Experimental evaluation of the effects of habitat structure and competitive interactions on the juveniles of two coral reef fishes

Abstract

Dascyllus aruanus (L.) and Pomacentrus amboinensis (Bleeker) are small and abundant planktivorous fishes that colonize small patch reefs in One Tree Lagoon (Great Barrier Reef, Australia). The former prefers a live Pocillopora damicornis (L.) coral substratum, and the latter is more common on Porites sp. reefs, whether live or dead. This paper describes an experiment which tests for effects of Pocillopora and Porites substrata on the survival and growth of juveniles of these two fish species. It also examines the outcomes of intra- and interspecific competition on these two coral types. Coral substratum was found to have much greater effects than competition on juvenile survival over a 1-yr period, with greater numbers of both fish species surviving on Pocillopora reefs. There were no indications of competitive dominance of one fish species over another, or a reversal of dominance in relation to substratum type. Slight positive effects of density on survival were obtained in P. amboinensis, and slight negative effects were apparent in D. aruanus. Effects of coral substratum on growth were less clear, although there is some suggestion that both species grow better on Pocillopora reefs. There were substantial effects of intraspecific competition on growth in P. amboinensis, but not in D. aruanus, and no effects of interspecific competition on growth in either species. Overall, the experiment suggests that the factors controlling growth and mortality are complex, and highlights the need for further multifactorial studies focused at the level of the species.