importance of zooplankton patches in foraging ecology of the planktivorous reef fish Chromis chrysurus (Pomacentridae) at Kuchinoerabu Island, Japan

Abstract

Short-term dynamics of prey-sized zooplankton and predation responses of the adult pomacentrid fish Chromis chrysurus were examined at Kuchinoerabu Island, near Yaku Island, Japan, from June 1986 to August 1987. Adult C, chrysurus fed primarily on larger planktonic copepods (ranging from 0.8 to 5.5 mm body length) that originated offshore, despite the dominance of smaller ones (<0.8 mm) in the water column. Siphonophores, appendicularians and chaetognaths were also utilized. Low overall densities (<50 m-3) of such zooplankton prey were common in the water column, whereas high densities (more than 500 m'3) occurred rarely. Dense aggregations of prey (3144 to 6475 m-3), including numerous large pelagic zooplankters, were recorded on 2 occasions. Differences in prey density were not detected among sampling times, nor among tide range classes. Pump-assisted plankton sampling from 07:OO to 19:OO h was conducted on 3 occasions of spring tides; however, prey densities remained extremely low (0 to 25 m-3) during almost all of the diurnal active period of the fish. A high proportion of fishes (31 to 59 %) sampled each month, regardless of season, had empty stomachs On the other hand, composition analysis of satiated stomachs showed that adult C, chrysul-us util~ze high density zooplankton patches when available. No die1 or tidal periodicity in feeding pattern was detected, either in feeding rates or in stomach fullness, although significant differences were detected both among feeding rates and among sets of stomach fullness of specimens sampled together. Hence, adult C. chrysurus feed intermittently on available prey, according to fluctuations in prey density, and are probably subject to extreme food deprivation. Zooplankton patches are therefore an indispensable food resource at Kuchinoerabu Island. The possibility is discussed that adult C. chrysurus may take positive action in foraging behavior, in order to efficiently utilize zooplankton patches.