Acanthaster planci on the Great Barrier Reef: Swain Reefs and northern surveys in 1975

Abstract

Surveys were undertaken in 1975 as part of a continued monitoring program of the distribution and abundance of Acanthaster planci and of the extent of coral damage on reefs of the Great Barrier Reef. Two extensive areas were chosen for which little or no information was previously available. A spot check technique was used which enabled us to survey vast tracts of reefs quickly and inexpensively, and which provided an index of starfish abundance. In July 1975, a 10 day survey was made of 85 of the approximately 340 charted reefs in the Swain Reefs Complex near the southern end of the Great Barrier Reef. One minute observations were made at 1205 spot checks or stations. Each observation was made from the surface on the reef perimeter, of a section of reef slope to a maximum depth of 20m. No starfish were observed on 59 reefs (69.4%) at 858 stations (71.2%). A total of 526 starfish were observed at 84 of the 347 stations on the remaining 26 reefs. Numbers of starfish observed per reef ranged from 1 to 104 with a maximum of 54 individuals counted at one station. The majority of starfish ( 489) and the greatest coral damage attributable to Acanthaster were observed on 28 reefs surveyed in the northeast sector of the Swain Reefs (between 21°00'S, 152°10'E and 21°16'S, 152°38'E). Nineteen reefs in this sector were assessed as being extensively damaged. In November 1975, an 11 day survey was made of 68 of the approximately 350 charted reefs near the northern end of the Great Barrier Reef between 12°36'S and 10°30'S. Observations were made at 1195 stations. Only 5 Acanthaster were observed. Eight reefs were assessed as being extensively damaged and all were located along the outer barrier between 10°46'S and 10°33'S. This damage, while not recent, we consider attributable to Acanthaster. To account for starfish infestations in the Swain Reefs Complex an hypothesis involving larval transport from infested reefs north of 20°S is suggested. Future developments of the infestations to the north of the Pompey Complex and in the Swain Reefs Complex are considered.