The Importance of Recruitment to the Dynamics of a Coral Reef Fish Population

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Abstract
An experiment on a coral reef fish is described that assesses the effects of recruitment success on adult abundance over a 3—yr period. The damselfish Pomacentrus amboinensis was examined on natural patch reefs of °8 m2 in the lagoon of One Tree Reef. Recruitment levels were adjusted to 0, 0.5, 1, and 2 recruits m—2°yr—1 by transplanting juveniles among reefs at the end of four successive recruitment seasons (with four replicates per treatment). Natural recruitment success was monitored on 16 undisturbed reefs, and adult numbers were monitored at irregular intervals on both experimental and undisturbed reefs. The adult population density at the end of the experiment increased significantly with recruitment within the 0—1 recruit/m2 range. However, doubling recruitment from 1 to 2 recruits/m2 did not lead to any further increase in adult density. The recruitment treatments did not affect adult density until a 2—yr period had elapsed (corresponding to average maturation time). At this time, treatments receiving 1 and 2 recruits/m2 showed a dramatic increase in adult numbers, whereas in the low—recruitment treatments, adult densities declined. The experiment indicated that a recruitment success of between 0.5 and 1 recruit/m2 was necessary to maintain adult numbers at pre—experimental levels. Natural recruitment success was highly variable among years, but adult densities remained relatively constant over time at °0.8 adult/m2. Differences in the final adult densities among reefs were correlated with average recruitment levels, but above 0.8—1.0 recruit/m2, adult densities did not increase with increasing recruitment success. The results from experimental and undisturbed reefs suggest that natural variation in recruitment success affects the importance of post—recruitment processes. In good recruitment years, adult densities on a substantial number of reefs may be limited by density—dependent processes, which may affect the majority of individuals in a region. Density—dependent juvenile growth may buffer adult populations on these reefs against the effects of poor recruitment years, since input may come from a number of immature age classes. Where recruitment success is always below a certain level, the dynamics of adult populations will reflect variation in recruitment success.