HOBSON and CHESS: TROPHIC RELATIONSHIPS AMONG FISHES 



swept into the lagoon thrives there and becomes 

 concentrated so that the average concentration 

 per cubic meter of the eleven most common animal 

 groups is about four times higher than outside." In 

 addition, by the time the incoming current passed 

 our Bogen Island station it presumably had picked 

 up lagoon materials upstream, so its contents 

 probably were of diverse origin. 



Of course, currents in themselves enhance the 

 planktivorous habit because planktivores holding 

 station above a reef receive more plankters in cur- 

 rents than in equally rich waters without cur- 

 rents. Most of these fishes, however, take shelter 

 by the time a current reaches 1 m/s, so that opti- 

 mal velocities are somewhat below this. As the 

 current increases, the advantage of receiving 

 more plankters is progressively outweighed by the 

 difficulty of holding station (as was pointed out for 

 Chromis punctipinnis in California by Hobson and 

 Chess 1976). 



The relatively few adult diurnal planktivores 

 that foraged where currents were weak probably 

 owed their low numbers to the lack there during 

 the day of calanoids and other zooplankters suit- 

 able as prey (Table 2). The many zooplankters that 

 tidal currents carried to planktivores elsewhere 

 were unavailable to fishes here, and those taken as 

 prey or otherwise lost were not quickly replaced. 

 Although the volume of zooplankters collected at 

 the weak-current site by day (Table 1) actually 

 exceeded the volume at the strong-current site 

 (Table 6), it consisted largely of swarming mysids 

 (Table 2) which are local residents seemingly un- 

 available as prey to diurnal planktivores (possibly 

 for reasons discussed below under Miscellaneous 

 Considerations). The strong-current site was in 

 fact much richer in copepods, caridean larvae, lar- 

 vaceans, and fish eggs — the major prey of the 

 diurnal planktivores (compare Tables 2 and 7). 



Locations in the lee of reefs, however, can be rich 

 in drifting debris from these reefs (Gerber and 

 Marshall 1974). This situation existed at the Walt 

 Island site, where Pomacentrus pavo and P. vaiuli, 

 the most numerous diurnal planktivores there, 

 subsisted largely on algal fragments. Further- 

 more, the only other diurnal planktivores numer- 

 ous in weak-current areas, Amblyglyphidodon 

 curacao and Dascyllus reticulatus, demonstrated a 

 capacity to utilize algae even though both species 

 are largely carnivorous. Gerber and Marshall 

 (1974), too, found that D. reticulatus fed on algal 

 fragments when zooplankters were sparse. Obvi- 

 ously, the capacity to utilize algae as food is highly 



adaptive for planktivorous fishes that would live 

 where drift from a reef is rich in algal fragments, 

 though relatively poor in zooplankters (Table 1). 

 Despite the adaptiveness of herbivory to plank- 

 tivores under these circumstances, most of the 

 fishes studied by us were strictly, or predomi- 

 nantly, carnivores. Drifting algal fragments were 

 plentiful in nearly all nearshore habitats, but 

 where zooplankters were also numerous the algae 

 were insignificant in the diets of most plankti- 

 vores. To be sure, certain species capitalized on 

 drifting algae even where zooplankters were 

 numerous. For example, P. vaiuli, which we fre- 

 quently observed plucking items from the water 

 column, was herbivorous and numerous at the 

 zooplankton-rich Bogen Island site, just as it was 

 at the zooplankton-poor Walt Island site. And P. 

 coelestus, which may have replaced P. pavo where 

 currents were strong, fed heavily on algal frag- 

 ments where zooplankters were readily accessible. 

 Yet the pattern is clear — zooplankters were fa- 

 vored by most. Generally Chromis spp. have been 

 reported as strictly carnivores even where other 

 planktivorous pomacentrids fed substantially on 

 drifting algae (e.g., in the Marshall Islands by 

 Hiatt and Strasburg 1960; in the West Indies by 

 Randall 1967; and in Hawaii by Hobson 1974). 

 Nevertheless, species of Chromis display some 

 capacity to accept algal fragments, as we found in 

 C. margaritifer and Gerber and Marshall (1974) 

 found in C caerulea. Thus, where waters are rich 

 in reef debris but poor in zooplankters, we should 

 expect to find Chromis spp. in relatively low num- 

 bers, just as we did at Walt Island. On the other 

 hand, Mirolabrichthys pascalus (a serranid) and 

 Pterocaesio tile (a lutjanid) are members of strictly 

 carnivorous families, a fact that probably limits 

 them to places adequately supplied with zoo- 

 plankters. This view finds support from Gerber 

 and Marshall (1974), who reported that M. pas- 

 cales (as M. tuka) and P. tile fed entirely on zoo- 

 plankters. They noted the same for A. curacao, C. 

 agilis, and C. lepidolepis but did not indicate 

 where any of these fishes had been collected, nor 

 whether anything but zooplankters had been 

 available to them. This may be important because 

 one of their major stations was in East Channel, 

 where their plankton collections were without al- 

 gae, and though they found A. curacao strictly 

 carnivorous, we found that it fed heavily on algal 

 fragments where zooplankters were in short sup- 

 ply (Table 4). Gerber and Marshall also noted that 

 P. vaiuli fed mainly on algal fragments while 



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