Mortality rates of small juvenile crown-of-thorns starfish Acanthaster planci on the Great Barrier Reef: Implications for population size and larval settlement thresholds for outbreaks

Abstract

Crown-of-thorns starfish Acanthaster planci are a significant predator of scleractinian corals, with outbreak populations continuing to impose large-scale mortality on coral reef communities. We measured mortality rates of small post-settlement juvenile A. planci in a caging experiment on the Great Barrier Reef. Starfish 3 mm in diameter suffered mortality rates of 2.6% d-1, of which 73.0% was attributed to mobile predators. Starfish 13 mm in diameter had a lower rate of mortality of 0.82% d-1. There were no differences in mortality rates between sites, and localised effects, such as the presence of specific predators like the shrimp Hymenocera sp., affected within-site variability in mortality rates. The coral rubble habitat where the experiments were conducted had a large suite of generalist putative predators including both fish and invertebrates. The results from this and previously published studies were used to develop a model of size- and age-dependent mortality, which, when applied with the commonly accepted destructive outbreak threshold for adult A. planci of 10 ha-1 at age 2.5 yr, would require larval settlement rates of 5 m-2. Parameterising the magnitude of likely settlement rates and rates of post-settlement mortality provides a significant advance in understanding and modelling the population dynamics of this important coral predator.