the same true in Kona (Hobson, in preparation) . 



Twilight Movements of Other Reef Fishes 



The patterns of movement described above are 

 readily recognized because each involves many 

 individual fish moving together. Most reef 

 fishes do not operate in large assemblages but 

 instead are active solitarily or in small groups. 



Unifying patterns in such species are difficult 

 to recognize, because many discrete units are si- 

 multaneously behaving independently at diff"er- 

 ent points on the reef. Nevertheless, limited 

 data show that patterns do exist. Probably the 

 movements of some fall into the broad categories 

 outlined above, but the actions of others, once 

 recognized, probably would represent additional 

 categories. Chaetodontids, labrids, pomacen- 

 trids, scarids, balistids, and other diurnal spe- 

 cies, as well as many holocentrids, apogonids, 

 scorpaenids, and other nocturnal fishes, all dis- 

 play distinctive nocturnal and diurnal habits. 

 The twilight activities of most of these fishes 

 do not include extensive movements; most 

 change from an active to a relatively inactive 

 state, or the reverse, within a limited area on 

 the reef. Data presented in this report show 

 well-defined temporal patterns in their seeking 

 and everging from cover. Limited additional 

 data indicate that in at least many of these fishes 

 the patterns also have a strong spatial element: 

 consider, for example, the circumstance that at 

 least some individuals of Thalassoma duperrey 

 and other species occupy specific resting spots 

 at nightfall. 



PATTERNS OF TWILIGHT ACTIVITY 



I have developed the thesis that activity of 

 most Kona reef fishes during twilight relates to 

 a transition between diurnal and nocturnal 

 modes of behavior. If these fishes were con- 

 cerned only with shifting from one mode of be- 

 havior to another, the timing of the various 

 transition events could well as not be random. 

 In fact, however, the transition events proceed 

 in an established, well-defined sequence, with 

 characteristics indicating that some force exerts 

 a strong controlling influence. The general pat- 

 tern has been outlined above (Figures 12 and 

 13). Some additional characteristics of the 

 transition events warrant discussion. 



Fish Size as Expressed in Transition Activity 



The time during the transition at which a 

 given fish joins certain of the activities is re- 

 lated to that fish's size. During evening twilight 



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