In 1989, reefs in the central GBR were surveyed to locate high density populations of larger (5-15 cm) juvenile crown-of-thorns starfish (Acanthaster planci). Preliminary surveys were undertaken at two inshore reefs (Hayman Island and Reef No. 20-019) in October 1989. During November 1989, searches were carried out at different depths at 45 sites on 12 reefs. The reefs surveyed were 19-123, 19-124, 19-131, 19-138, Bait, Bowden, Davies, Hope, Little, Oublier, Rafter and Stanley.
All reefs surveyed were relatively small and had supported large numbers of starfish during the 1970s. At the time of survey they either supported large numbers of adult starfish or were located immediately to the south of the main concentration of reefs with outbreaks. A range of reef types across the continental shelf were surveyed. The perimeter of each reef was surveyed by manta tow to identify sites with relatively high live coral cover and small areas of dead coral then 3 or 6 sites were selected for more intensive SCUBA swim searches.
SCUBA searches were undertaken by divers along 4 depth contours at each site: reef crest/flat, 5m, 10m and 15m. Where this was impractical, the divers swam parallel to each other and covered the maximum practical depth range between the crest and the base of the reef slope. The four divers swam slowly along their respective depth contours for a set period of twenty minutes, covering a distance of approximately 100m. Whilst swimming, each diver scanned approximately 1m either side of the swim path looking for any small starfish or evidence of their presence. All small areas of recently dead coral were closely examined and if no starfish were found, then an attempt was made to determine the cause of mortality.
The following observations were recorded during each 20 minute swim search: number of adult crown-of-thorns starfish, number and size of juvenile (<150mm), crown-of-thorns starfish, number of small coral scars attributed to the gastropod Drupella and the number of small coral scars for which no cause of mortality could be identified.
Additional dives were undertaken at night on Stanley and Bait Reefs, in areas where numerous small coral scars had been recorded during the day. These involved 5 divers who searched for juveniles within an area approximately 100x50m, over a 90 minute period.
