Report to GBRMPA (1990)

Preliminary survey of giant triton (Charonia tritonis) on selected reefs in the Cairns region (Hastings, Saxon, Norman), during January 1990.

1. INTRODUCTION

1.1 General

The crown of thorns starfish (Acanthaster planci) is hereafter referred to as starfish except where a distinction is required between different species of starfish. Over the past 30 years, population explosions (outbreaks) of starfish have occurred on coral reefs of the Indo-West Pacific region and these outbreaks have been the subject of extensive research effort and protracted discussion (refer Moran, 1986). Most of the research has centred on establishing the scale of these outbreaks and the effect of starfish predation on the coral reef community.

Little is known concerning the factors which have caused the outbreaks and two schools of thought have evolved. One is based on a premise that such events are natural phenomena (Birkeland, 1982; Sale, Potts and Frankel, 1976) and the other is based on a premise that the observed recent outbreaks are in some way related to human activities. Prominent among the latter is the set of hypotheses based on regulation of starfish abundance by predators, and Endean (1969) discussed the possible causes of starfish outbreaks with particular emphasis on the removal by humans of the predators of adult and juvenile starfish. These latter hypotheses now fall into two subsets. One proposes that under normal conditions critical regulation of juvenile and sub-adult starfish by fish predators results in relative stability of adult starfish numbers (Ormond et al., 1990). The other proposes that, in addition to its role of predatory regulation, the foraging behaviour of the triton prior to starfish spawning essentially precludes starfish egg fertilisation by dispersing most small breeding aggregations of the starfish (present study).

Both subsets of predator regulatory hypotheses are in critical need of field testing. On ecological and also purely scientific grounds, the testing for a role of starfish predators in the dynamics of the starfish outbreak pattern should be regarded as essential information that is required for reef management. One of the reasons for the lack of this strategic information is that predation is difficult to observe in the marine environment. Also, there appears to have been a general view that starfish outbreaks are regular cyclical events, the behaviour of which are independent of predator densities.

The triton is an established predator of the starfish and this shell has been collected by humans for most of recorded history. It is difficult to determine the extent to which the abundance of the triton has been reduced by human activities but it has generally been regarded as uncommon on the Great Barrier Reef (Endean, 1969). Some recent work (Appendix 3,4,5) suggests that the triton may be more abundant, but still not common, in localised areas of high starfish abundance on a reef. This may be a result of the attraction of tritons to their prey which would cause triton aggregation in the vicinity of starfish aggregations. If this is true, it will assist the estimation of triton numbers on a reef by predicting certain areas of higher triton abundance that are searched in detail while other areas of lower abundance are sampled less rigorously.

1.2 Predation on starfish

The idea that predation on starfish plays an important role in the maintenance of a relatively stable population density was initially voiced by Endean (1969). Tritons are confirmed predators of many species of starfish (Chesher, 1969; Endean, 1969; Laxton, 1971; Noguchi et al.,1982; Percharde, 1972), but the preferred prey species appears to vary. Endean found the preferred prey to be a species of Nardoa. Both Chesher and Endean found that a triton eats an average of one starfish per week. Further discussion seemed to conclude that the feeding rate and feeding preference of the triton were insufficient to have any significant effect on adult starfish numbers.

Preliminary data (Appendix 5) suggest that a prey preference for Acanthaster planci may exist but that the increased mobility of this starfish may result in a confounding of experimental variables when the experimental design is not sufficiently precise. This proposition is based on the observation that large starfish can repeatedly escape complete predation (mortality). Their survival following predator attack in experimental enclosures appears to have resulted in confusion between the estimation of prey preference and that of prey capture in previous experiments. With respect to the role of the triton, this distinction between prey preference and prey capture appears to have been overlooked in the assessment of the triton’s ability to effectively regulate starfish numbers at low starfish density.

1.3 Role of aggregation and effect of triton

Chesher noted that the triton can detect and actively seek out its prey from a distance of at least one metre and when contact is made the starfish recognises the predator and moved away rapidly. Endean did not observe such avoidance reaction by the starfish to the presence of the triton. Recent studies (Appendix 3) demonstrate a strong avoidance reaction from the starfish when any one of its sensory tentacles make physical contact with the body of a triton.

