Sublethal damage to Acanthasterplanci as an index of predation pressure

Almost 40 % of adult crown-of-thorns starfish Acanthaster planci examined at Holbourne Island, Great Barrier Reef, showed evidence of predator attack, with arms either missing or regenerating.The identity of the injury-causing agents is unknown.At the time of the survey, Holbourne Island was carrylng an active starfish outbreak consisting of a high density 'front' population, made up of mostly large individuals, and lower density populations of smaller individuals away from the front.Starfish were surveyed on 13 transects chosen to cover as wide a range of local starfish densities as possible.There was significant heterogeneity between transects in the proportion of injured starfish.This could b e explained statistically either as being inversely related to starfish density on the transect or inversely related to mean starfish diameter.As mean dameter and density were strongly positively correlated, these effects were difficult to disentangle These data are consistent with a Type I1 functional response on the part of the predators.This ind~cates that the impact of predation on A. planci populations 1s likely to be reduced at very high population densities, but may be large indeed in 'normal' low density populations.