Distribution, Abundance, and Reproductive Success in the Coral Reef Fish Acanthochromis polyacanthus

The relationships between population density, adult size, and various reproductive parameters (including summed reproductive effort and reproductive success) were examined for the planktivorous coral reef fish Acanthochromis polyacanthus. Data were based on multiple censuses conducted over a 2—yr period on One Tree Reef (southern Great Barrier Reef), at 10 sites that cover the range of habitats occupied by the species. Although several reproductive parameters varied significantly over this habitat range, between—site differences in mean reproductive effort per pair were not significant, and mean reproductive success per pair were significantly reduced at only 2 of the 10 sites. Multivariate analysis suggested that heterogeneity among sites in the mean number of broods produced per pair per spawning season is the major determinant of between—site differences in reproductive success, alone accounting for 69% of the among—site variance in reproductive success in 1979—1980, and 67% in 1980—1981. Principal components analysis of nine apparently independent population and fitness parameters over the 10 sites indicates 82.9% of the communal variance can be accounted for by three orthogonal source variables. These appear to relate to food availability, rates of predation, and, most likely, seasonal variation in water temperature. Results are consistent, overall, with the hypothesis that local populations of A. polyacanthus are equilibrial and resource limited. If this hypothesis is correct, adult density is a valid index of habitat quality. Theoretical considerations suggest that this is not likely to be true for most other small, site—attached species of reef fishes.