coocurring pomacentrids concentrated on zoo- 

 plankters, but concluded from this that the species 

 is a benthic grazer. 



Nocturnal Relationships 



Nocturnal planktivores probably concentrated 

 where currents were weak because their prey — 

 including polychaetes, large calanoids, mysids, 

 isopods, gammarids, postlarval carideans, and 

 brachyuran megalops — were most numerous 

 there (Table 2). With the probable exception of at 

 least most of the calanoids (see below), most of 

 these zooplankters were local residents that rose 

 into the water column at night after spending the 

 daytime sheltered on or near the sea floor. This 

 pattern has been adequately documented among 

 these groups of organisms from both Atlantic and 

 Pacific Oceans (Emery 1968; Williams and Bynum 

 1972; Alldredge and King 1977), and its impor- 

 tance in shaping the activities of nocturnal 

 planktivorous fishes has been stressed (Hobson 

 1968, 1972, 1974; Hobson and Chess 1976). Food- 

 habit studies have shown that these groups in- 

 clude the major prey of apogonids, holocentrids, 

 and other tropical nocturnal planktivores (Atlan- 

 tic Ocean: Randall 1967; Indian Ocean: Vivien 

 1973, 1975; and Pacific Ocean: Hobson 1974). 



Only a relatively few nocturnal planktivorous 

 fishes occurred where currents were strong, prob- 

 ably because prey suitable to them were relatively 

 scarce there (Table 7). Many of the organisms on 

 which these fishes feed most likely find conditions 

 in places with strong currents adverse. For exam- 

 ple, those nocturnal zooplankters that return each 

 morning to shelter in specific habitats would 

 likely be transported to foreign surroundings 

 should they encounter strong currents while in the 

 water column. The mysids, which include some of 

 the strongest swimmers, probably cannot hold sta- 

 tion in currents much over 15 cm/s (based on the 

 maximum swimming speeds of several species: 

 Steven 1961; Clutter 1969) and currents at the 

 Bogen Island station regularly exceeded this six- 

 fold. Organisms that need to spend only a few 

 hours in the water column each night might time 

 their emergence to avoid currents, as pointed out 

 by Alldredge and King (1977), but probably even 

 these would find it advantageous to live without 

 this complex timing problem. Furthermore, many 

 of these nocturnal forms rest in sediments by day 

 (Hobson and Chess 1976; Alldredge and King 

 1977) and might find the coarse, unstable sand 



148 



FISHERY BULLETIN: VOL. 76, NO. 1 



characteristic of strong-current areas unfavor- 

 able. 



Only part of the increased numbers of zoo- 

 plankters at night were suitable prey of the noc- 

 turnal planktivores. These were individuals more 

 than about 2 mm long, which predominated 

 among the nocturnal visitors at the weak-current 

 site but which were a much smaller segment of the 

 zooplankters that appeared after dark at the 

 strong-current site. Among calanoids, for exam- 

 ple, only individuals longer than 2 mm (mostly 

 Euchaeta marina, Pleurommama xiphias, and 

 Undinula vulgaris) were important prey of such 

 larger nocturnal planktivores as Myripristis spp., 

 and while these larger calanoids were never seen 

 or collected by us at the weak-current site during 

 the day, they were more numerous than the small- 

 er ones at that station after dark (Table 3). On the 

 other hand, most of the dramatic increase in 

 calanoids at the strong-current site involved only 

 slightly larger individuals of essentially the same 

 species that were there by day, including Acartia 

 sp., Candacia sp., and E. marina (Table 8), and 

 these were largely unexploited by nocturnal 

 planktivores. At 3 mm or less, the majority may be 

 too small to be taken by the relatively large 

 mouths of most of the nocturnal fishes considered 

 here (see Hobson and Chess 1976), although they 

 were important prey of some of the smaller 

 species, such as Apogon nouaeguinae. 



The daytime location of the many calanoids 

 which appear above the reefs at night remains in 

 question. Our nearshore plankton collections in 

 southern California (Hobson and Chess 1976) 

 showed far less increase in calanoids after dark, 

 and we concluded they were in the nearshore 

 water column day and night. But the dramatic 

 increase in calanoids nearshore after dark at 

 Enewetak suggests a different situation. We rec- 

 ognize one or a combination of two possibilities: 1) 

 that some calanoids reside under shelter on the 

 sea floor by day, and join the plankton at night, or 

 2) that some calanoids reside elsewhere by day, 

 and migrate, or are transported, to the nearshore 

 waters only after dark. There is evidence for both 

 possibilities. The large calanoids that swarmed 

 around our lights shortly after last evening light 

 (but not taken in our collections) could not have 

 traveled far. Alldredge and King (1977) reported 

 calanoids emerging at night from nearshore 

 benthic substrata on the Great Barrier Reef in 

 numbers that could readily account for the in- 

 crease in calanoids we observed after dark at 



