Test of a Model of Regulation of Crown-of-Thorns Starfish by Fish Predators

Both a population dynamic model and simplified calculations of the numbers of fish predators that would be required to control heavy recruitment of the coral-eating crown-of-thorns starfish (Acanthaster planci L.) predict threshold densities of fish predators below which starfish population outbreaks may occur. This prediction has been tested through comparable surveys of putative fish predators in the Red Sea, where no major outbreaks of A. planci were found or are known to have occurred, and on the Great Barrier Reef, where two major series of outbreaks have occurred during the last 25 years. In the Red Sea densities of presumed fish predators were found to be well above the predicted threshold. By contrast on the Great Barrier Reef mean densities both of proposed fish predators of juvenile and subadult A. planci, and of know fish predators of adult A. planci, were below the predicted threshold. Moreover the densities of fish predators on four mid-shelf reefs that are believed to have escaped major outbreaks of A. planci were found to be significantly higher than those on otherwise similar but impacted reefs. These data are compatible with the hypothesis that current and recent outbreaks of the crown-of-thorns starfish on the Great Barrier Reef have been facilitated by the presence of only relatively low numbers of fish predators, the numbers of which may have been decreased as a result of the intensification of fishing pressure that has occurred from the 1960s onwards.