Factors Influencing the Juvenile Demography of a Coral Reef Fish

A notable recent development in marine ecology has been the suggestion that the size of demersal populations is limited and that patterns in demersal abundance are determined by the settlement of pelagic larvae (recruitment). Here I examine some factors potentially limiting and determining population density in a small coral reef fish. An experimental study of the demography of juveniles of a small planktivorous damselfish, the humbug Dascyllus aruanus (Pomacentridae), was done at One Tree Reef, southern Great Barrier Reef, Australia. Recently settled fish were transplanted to replicate units of habitat at three lagoonal sites. The interactive effects of initial recruit density, the presence of adults, and supplemental feeding on the growth, survival, migration, and maturation of the recruits were examined over the following 10 mo. Migration was apparently rare and thus unimportant. Effects of the factors on the growth and survival of recruits were complex. Survival of recruits was generally inversely related to their density but enhanced in the presence of adults. Average survival also varied among locations. Mean growth was generally depressed at high recruit densities and in the presence of adults but was enhanced by supplemental feeding. Maturation was related to size and so was influenced by effects on growth. The results suggest that regulatory interactions and shortages of food may limit the size of adult populations via their effect on growth rates. Effects of the factors on survival were slight. Total abundance may thus be limited primarily by recruitment unless effects on growth are ultimately translated to effects on demersal mortality. As a result of the effects on survival, there was some modification of the initial pattern of recruit abundance. The importance of these effects in determining patterns of abundance in unmanipulated populations depends on variation in recruitment at similar scales. The number of recruits entering the adult population under different treatments was highly modified from the pattern of initial recruit density. The number of recruits that became mature by the end of the experiment was totally unrelated to their initial density. In addition, that number was reduced in the presence of adults, and was limited by the availability of food. The results suggest that complex effects of these factors on juvenile demography can modify recruitment patterns at small spatial scales. They also suggest that these factors may have different effects on the adult and total demersal populations. In general terms, both recruitment and postrecruitment processes influence patterns in abundance.