JUVENILE GROUPS IN A CORAL-REEF DAMSELFISH - DENSITY-DEPENDENT EFFECTS ON INDIVIDUAL FITNESS AND POPULATION DEMOGRAPHY

Costs and benefits to group living in animals may affect the fitness of individual group members and also demography of the population. The effects of grouping on the growth, survival, and attainment of maturity of juveniles of an Hawaiian coral-reef damselfish (Dascyllus albisella) were evaluated from 1987 through 1989. In this species, pelagic larvae settle (at approximate to 14 mm total length) on coral heads, joining temporary groups of up to 15 juveniles. Group members establish a dominance hierarchy based on size, and fish leave these groups upon reaching mature size (70 mm total length) to enter the nearby adult population. Previously, I had demonstrated that larvae preferentially join larger groups and I expected to find clear advantages to group membership. Survival, especially of smaller fish, was enhanced in large groups, but growth, especially by individuals of low social status, was reduced. Consequently, the time to reach maturity increased with group size in both years of the study, suggesting a more rapid entry into the adult population of fish in smaller groups or living alone. However, the probability of reaching mature size (a function of size-specific growth and survival) increased with group size in 1988 but not in 1987, thus indicating a benefit to group living during only one of the two years of the study. Although this study demonstrated density-dependent juvenile growth and survival, with consequent fitness effects of group living, it has also highlighted considerable inter- and intra-annual variability in these relationships. Such variability will have major consequences for predictions of the effects of juvenile ecology on the demography of organisms.