HOBSON and CHESS: TROPHIC RELATIONSHIPS AMONG FISHES 



items ofeach food type: 1) number, 2) size range, 3) 

 state of digestion (subjectively assessed on a scale 

 of 5, from fresh to well-digested), and 4) an esti- 

 mate of their representation among the gut con- 

 tents as percent of the total volume. 



RESULTS 



Our widespread observations along the sandy 

 shelf which rims the lagoon established that the 

 planktivorous fishes were centered about the iso- 

 lated patch reefs. At least a few planktivores for- 

 aged in the water column above virtually every 

 reef, but more of them were above some reefs than 

 others and there were clear patterns to their dis- 

 tributions. For example, during the day there 

 tended to be more planktivores above reefs at the 

 outer edge of the shelf than above similar reefs at 

 comparable depths, and shallower, shoreward on 

 the shelf. But diurnal planktivores were most 

 numerous where strong tidal currents flowed 

 through passes from the open sea, and least 

 numerous where reefs or islands blocked the flow 

 of water into the lagoon. On the other hand, the 

 reverse was true of the nocturnal planktivores. 

 Because the distributions and activities of these 

 fishes proved to be closely related to current pat- 

 terns, we judged that the contributing influences 

 are best isolated by concentrating on the more 

 extreme current situations. This was true even 

 though in most places over the range of our obser- 

 vations currents were variably moderate, and 

 prevailing conditions intermediate between the 

 two extremes. 



Where Currents Were Weak 



General Observations 



There is relatively little water movement near 

 the lee shores of the islands and close behind the 

 interisland reefs that block entry of water into the 

 lagoon from the open sea. In some of these loca- 

 tions there is enough circulation to permit rich 

 coral growth and underwater visibility that ex- 

 ceeds 15 m, but in other places the circulation is 

 more limited, and living corals exist as small 

 heads or encrustations on otherwise dead reefs, 

 while underwater visibility often is <5 m. The 

 lagoon floor in these regions generally is of rela- 

 tively undisturbed, fine-grained sand. (A sample 

 of sediment from the Walt Island site proved to be 



75% foraminiferans, with a density of 1.32 g/ml. 

 Grain size in over 80% of this sample was < 1 mm.) 



PLANKTON.— Usually we made no effort to 

 detect the smaller plankters during our general 

 diurnal observations, even though many of the 

 copepods and others were visible with close inspec- 

 tion. Dense swarms of mysids, however, were out- 

 standing features of the daytime scene in many 

 places where currents were weak, especially above 

 sand close to the patch reefs. With increasing dis- 

 tance from the bottom, their swarms were smaller 

 and less numerous, though swarm-members al- 

 ways were closely spaced. Juveniles predominated 

 at the lower levels, adults were more numerous 

 above. The swarms dispersed at night, when both 

 adults and juveniles scattered near the bottom and 

 at middepth, but only adults were near the sur- 

 face. 



Although mysids were the only plankters 

 routinely noted during the day, others were prom- 

 inent after dark. Most conspicuous were large 

 calanoid copepods — larger than any copepods pre- 

 sent in daylight — that for a few hours after last 

 evening light swarmed around us in dense num- 

 bers whenever we turned on our diving lights. 

 Highly motile epitokous nereids, as well as an 

 opheliid, Polyophthalmus sp., were numerous 

 polychaetes, with other forms including hyperid 

 and gammarid amphipods, stomatopod larvae, 

 reptantian zoea, and brachyuran megalops. None 

 of these forms were seen in daylight. 



FISHES. — Adult diurnal planktivorous fishes 

 were relatively few in these surroundings com- 

 pared with their numbers elsewhere. Neverthe- 

 less, this seemed a favored habitat for at least one 

 species, Pomacentrus pavo, which was widespread 

 in groups of four to six individuals 2 to 5 cm above 

 low coral-rock outcroppings in the sand, usually in 

 the vicinity of patch reefs. Pomacentrus vaiuli, 

 another abundant species, was present only as 

 solitary individuals that rarely moved more than 

 a few centimeters from the larger patch reefs, yet 

 most of its food was small organisms swimming or 

 drifting free in the water immediately adjacent to 

 the substrate. Dascyllus reticulatus was numerous 

 by day in feeding aggregations up into the mid- 

 waters, usually above large heads of branching 

 corals, while at the same time Amblyglyphidodon 

 curacao, which usually fed in groups of <10, often 

 ranged up to the water's surface. Of the diurnal 

 planktivores considered here that ranged into the 



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