Pattern of outbreaks of crown-of-thorns starfish (Acanthaster planci L. ) along the Great Barrier Reef since 1966

Abstract

Two series of outbreaks of the crown-of-thorns starfish, Acanthaster planci (L.), have been recorded on the Great Barrier Reef since the early 1960s. Data from scientific surveys and reliable unpublished observations were analysed statistically to quantify the pattern of movement of these outbreaks. Data on outbreak populations were obtained from 84 and 97 reefs for the periods 1966-74 and 1979-91, respectively. Outbreaks during the former period were found to occur further south with time, their rate of movement increasing from 49 km year^-1 in 1966 to almost 81 km year^-1 by 1974. Two sets of outbreaks were identified during the 1979-91 period, one occurring to the north, and the other to the south, of latitude 16ºs. The 'southern drift' in outbreaks was found to be the more dominant of the two, 'travelling faster' for a greater period. Its initial rate of movement was estimated at 77 km year^-1 compared with 57 km year ^-1 for the 'northern drift'. The latter travelled 150 km before it stopped in 1984. In contrast, the 'southern drift' had travelled a distance of 560 km by 1991, although it had also become almost stationary by then. Outbreaks that moved south during 1979-91 followed a path similar to those that occurred in the 1966-74 period. This similarity in the outbreak paths was particularly evident during the first 6 years of each period. In all, 35 reefs were recorded as experiencing both outbreaks, the average time between each being 14.68 years. The origin of the northern and southern patterns in the 1979-91 period was found to coincide, suggesting that the likely epicentre for primary outbreaks is close to latitude 16ºs.