Estimates of the abundance of the crown-of-throns starfish Acanthaster planci in outbreaking and non-outbreaking populations on reefs within the Great Barrier Reef

Abstract

Criteria used to define outbreaking and non-outbreaking populations of Acanthaster planci are described. Estimates of starfish density were derived for these population types from a series of intensive underwater surveys and from calibrated manta-tow surveys of 334 reefs within the Great Barrier Reef, from January to October 1987. By comparison of these data, density differences were identified between the two populations. Both the mean density and total numbers of non-cryptic crown-of-thorns starfish in outbreaking populations are at least one order of magnitude greater than that in non-outbreaking populations. Non-outbreaking populations may comprise up to 1500 starfish km-2, an order of magnitude less than some previous estimates for reefs within the Great Barrier Reef. Densities of ≳1500 starfish km-2 indicate that an individual reef may be experiencing an outbreak. This density is equivalent to an average of 0.22 starfish per 2 min manta tow over the whole reef. Approximately 90% of reefs classified as out-breaking on the Great Barrier Reef had densities of >1500 starfish km-2, while about 96.5% of reefs classified as non-outbreaking had densities below this level.