TROPHIC RELATIONSHIPS AMONG FISHES AND PLANKTON IN 

 THE LAGOON AT ENEWETAK ATOLL, MARSHALL ISLANDS 



Edmund S. Hobson and James R. Chess' 



ABSTRACT 



Trophic relationships among fishes and zooplankters in the nearshore lagoon at Enewetak differ 

 sharply between day and night, and are strongly influenced by current patterns. Adults of most diurnal 

 planktivorous fishes are numerous in certain places where tidal currents are strong, but few where 

 such currents are consistently weak. Thus, the sea bass, Mirolabrichthys pascalus; the snapper 

 Pterocaesio tile; and the damselfishes (Chromis agilis, C. caerulea, C. lepidolepis, C. margaritifer , and 

 Pomacentrus coelestus) are numerous in strong-current areas near interisland passes, but relatively 

 few or absent in weak-current areas close in the lee of islands or interisland reefs. The former areas are 

 rich, the latter poor, in the major prey of these fishes — copepods, larvaceans, and fish eggs. On the other 

 hsind, the zooplankton-poor waters close in the lee of islands and interisland reefs are rich in debris 

 from the reefs, and fishes that can subsist on these materials are abundant. Dascyllus reticulatus is 

 numerous here, although less so than where currents are strong, and takes algal fragments as an 

 important, if secondary, part of its diet; Pomacentrus vaiuli, equally abundemt in both strong- emd 

 weak-current areas, feeds largely on algal fragments, as does P. pavo, which is more numerous here 

 than where currents are strong. 



In contrast, the major nocturnal planktivores are concentrated where currents are weak, but 

 relatively sparse where these currents are strong. Included are: the soldier fishes Myripristis pralinus 

 eindM. violaceus, and the cardinalfishes Apogo« ^nicj/is (youngalsofeedby day), A. novaeguinae, and 

 A. savayensis. They are strictly carnivores that prey mostly on larger zooplankters — including large 

 calanoids, mysids, isopods, gammarids, postlarval carideans, and brachyuran megalops — absent (ex- 

 cept for the mysids) in the nearshore water column by day. These prey organisms generally find 

 conditions unfavorable where strong currents flow. Most of them are sheltered on or near specific 

 nearshore substrata during the day and enter the water colunm only at night; but others are in deeper 

 water offshore by day and move inshore at night after rising toward the surface. 



Limited evidence indicates that planktivorous juvenile and larval fishes, as well as the tiny 

 plankters on which they feed, follow patterns different from those followed by larger individuals. 



Many nearshore fishes find most of their food 

 among the plankton. Clearly, the water column is 

 a rich feeding ground. Nevertheless, fishes that 

 would take plankters face problems perhaps not 

 immediately apparent. Consider, for example, the 

 feeding-related morphologies of planktivorous 

 fishes, which obviously are products of strong 

 selection pressures. Fishes that take plankters by 

 day are characterized by modifications of head and 

 jaws, including dentition, that permit even rela- 

 tively large individuals to effectively consume 

 tiny organisms in midwater, whereas fishes that 

 take plankters at night tend to be large-mouthed 

 species with specialized means to detect, and cap- 

 ture, the larger organisms that are in the near- 

 shore water column only after dark. Awareness of 



'Southwest Fisheries Center Tiburon Laboratory, National 

 Marine Fisheries Service, NOAA, 3150 Paradise Drive, Tiburon, 

 CA 94920. 



Manuscript accepted May 1977. 



FISHERY BULLETIN: VOL. 76, NO. 1, 1978. 



these facts evolved from studies in tropical seas 

 (Hobson 1965, 1968, 1972, 1974; Starck and Davis 

 1966; Davis and Birdsong 1973) and was em- 

 phasized in more detailed study in warm temper- 

 ate waters of southern California (Hobson and 

 Chess 1976). Additional study has shown that 

 many fishes which take plankters by day accen- 

 tuate fusiform bodies and deeply incised caudal 

 fins — features that promote rapid swimming, and 

 which, significantly, are undeveloped among their 

 nocturnal counterparts. Increased speed, it was 

 suggested (Hobson 1974, 1975; Hobson and Chess 

 1976), has given diurnal planktivores that swim 

 in the water column quicker access to shelter in 

 response to severe pressures from piscivorous 

 predators; that these speed-inducing features are 

 comparatively undeveloped among the nocturnal 

 species, the suggestion continued, reflects a sharp- 

 ly reduced threat from piscivorous predators in the 

 water column after dark. 



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