Background mortality rates for recovering populations of Acropora cytherea in the Chagos Archipelago, central Indian Ocean

This study quantified background rates of mortality for Acropora cytherea in the Chagos Archipelago. Despite low levels of anthropogenic disturbance, 27.5% (149/541) of A. cytherea colonies exhibited some level of partial mortality, and 9.0% (49/541) of colonies had recent injuries. A total of 15.3% of the overall surface area of physically intact A. cytherea colonies was dead. Observed mortality was partly attributable to overtopping and/or self-shading among colonies. There were also low-densities of Acanthaster planci apparent at some study sites. However, most of the recent mortality recorded was associated with isolated infestations of the coral crab, Cymo melanodactylus. A. cytherea is a relatively fast growing coral and these levels of mortality may be biologically unimportant. However, few studies have measured background rates of coral mortality, especially in the absence of direct human disturbances. These data are important for assessing the impacts of increasing disturbances, especially in projecting likely recovery.

► Background rates of mortality for Acropora cytherea in Chagos were quantified.
► Chagos is an important study location isolated from most anthropogenic disturbances.
► Estimated rates of annual mortality were between 1.01 and 16.38%.
► Probability of whole colony mortality did not decline with increasing size.
► Background mortality rates are important for projecting population recovery.