Peter Moran

Estimates of the abundance of the crown-of-throns starfish Acanthaster planci in outbreaking and non-outbreaking populations on reefs within the Great Barrier Reef

Criteria used to define outbreaking and non-outbreaking populations of Acanthaster planci are described. Estimates of starfish density were derived for these population types from a series of intensive underwater surveys and from calibrated manta-tow surveys of 334 reefs within the Great Barrier Reef, from January to October 1987. By comparison of these data, density differences were identified between the two populations. Both the mean density and total numbers of non-cryptic crown-of-thorns starfish in outbreaking populations are at least one order of magnitude greater than that in non-outbreaking populations. Non-outbreaking populations may comprise up to 1500 starfish km-2, an order of magnitude less than some previous estimates for reefs within the Great Barrier Reef. Densities of ≳1500 starfish km-2 indicate that an individual reef may be experiencing an outbreak. This density is equivalent to an average of 0.22 starfish per 2 min manta tow over the whole reef. Approximately 90% of reefs classified as out-breaking on the Great Barrier Reef had densities of >1500 starfish km-2, while about 96.5% of reefs classified as non-outbreaking had densities below this level.

Distribution of recent outbreaks of the crown-of-thorns starfish (Acanthaster planci) along the Great Barrier Reef: 1985–1986

A large survey program was conducted during 1985/1986 to determine the extent of activity of the crown-of-thorns starfish, Acanthaster planci, and its broad effects on the coral communities of the Great Barrier Reef (GBR). The perimeters of 228 reefs (about 9% of reefs in the GBR system) were surveyed within 1 year using rapid survey, manta tow techniques. These reefs encompassed the broad latitudinal and longitudinal gradients within the GBR. Approximately 27% (62 reefs) of the reefs surveyed had recently experienced (18%), or were experiencing (9%), an outbreak of the crown-of-thorns starfish. These outbreaks were mainly confined to reefs in the central third of the GBR (between Lizard Island and Townsville) and had affected, to varying degrees, approximately 65% of the reefs surveyed within this region. A greater proportion of mid-shelf reefs had experienced outbreaks than outer-shelf reefs, although this difference was not statistically significant. Of the small number of inner-shelf reefs surveyed, none had been recently affected by an outbreak. Large active outbreaks of starfish were reported on many of the reefs located off Townsville while much smaller outbreaks were found on several reefs at the southern end of the GBR, in the Swain Reef complex. Almost 86% of reefs currently experiencing an outbreak had moderate to high coral mortality over at least a third of their perimeters. Only 10% of reefs with active outbreaks had high coral mortality over most of their windward and leeward margins. A similar proportion of reefs had low to moderate coral mortality over less than a third of their perimeters.

Coral reef communities and the crown-of-thorns starfish: Evidence for qualitatively stable cycles*

The interaction between the crown-of-thorns starfish, Acanthaster planci , and its prey, the coral community, is a major source of disturbance on coral reefs. It displays characteristic dynamics with variable time scales and is found with the same dynamics on reefs with different types of coral communities. The cyclicity and stability of these dynamics agree qualitatively with the stable limit cycles of current ecological theory. We argue that the analogy of these qualitatively stable cycles with stable limit cycles is compelling and warrants the qualitative extension of the theory to inherently variable systems such as coral reefs. We show that the evidence for what drives these cycles is ambivalent. The cycles may be purely endogenous, probably driven by asymmetries in response of prey and predator to each other induced through the archipelagic nature of the reef ecosystem. On the other hand, the cycles may be the result of exogenous perturbations mitigating an otherwise endogenous spiral to a qualitatively stable point—here called a metastable state. The existence of such qualitatively stable cycles and metastable states may cause problems for management of infestations since intervention could produce counter productive results.

Distribution of Acanthaster planci outbreaks on the Great Barrier reef between 1966 and 1989

Analysis of data from 1966 to 1989 indicates 2 periods of abundant starfish outbreaks on the Great Barrier Reef (GBR). While the data for the first peak of activity (1966–1975) are relatively limited, the data for the most recent peak of activity (1981–1989) support the hypothesis of southward moving waves of outbreaks. The southward drift of outbreak activity is consistent with speed and direction of average summer currents on the GBR but the concept of a discrete seed area to initiate the wave is not substantiated, nor testable, with presently available data. As the present wave of outbreaks appears to be declining in the central section of the GBR (17–19°S) small residual populations may remain. If the outbreaks are coupled to coral recovery patterns then the next period of high starfish activity in the central section would be expected in the late-1990s.

