HOBSON and CHESS: TROPHIC RELATIONSHIPS AMONG FISHES 



Table 7. — Occurrence, number ( actual and adjusted for current velocity), and size of zooplankters collected day and night at Bogen 



Island, Enewetak Atoll, site of strong currents. 



' Most of them planktonic stage of Tretomphalus 

 ^A 90-mm leptocephalus iarva, 



TABLE 8. — Size distribution of calanoid copepods, day and night, 

 at Bogen Island, Enewetak Atoll, site of strong currents. 



'Numbers from collections in varying currents adjusted for equivalence to 

 collections from the Walt Island site. 

 ^Including Euchaeta marina. 

 ^Including Candacia sp and E marina 



"Including Candacia sp,, E. marina. Neocalanus sp, and Undinula vulgaris. 

 5 Including Acartia sp, and Euchaeta sp, 

 'Including Acartia sp,. E. marina, and Metridia sp. 



supplemented by widespread observations else- 

 where, permit a synthesis that we hope stimulates 

 needed additional study. The following discussion 

 pertains to adults of the planktivorous fishes and 

 to plankters collected by our 0.333-mm mesh 

 meter net. All food items found in the fish guts 

 occurred in these plankton collections, so the com- 

 bined assemblage can be considered a trophic unit. 

 The situation described from these data, however, 

 may not apply to smaller individuals. Limited 

 data, including that from Apogon gracilis, the 

 only planktivore studied as an early juvenile, 

 suggest that the smaller plankters which passed 

 through our net, and their predators among 



juvenile and larval fishes, may follow significantly 

 different patterns (see Miscellaneous Considera- 

 tions below). 



Diurnal Relationships 



Probably diurnal planktivores concentrated 

 where strong tidal currents fiowed into the lagoon 

 through the passes because these waters were rich 

 in zooplankters, particularly calanoid copepods 

 (Table 7). We presume that at least many of these 

 were oceanic zooplankters carried to within reach 

 of inflowing tidal currents on the eastern side of 

 the atoll by the westward flowing North Equato- 

 rial Current — a phenomenon amplified by the 

 trade winds. In addition, some of the materials 

 carried from the lagoon on the preceding ebb tide 

 probably return. Although this outflow is minimal 

 on the windward side of the atoll, at least during 

 the trade- wind season (see von Arx 1948), it prob- 

 ably contains significant amounts of certain kinds 

 of organisms. Gerber and Marshall (1974) noted 

 that the waters of the Enewetak lagoon are much 

 richer in zooplankton than the surrounding ocean. 

 Describing the same condition at Bikini, Johnson 

 (1949) stated: "Much of the oceanic plankton 



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