MEPS 169:251-261 (1998)  -  doi:10.3354/meps169251

Effects of predators on growth, mortality and abundance of a juvenile reef-fish: evidence from manipulations of predator and prey abundance

S. D. Connell*

School of Biological Sciences A08, University of Sydney, New South Wales 2006, Australia
*Address for correspondence: Centre for Research on Ecological Impacts of Coastal Cities, Marine Ecology Laboratories A11, University of Sydney, New South Wales 2006, Australia. E-mail:

ABSTRACT: Studies of open populations, in which offspring are dispersed away from their natal site, often emphasise density-independent processes, namely stochastic variation in the input of newly arrived young (recruitment). Studies of closed populations, in which offspring remain close to their natal site, emphasise the role of density-dependence, particularly through competition and predation. The usefulness of this dichotomy was tested on spiny chromis Acanthochromis polyacanthus, which is one of the few marine fish that lack a pelagic larval phase and is thought to be the archetypal species for closed population status among reef-fishes. Experimental manipulations showed that juvenile A. polyacanthus suffered lower rates of mortality in locations where large predatory fish were excluded. The magnitude of differences in mortality between treatments (cages and open plots) matched natural differences in mortality between locations representing low and high numbers of predators, suggesting that large predatory fish were a primary source of juvenile mortality. Growth did not increase when prey and predator abundances were reduced. This finding rejected 2 models: that growth was reduced in large schools and these slower growing individuals suffered greater rates of predation; and that intraspecific competition occurred. Growth of prey increased when released from predation pressure, a process not previously considered in studies of growth in reef-fish. The repression of growth in response to predation pressure indicates that predators may have a more far-reaching effect on demography of reef-fish than previously thought. Predation was density-dependent in that juveniles in larger schools suffered greater rates of mortality in the presence of predators but not in their absence. Predation appeared to limit the upper size of schools, but this did not eliminate a linear relationship between input and subsequent abundance. Hence, density-dependent predation was weak and large fluctuations in recruitment (births) persisted and contributed importantly to fluctuation in prey density. These results support the model that when density-dependent mortality is weak, both input and subsequent mortality will be important limiting factors. Consequently, this study questions the assumption that variation in predation and competition has greater consequences for closed populations.

KEY WORDS: Reef-fish · Predation · Mortality · Open populations · Growth · Experiment

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