FISHERY BULLETIN: VOL 76, NO 1 



GUT CONTENTS OF THE PLANKTIVOR- 

 OUS FISHES.— The gut contents of diurnal fishes 

 collected at the same time, and in the same loca- 

 tion, as the daytime plankton collections are listed 

 in Table 4, and those from the nocturnal species, 

 which were collected between midnight and first 

 morning light on nights when the plankton were 

 sampled, are listed in Table 5. 



Where Currents Were Strong 



General Observations 



Currents were periodically strong near the 

 passes from the open sea, and here, where patch 

 reefs and other hard substrata typically are co- 

 vered with living corals, underwater visibility 

 consistently exceeded 20 m.^ The lagoon floor in 



these areas generally is coarse, well-sorted sand ( a 

 sample of the sediment at the Bogen Island site 

 proved to be about 60% fragments of calcareous 

 algae, Halimeda spp., with a density of 1.25 g/ml; 

 grain size in over 80% of this sample was greater 

 than 1 mm). 



PLANKTON. — Plankters were noted infre- 

 quently during casual diurnal observations where 

 currents were strong. Nevertheless, the mysids so 

 prominent where currents were weak occurred 

 here only in small, inconspicuous swarms that 

 concentrated close in the lee of patch reefs when 

 currents were running. The larger zooplankters, 

 frequently so prominent after dark in weak- 

 current areas, were not noted here in any abun- 

 dance, although nocturnal observations underwa- 

 ter in this habitat were limited. 



■•Our concept of strong-current locations does not include those 

 breaks in the interisland reefs where the lagoonward flow of 

 water crossing the reef concentrated and spilled into the lagoon 

 at sometimes exceptionally high velocities. These currents were 

 localized and relatively shallow. Planktivorous fishes present 

 were essentially those of nearby weak-current locations in the 

 lee of these reefs, and although no collections were made here we 

 would not have expected such currents to be rich in zooplankters, 

 for reasons developed in the Discussion. 



FISHES. — During the day planktivorous fishes 

 were especially numerous in these surroundings. 

 Many diurnal species were concentrated here, the 

 more prominent being: the serranid Mirolab- 

 richthus pascalus, the lutjanid Pterocaesio tile, 

 and the damselfishes Chromis agilis, C. caerulea, 

 C. lepidolepis , C. margaritifer, Pomacentrus coe- 

 lestus, and Dascyllus reticulatus. Pomacentrus 



Table 4.— Food habits of diurnal planktivorous fishes from Walt Island, Enewetak Atoll, site of weak currents. The value outside the 

 parentheses is the rank of the item as food of that fish species ( based on incidence and volume in diet); of the two values in parentheses, 

 the first is the percent offish of that species containing the item, the second is the mean percent of the total diet of that fish species 

 represented by the item. 



1 Apogon gracilis (juveniles) n = 10, 17-37, x = 27 mm SL 



2. Pomacentrus pavo n =_5: 46-65, x = 57,2 mm SL 



3. P. vaiuli n = 6. 40-51, x = 50 mm_SL 



4 Dascyllus reticulata n = 5; 50-74, x = 63 7 mm SL 



Categories present Mean no.' 1 



5. Amblyglyphidodon curacao n_= 5; 67-82, x = 74.2 mm SL 



6. Chromis agilis n = 2, 50-54, x = 52 mm SL 



7. C. caerulea n = 5: 44-73, x =58.6 mm SL 



8. C. margaritifer n = 3; 43-50, x = 46 mm SL 



3 4 5 6 7 



'Numbers of plankters (from Table 2) provided only for rough measure of relative abundance. 



^Calanoids and cyclopoids not separated in gut contents: both occurred in all fish species but calanoids predominated. 



^Larvaceans not present in plankton collections but in two fish guts. 



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