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 o f individual group members and also demography of the population. The effects of grouping on the growth, survival, and attainment of maturit y of juveniles of an Hawaiian coral-reef damselfish (Dascyllus albisel la) were evaluated from 1987 through 1989. In this species, pelagic la rvae settle (at approximate to 14 mm total length) on coral heads, joi ning temporary groups of up to 15 juveniles. Group members establish a dominance hierarchy based on size, and fish leave these groups upon r eaching mature size (70 mm total length) to enter the nearby adult pop ulation. Previously, I had demonstrated that larvae preferentially joi n larger groups and I expected to find clear advantages to group membe rship. Survival, especially of smaller fish, was enhanced in large gro ups, but growth, especially by individuals of low social status, was r educed. Consequently, the time to reach maturity increased with group size in both years of the study, suggesting a more rapid entry into th e 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-dependen t juvenile growth and survival, with consequent fitness effects of gro up living, it has also highlighted considerable inter- and intra-annua l variability in these relationships. Such variability will have major consequences for predictions of the effects of juvenile ecology on th e demography of organisms.