FISHERY BULLETIN: VOL. 76, NO. 1 



land had been carried in through East Channel 

 from outside the lagoon — ^just as during the day. 

 The greatly increased numbers at night probably 

 followed a general rise of zooplankton toward the 

 surface waters in the open sea. Some of these zoo- 

 plankters were larger than any that were present 

 by day, but such forms represented a lesser propor- 

 tion of the nocturnal plankton here than they did 

 at the weak-current site. Presumably the collec- 

 tions also included lagoon organisms from up- 

 stream, but we would expect these to be relatively 

 few because the entrance to East Channel is only 

 about 1.2 km away (Figure 1). Although the in- 

 coming tidal currents probably carried materials 

 that had been transported from the lagoon on ear- 

 lier ebb tides, we would not expect many of the 

 larger mobile organisms to be among them. Most 

 large mobile forms, it would seem, could avoid 

 being transported from the lagoon by the com- 

 paratively weak outgoing currents. But certainly 

 the incoming tide could be returning substantial 

 numbers of passive drifters, like fish eggs and 

 algal fragments, in addition to forms like the 

 smaller calanoids. In any event, we can under- 

 stand the relative scarcity in the flooding tidal 

 currents of the relatively large nearshore resi- 

 dents (e.g., polychaetes, mysids, and postlarval 

 carideans) that are so important in the diets of 

 nocturnal planktivores. 



Probably at least some zooplankters from the 

 deeper waters of the lagoon visited the Bogen Is- 

 land site at night during periods between flooding 

 tides, but we made no collections at these times. 

 Nevertheless, it would seem that the impact of 

 such forms on the area would be limited, consider- 

 ing how long it takes them to travel without ben- 

 efit of transport by current, and the fact that a 

 flooding tide sweeps through here during much of 

 most nights. 



Miscellaneous Topics 



The Nocturnal Increase in Fish Eggs 



Planktonic fish eggs represent a special case. 

 Unlike most other zooplankters, which are mobile 

 forms that strongly influence their own distribu- 

 tions, fish eggs are passive drifters that are 

 quickly carried from where they are released if 

 there is any current. Presumably their relative 

 numbers in the water column closely follow the 

 incidence of their release by fishes on the reefs 

 below, and certainly the circumstances of this re- 



150 



lease have been strongly influenced by the threat 

 from predators that abound over the nearshore 

 reefs. Planktonic fish eggs were a major food of 

 diurnal planktivores (Tables 4, 9) but, despite an 

 almost sevenfold increase in numbers at night 

 (Tables 2, 7), they were insignificant in the diets of 

 nocturnal planktivores (Table 5). Clearly these 

 largely transparent eggs are relatively safe from 

 predatory fishes after dark, probably because they 

 are then invisible. Thus, it would be highly adap- 

 tive for reef fishes to release planktonic eggs late 

 in the day, or early in the night, when the eggs 

 have maximum time for dispersing in the dark, 

 relatively free of threat from planktivorous reef 

 fishes. 



Possible Influences of Water Depth and Size 



Among the promising topics we lacked time to 

 pursue during our short stay at Enewetak were 

 ways that water depths, and the sizes of interact- 

 ing fishes and zooplankters, may influence trophic 

 relationships. 



We believe that the difference in water depth 

 between our primary collecting sites (7 vs. 13 m) 

 did not significantly influence our findings, espe- 

 cially as the deeper station was well away from the 

 deep part of the lagoon (Figure 1) — farther, in fact, 

 than the shallower station. It was apparent to us, 

 nevertheless, that water depth in the lagoon can, 

 directly or indirectly, influence fish-zooplankton 

 interactions. Obviously both fishes and zoo- 

 plankters are physically limited in extreme shal- 

 lows, especially in turbulent waters above shallow 

 reefs. But probably the major depth-related 

 influence stems from the general tendency of la- 

 goon zooplankters to seek deeper water during the 

 day (e.g., Johnson 1949; Hobson and Chess 

 1973) — a tendency that apparently increases with 

 size. We suggest above that many of the larger 

 zooplankters active above the nearshore shelf at 

 night were in the deeper lagoon waters by day, 

 when the water column of the nearshore shelf was 

 largely without such forms. Perhaps the concen- 

 trations of planktivores along the outer edge of the 

 nearshore shelf during the day were in contact 

 with the fringe of these deep zooplankton popula- 

 tions. 



This leads to a possible influence related to size. 

 Very small zooplankters (those passing through 

 the mesh of our net, and so unrepresented in the 

 collections), and their predators among juvenile 

 and larval fishes, may follow patterns sig- 



