In April 1985, massive corals on 5 mid-shelf reefs (Green Island, Feather Reef, John Brewer Reef, Rib Reef and Wheeler Reef) and 1 outer-shelf reef (Potter Reef) were surveyed for feeding scars caused by the crown-of thorns starfish (Acanthaster planci). Each reef, with the exception of Wheeler Reef, had experienced crown-of-thorns starfish outbreaks between 1962 and 1985.
The survey involved 2 divers using SCUBA equipment swimming along randomly placed 200 m (approximately) sections of the reef perimeter. These swims were carried out in a slow meandering pattern covering as much of the reef slope as possible to a depth of approximately 12m. Various size classes of Diploastrea heliopora and a number of species of Porites were surveyed. Each individual colony encountered was recorded in 1 of 4 diameter categories and 1 of 4 proportional damage categories. The height difference between the scarred surface and the living surface was measured using a specially built PVC height gauge.
Shallow cores (to 20 cm) were taken in the dead surfaces of some Porites colonies for the purpose of dating the dead surface. For each colony, measurements were made of total height, maximum diameter, the diameter perpendicular to the maximum diameter, and 5 independent measurements of the heights between adjacent 'steps', where a 'step' is the distance between adjacent dead or living and dead surfaces normal to the lower surface. The cores were bleached, sun dried and logged. The dead surfaces were then dated using a fluorescent banding technique.
This study set out to examine the feasibility of eliciting reliable data on previous crown-of-thorns (Acanthaster planci) aggregations by dating surface scars on massive corals. The objectives of the surveys were to assess the level of damage caused by the most recent Acanthaster planci outbreaks on various size classes of Porites and Diploastrea, and to locate suitable specimens for detailed measurement and dating of scars.
This research was a component of 'The Crown-of thorns Study', which was supported by the Commonwealth Community Employment Program'
Statement: Size categories:
0.6 to 1m
1 to 2m
2 to 3m
1/3 to 2/3
Two terms were used to describe the measurements taken of the level of the dead surfaces in relation to the living surface. 'Step' is the distance between adjacent dead or living and dead surfaces normal to the lower surface. The 'level' is the distance between the dead surface and the living surface, and is the sum of the steps.
Only Porites was cored as the Diploastrea skeleton was very dense and quickly dulled the corer. The core was taken as close as possible to the step of the adjacent living or dead surface and the height of the step was recorded. Location of the core close to the adjacent step assured that the step height provides the best possible measurement of the amount of coral growth between adjacent levels. This measurement was used in a regression analysis to correlate step measurement with the time of death.
The coring apparatus was a hand held pneumatic drill driven by air from a SCUBA tank. The cores were taken with a 20 mm diameter hole saw attached to a stainless steel barrel 20 cm long. Cores were removed using a tapered split ring which was driven down the core barrel after the full depth had been reached. The barrel was supported by a tripod stand that was firmly attached to the colony with 100 mm 'Ramset' nails. The cores were labelled for vertical orientation and, if broken, the pieces were marked accordingly. The time taken for each core ranged from 5 to 20 minutes depending on the operator, and the air pressure in the scuba tanks.
The dead surfaces were dated using the fluorescent banding technique developed by Isdale:
Isdale PJ (1984) Fluorescent bands in massive corals record centuries of coastal rainfall. Nature, 310 (5978): 578-579.
Yellow-green bands which appear fluorescent under ultra-violet light correlate strongly with known summer monsoonal rainfall and coastal runoff. The technique allowed estimation of the year at death by comparing the intensity and spacing of fluorescent bands with those on standard reference cores held by the Institute. The reference cores, provided by P. Isdale, were taken on or close to each of the 6 reefs included in this study. Their fluorescent banding patterns had been calibrated by P. Isdale, and particular distinctive fluorescent sequences were clearly characteristic for different year sequences. All cores were sectioned longitudinally, and polished to prevent shadowing. Those cores in which the banding was still not clearly defined were then acid etched and oven dried.
To aid dating of the cores, certain other useful indicators of age were used, including the degree of fouling on surfaces. These indicators were derived directly from the specimen and field notes, and were used along with the level measurements to guide direct comparisons with appropriate periods on the reference core.
Done, Terence J (Terry), Dr (Principal Investigator)