Competitive Interactions Among Adults and Juveniles in a Coral Reef Fish

Considerable discussion has arisen over whether population limitation in coral reef fishes is achieved through postrecruitment processes or through factors operating prior to recruitment that lead to insufficient numbers to saturate resources. This paper presents the results of two field experiments designed to document and measure the relative effects of recruitment and subsequent intraspecific competition on populations of the planktivorous damselfish Pomacentrus amboinensis, a common species on the Great Barrier Reef. I tested for effects of conspecifics on juvenile mortality, growth, and maturation by manipulating population densities on coral rubble patch reefs. Each experiment examined the potential for competition among juveniles and adults. The first experiment manipulated the densities of recently settled recruits; the second manipulated 1—yr—old juveniles using a similar experimental design. There were no consistent effects of juvenile density or of adult presence/absence on juvenile survivorship, during either the 0 + yr or the 1 + yr. However, there were substantial effects on growth and maturation. During the 1st 8 mo of experiment 1, grow was inversely related to initial juvenile density, but was not affected by the presence of adults. A marked negative effect of adults on juvenile growth developed as juveniles approached an age of 1 yr. This effect developed early in experiment 2. Both the mean and the variance in 1 + yr growth were reduced in the presence of adults. Adults primarily affected the growth of the initially largest, behaviorally dominant juveniles. A significantly greater number of 0 + yr and 1 + yr juveniles reached a mature size after 1 yr in treatments without adults present. The numbers maturing were not related to initial density, either with adults present or absent. It is concluded that postrecruitment competitive processes play an important role in restricting adult population size in P. amboinensis, at least over the range of densities and on the spatial scale examined. The mechanism, however, is not one of those traditionally considered in equilibrial models. That is, competition does not appear to modify recruitment or mortality patterns directly, but influences numbers maturing through density dependent effects on maturation time.