CRC REEF RESEARCH CENTRE TECHNICAL REPORT No. 30

Fine-scale surveys of crown-of-thorns (Acanthaster planci) in the central Great Barrier Reef region.

U. Engelhardt, CRC Reef Research Centre Ltd
M. Hartcher, CRC Reef Research Centre Ltd
J. Cruise, CRC Reef Research Centre Ltd
D. Engelhardt,CRC Reef Research Centre Ltd
M. Russell,CRC Reef Research Centre Ltd
N. Taylor, CRC Reef Research Centre Ltd
G. Thomas, CRC Reef Research Centre Ltd
D. Wiseman, CRC Reef Research Centre Ltd

EXECUTIVE SUMMARY

In 1998-99, intensive transect-based surveys of crown-of-thorns starfish (Acanthaster planci) and associated live hard coral cover were conducted on 21 mid-shelf reefs located in the Cairns and Central Sections of the Great Barrier Reef Marine Park.

The following listing is a summary of our key results:

1. We recorded a total of 4,032 A. planci on the 21 reefs surveyed. Juvenile starfish (est. age 1) accounted for 2,639 of these with a further 445 individual sub-adults and 948 adult starfish recorded inside the 800 benthic transects sampled. This is the first time in the five-year history of the surveys that juvenile starfish have dominated the sample.

2. Juvenile starfish (est. age 1): The average density of juvenile A. planci across all reefs surveyed in 1998-99 was estimated at 3.30±0.20 individuals per 250 m², which is approximately 13.75 times the previous highest density recorded during the 1995-96 surveys when an average of 0.24±0.02 juveniles was observed across all reefs. Comparable average densities of small crown-of-thorns starfish have, to our knowledge, never before been recorded by any other sampling or monitoring program within the Great Barrier Reef region.

   Highest densities of juvenile A. planci were recorded on reefs located in the offshore Port Douglas and Cairns area. Possible Future Spot Outbreaks (FSO) were detected on 9 individual survey reefs (15-070, 15-084, 16-023, 16-024, 16-057, 16-068, 16-071, 17-004 and 17-034). At each of these reefs juvenile densities above the critical threshold of 2.5 individuals per 250 m² were found within the exposed front reef zone. At Michaelmas Reef (16-060) both reef zones had juvenile densities above the threshold resulting in its classification as a possible Future reef-wide Outbreak (FO). Future outbreaks are expected to develop on the abovementioned reefs within the next 18-24 months.

   Significant or unsustainably high juvenile densities were found in reef areas both affected and unaffected by recent A. planci outbreaks. In areas that had already suffered significant starfish-induced coral mortality over recent years (i.e. remnant live hard coral cover of <10%) we noted an obvious preference of juvenile starfish for feeding on the smallest most recently recruited hard corals. These observations suggest that this latest starfish cohort has the potential to significantly impact on the onset and progress of the coral recovery phase on previously outbreaking reefs.

3. Adult starfish (est. age 3 or older) were significantly more abundant on reefs in the Innisfail to Townsville region compared to adult densities recorded in the more northern regions from offshore Cooktown to offshore Port Douglas and Cairns. Active Spot Outbreaks (ASO) dominated by adult starfish were detected on 9 individual survey reefs (15-024, 15-070, 16-023, 16-071, 17-004, 17-023, 17-034, 17-064 and 18-031). At each of these reefs A. planci densities within the protected back reef zone were found to be above the upper limit of a sustainable population. At Eddy Reef (17-047) adult densities exceeded the sustainable threshold in both the back and front reef zones leading to its classification as and Active reef-wide Outbreak (AO).

4. We found significant size-specific patterns of distribution within reefs. Small juvenile starfish were significantly more abundant in exposed front reef zones. Conversely, adult starfish were more common in protected back reef zones. These findings have important implications for future monitoring studies. If the main objective is the early detection of developing outbreaks (forecasting capability), then it would appear that considerable effort should go into sampling reef front environments. In contrast, if the main objective is an assessment of past recruitment events on reefs (hindcasting capability), then back reef environments may provide a more complete insight into the probable age structure of resident starfish populations.

Our survey results clearly demonstrate the capacity of intensive transect-based surveys to reliably detect the early signs of possible future outbreaks of Crown-of-thorns starfish. The record numbers of small juvenile starfish recorded in 1998-99 provide a strong indication of possible renewed outbreaks likely to develop on many reefs in the Cairns Section of the GBR Marine Park over the next 18 to 24 months. As the identified cohort of small A. planci is not only the largest but also the geographically most widespread age class of young starfish recorded on the Great Barrier Reef to date there should be serious concern about their likely future impact on local and regional coral reef communities.

If renewed outbreaks develop within the predicted time frame a second peak in A. planci activity would have affected reefs in the central GBR region within the space of only 4 to 5 years. Such a pattern would suggest a possible shift away from the previously observed 15 to 17 year cyclicity of outbreak episodes in this region. A significant shortening of cycle periodicity is likely to be unsustainable in the long term as hard coral communities on affected reefs would have insufficient time for complete recovery and regeneration. Such a scenario could result in an increased number of permanently degraded reef areas unable to recover from an increasingly chronic source of disturbance. As our survey area includes many reefs located directly offshore the Reef tourism centres of Port Douglas and Cairns any such trends could have serious implications for the future operations and sustainability of the regional tourism industry.

Efforts aimed at assessing possible indications of such detrimental trends and conditions should be given a high priority. Intensive fine-scale monitoring of A. planci and associated live hard coral cover should not only be continued but extended to maximise the chances of identifying possible signs of further reef degradation and reduced coral recovery rates in the study area in a timely manner.

THIS PUBLICATION IS CITED AS:
Engelhardt, U., Hartcher, M., Cruise, J., Engelhardt, D., Russell, M., Taylor, N., Thomas, G., & Wiseman, D. (1999)
Fine-scale surveys of crown-of-thorns (Acanthaster planci) in the central Great Barrier Reef region
CRC Reef Research Centre Ltd
Technical Report No. 30
Townsville; CRC Reef Research Centre Ltd, 97 pp.

ISBN 1 876054 40 9

A full copy of this report may be obtained from the author(s), and through the following libraries: