Population dynamics of the reef-building coral Acropora hemprichii as an indicator of reef condition

Population dynamics of stony corals may reveal processes of change in the community structure of coral reefs, yet little information is available on species-specific patterns of recruitment, growth, and mortality in reef corals. We report here on population dynamics of the common reef-building coral Acropora hemprichi on a fringing reef slope in the northern Red Sea. Fusion, fission, and partial mortality of colonies were rare in this population. Analysis of population size structure and colony growth and mortality rates revealed a maximum colony age of 13 to 24 yr and rapid population turnover of 10 to 20 yr. A deficiency of juveniles resulted in older, larger, and less abundant coral colonies than expected for a population with stable age structure. Colonies of this major reef-builder appeared to be aging without sufficient replacement, leading to a decline in abundance. Several processes may have contributed to the patterns observed, including variable recruitment over space and time, long-term recruitment failure, and/or low adult mortality. Total percent cover, which was determined mainly by large colonies, was little affected by the paucity of juveniles. We conclude that monitoring of live coral percent cover, even at high taxonomic resolution, cannot detect rapid change in coral assemblages, because it is not sensitive to variation in juvenile abundance and population dynamics. More extensive information on demographic processes, especially species-specific coral recruitment, is urgently needed for the effective and timely management of coral reefs.