CRC REEF RESEARCH CENTRE TECHNICAL REPORT No. 10

An Investigation To Describe The Population Dynamics Of Acanthaster Planci (L.) Around Lizard Island, Cairns Section, Great Barrier Reef Marine Park .

Richard Stump, Marenre Marine Environment Research

EXECUTIVE SUMMARY

Renewed reports of Acanthaster planci (L.) in the northern Cairns Section of the Great Barrier Reef Marine Park (GBRMP) have prompted opinion that a third episode of outbreaks, since about 1960, is developing in this region. Despite three decades of research effort there is little information about the early stages of outbreaks or their larval sources. This is primarily due to a lack of data on population dynamics and inconsistent use of survey methods that have failed to identify the early stages of outbreaks. To address this issue, this project aimed to investigate methods to describe A. planci population dynamics off Lizard Island through a series of three field exercises using mark/release/recapture (MRR) techniques and the application of a novel method of age determination. The specific aims of the study were to:

1. determine changes in densities of A. planci by conducting adult (daytime) and juvenile (night) surveys and assess their impact on the benthic community.

2. develop the methods of MRR and obtain data for population dynamics parameter estimation and analyses (i.e., age structure, population size, mortality, and recruitment).

3. assess the method of identification of recaptures and establish a reliable protocol for identification.

4. obtain morphometric data for growth analyses.

5. analyse the capture/recapture data and investigate the use of the Jolly-Seber model (JOLLY) to determine population dynamics parameters.

The method to determine age in adult A. planci uses counts of pigment bands on cleaned aboral spine ossicles. Juveniles do not exhibit spine pigment bands but age can, in general, be determined from body size, due to the rapid growth phase leading to maturity. In the present study, individual ages were estimated assuming the validity of the method from evidence obtained in the first MRR study using A. planci on Davies Reef (Central Section). The recommended guidelines for field validation of annual bands involve recapturing marked individuals released for at least 12 months in all band classes from the population. This objective remains a priority for continuing research on A. planci populations.

Core research involved the development of a reliable procedure for individual identification based on coded madreporite patterns and an assessment of the potential for recapturing starfish marked with tetracycline. A. planci were collected from North Point and Granite locations and transported to the Lizard Island Research Station (LIRS) for processing (identification and injecting with tetracycline to mark spine ossicles). They were maintained in aquaria prior to their release at the North Granite site. Of the 806 processed, 718 madreporite patterns were identified and 230 individuals shared 70 of these patterns (i.e., with one or more starfish). Therefore, 68% (488) of the starfish had unique madreporite patterns. Individuals with identical madreporite patterns can, in general, be distinguished using other characteristics (i.e., spine ossicle length, pigment band patterns on spine samples, arm-damage, colour patterns on the aboral body surface, body size and sex). A stepwise protocol was developed for identifying recaptures using these characteristics. Recapture rates derived from the estimated number alive following their release were 7.3% (8 recaptured) and 27.9% (80 recaptured) for the July and October field trips, respectively.

A broad, estimated age structure was determined from the spine ossicle samples, between 14 and 146 months (under the simplifying assumption that recruitment occurs in January each year). The oldest estimated age (14+ years) suggests a potential for this species to persist between recent outbreak episodes, with a similar interval to their estimated longevity (approximately 15 - 17 years). Assuming low adult mortality, the pattern in age structure reflects consecutive annual recruitment, increasing exponentially, over the past 6 to 8 years. This pattern is unlike those previously hypothesised from observations suggesting the sudden development of primary outbreaks.

Temporal changes in starfish densities showed a small but significant decline over the period of the three trips (nine months), particularly in the 1+ year class. The starfish relocated by the MRR experiments may have contributed to this pattern. Highest densities of both juveniles and adults were found at North Point (up to 867 ha-1) and the North Granite release site (up to 1267 ha-1) at 3 m depth where highest live hard coral abundances were found. Apart from the release site, high densities resulted from larval recruitment (1+ juveniles were found at all northern sites) and movement into those areas with higher resources (highest densities in areas of highest coral cover). Analyses of live hard coral and dead coral cover revealed no significant change in adjusted relative abundances with respect to site or depth over the study period, including the release site. The parameters relating to population dynamics were investigated using the general Jolly-Seber model for an open population. The large variation in recapture rates reflected the variation in mortality between the samples. This was considered to be the greatest influence on the error in the model calculations and the parameter estimates should be considered as first approximations only. Recommendations for marking starfish in situ were made to improve the accuracy of population dynamics parameter estimates in future studies.

The results have clear management implications for it is only through this type of study that accurate estimates can be made for recruitment, mortality and population size. These can be used to determine; the timing and effort required to implement effective control exercises (i.e., using the ratio of juvenile to adult sub-populations), the period of time taken to cause a reduction in the levels of hard coral cover (i.e., age structure), and the time taken for the population to attain its present size (i.e., age structure/MRR analyses). There is also potential for new applied research (i.e., before-and-after MRR studies for assessment of the efficacy of experimental control actions).

This study has demonstrated the potential significant contribution of MRR population dynamics studies in understanding how A. planci outbreaks occur. Therefore, with reasonable expectation of the third large-scale outbreak episode since the 1960's, a unique opportunity exists to describe the patterns of recruitment (temporal and latitudinal), population dynamics and growth. In addition, complementary data are required to assess the variation in the life-history characteristics of growth, mortality and reproduction. This could be achieved by immediate implementation of population dynamics and morphometry studies (including the MRR exercises) on representative reefs in the Cairns and Central Sections of the GBR.

THIS PUBLICATION IS CITED AS:
Stump, Richard. (1996)
An investigation to describe the population dynamics of Acanthaster planci (L.) around Lizard Island, Cairns Section, Great Barrier Reef Marine Park.
CRC Reef Research Centre Ltd
Technical Report No. 10
Townsville; CRC Reef Research Centre Ltd, 56 pp.

ISBN 1 876054 09 3

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