Bias in manta tow surveys of Acanthaster planci

To investigate the biases associated with manta tow surveys of Acanthaster planci, counts obtained by manta-towed observers were compared with counts made on SCUBA swims under a limited range of conditions. Five 10 m wide strip transects on different parts of two reefs and with different densities of A. planci were surveyed. On average, 22.7% of starfish counted on SCUBA searches were counted on manta tows over the same transect (SD=12.0%, n=69). This proportion is termed sightability. As sightability decreases with increasing transect width, we estimate that, on average, less than 5% of the A. planci present are counted on routine manta tows which are conducted over transect of undefined width. Multiple regression analysis was used to determine which of 33 variables explained most of the variation in sightability in 10 m wide transects. The most influential variables were the proportion of cryptic starfish and an index of the degree of reef complexity. A regression equation designed to improve estimates of the abundance of A. planci on routine manta tows was developed. However, as it explained only 39% of the variation in sightability, this equation is of limited value in stabilising the negative bias associated with manta tow counts. In view of the variability of this bias, the manta tow technique is not suitable for estimating absolute densities of A. planci. Manta tow surveys are appropriate for identifying gross relative differences between densities of A. planci, and thus for determining broad-scale patterns of abundance. As such, they are a cost-effective method of estimating the geographical extent of A. planci outbreaks.

Evaluation of a crown-of-thorns starfish (Acanthaster planci) control program at Grub Reef (central Great Barrier Reef)

A crown-of-thorns starfish control program was conducted at Grub Reef (central Great Barrier Reef) in an area (0.64 km2) which encompassed 53 individual patch reefs. During a two week period, 15 divers injected 3175 starfish with copper sulphate. The program was considered unsuccessful. Although starfish abundance had declined significantly after the control efforts, biological surveys indicated that a relatively large number of starfish remained. The surveys also indicated a general decline in the number of starfish along the reef perimeter, outside the control area. The total cost of the control program was

An assessment of the geological evidence for previous Acanthaster outbreaks

35 per starfish. These results have important implications for the implementation of future control programs and highlight the need to undertake before and after biological surveys to assess the effectiveness of the control efforts.

ACANTHASTER-PLANCI (L) - BIOGRAPHICAL DATA

Much debate has surrounded the notion that outbreaks of the crown-of-thorns starfish (Acanthaster planci) have occurred in the geological past and hence are natural phenomena. As this debate has recently been renewed, we have reassessed statistically data presented by Frankel (1977, 1978) as evidence for the occurrence of past outbreaks. This was done using Frankels data as well as those from extensive starfish surveys conducted prior to the commencement of his research. Our analysis of these data indicates that the occurrence of A. planci remains in recent sediments is independent of whether or not the reef from which the sample was collected had experienced a recent outbreak. Based on this premise, it is not possible to infer from Frankels data the occurrence of past outbreaks from similar material in much older sediments. Thus while the data presented by Frankel (1977, 1978) may show that A. planci has existed within the Great Barrier Reef for at least several thousand years it does not demonstrate that outbreaks of this starfish have occurred in the geological past.

Acanthaster planci outbreak initiation: A starfish-coral site model

Systematics. Common names: crown-of-thorns starfish (e.g. Australia, USA), bula (e.g. Fiji), rrusech (e.g. Palau), alamea (e.g. Samoa, Tonga). First recorded: George Rumphius in 1705. Classified: 1758 by Linnaeus. Close relatives: A. brevispinus, A. ellisii probably not a valid species b. Possible ancestors: few fossil records, hypothesised that A. planei may have evolved from an A. brevispinus-type ancestor b.

The crown-of-thorns starfish, Acanthaster planci, on the great barrier reef

Abstract A lumped parameter model of the dynamics of the crown of thorns starfish (predator) and hard corals (prey) based on simple recruitment, growth and feeding equations was used to simulate the conditions that would lead to a starfish outbreak. The logical consistency of hypotheses concerning the onset of starfish outbreaks were tested by simulation. Results showed that adult starfish populations could change from extremely low to outbreak density within 1 year under the ‘runoff’ hypothesis and the ‘predation of larvae’ hypothesis. All other hypotheses showed that exponential increase of starfish populations could lead to outbreaks within a few years. Therefore, the testing of several of the hypotheses depends on whether outbreaks are the result of single massive recruitment to an otherwise small population or whether the outbreaks build over several years at an exponential rate and future field data collection should concentrate on low density starfish populations in the pre-outbreak phase. The model demonstrates not only the intuitively obvious result that processes influencing larval mortality can have a radical effect on the size of adult populations, but also the less obvious result that small changes in adult mortality and possibly juvenile mortality, can also cause the starfish to outbreak owing to density-dependent effects. The predation hypotheses are logically sound if the rate of predation on the starfish varies markedly with starfish population size. This density-dependent predation result is similar to that found for other outbreaking species such as the spruce budworm.