Ormond et al. (1973) discussed the consequences of spawning aggregations of starfish and suggested that the increased proximity of adult starfish may enhance the chances of fertilisation, especially if synchronous spawning takes place, as has been described for other echinoderms. Further, they suggested that the population density of starfish at which aggregation into groups begins may constitute a threshold beyond which a population explosion (outbreak) is likely to occur, and that populations of starfish may therefore be particularly sensitive to small changes of significant environmental factors which could result in densities in the region of this threshold.

The effect of sperm dilution, adult aggregation and synchronous spawning upon the fertilisation of sea-urchin eggs was reported by Pennington (1985). Pennington concluded that significant fertilisation occurred only when spawning individuals are closer than a few metres. Percharde (1972) described the attack of the triton upon a breeding aggregation of a starfish Echinaster sentus and concluded that “this mollusc must play an important role in the ecological balance of the extensive areas of its habitat”.

1.4 Summary

Starfish aggregation is reduced by the foraging activities of the triton, when escape by the starfish following attack by the triton causes an increased distance between individual starfish as occurs when starfish are present in small discrete aggregations. The fertilisation of starfish eggs depends on the close proximity of spawning individuals and the disruption of aggregations by the foraging behaviour of tritons may reduce the number of fertilised eggs and thereby provide a limit to future recruitment.

The triton has the potential to play a significant ecological role in low-density population dynamics by direct predation on juvenile and adult starfish and by limiting the aggregation of starfish prior to spawning and thereby limiting egg fertilisation and subsequent outbreak potential.

2. METHODS

The AUSLIG survey vessel MV Febrina was stationed at Hastings Reef and Norman Reef during week 1 and week 2 of this survey respectively. Figures 1 and 2 show positions of manta tows and scuba belt transects for Hastings Reef and Norman Reef respectively.

2.1 Development of sampling methodology

Two methods for estimation of triton and starfish abundance were compared on two reefs. These were conducted with the following frequency:

REEF                                              Hastings       Norman
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METHOD OF SAMPLING

Manta tow                                               2              2

Belt transect                                          11             10

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2.2  Sample Stratification Parameters

2.2.1     Prey Abundance

Three categories of prey abundance were identified in the
field. These were sampled with the following frequency:

CATEGORY                                Manta series        Transects
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ABUNDANCE CATEGORY

Acanthaster    low                                   4              21

Linckia             high                                  2              6
Linckia               low                                  2              15
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2.2.2     Physical Relief

Two categories of physical relief were identified in the
field. These were sampled with the following frequency:

CATEGORY                              Manta series              Transects
____________________________________________________________

RELIEF CATEGORY

Relief high                                       2                                   15

Relief low                                         2                                    6
____________________________________________________________

2.3 Selection of Sampling Sites

A series of manta tows along back-reef and fore-reef slopes was conducted to locate areas of high starfish abundance for subsequent belt transect surveys. Despite extensive manta towing, no sub-tidal area on either reef could be selected for belt transect sampling on the basis of high number of starfish (either Acanthaster or Linckia) and sub-tidal sites for belt transects were selected solely on the basis of high physical relief.

A further series of manta tows were undertaken across the inter-tidal reef flat and reef crest so that some sites could be selected for belt transect sampling on the basis of high starfish abundance. An intertidal spot search by snorkel (diameter approximately 30 metre) was conducted prior to belt transect sampling.

In addition to starfish numbers, the position on the reef, coral cover, life forms and an index of physical relief (substratum complexity) were recorded for each 2 minute manta tow (within constraint of observer experience).

2.4 Belt Transects (4 metre width)

Nine belt transects (100 metre) and two belt transects (75 metre) were conducted on Hastings Reef. Nine belt transects (100 metre) and one belt transect (50 metre) were conducted on Norman Reef.

REEF                                                              Hastings       Norman
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STRATIFICATION PARAMETERS

High starfish, low relief (100 metre)            4              2

Low starfish, high relief (100 metre)            5              7
Low starfish, high relief   (75 metre)            2
Low starfish, high relief   (50 metre)                             1
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3. RESULTS

A preliminary inspection by manta tows and belt transects of the two reefs (Hastings and Norman Reefs) found no live tritons and found no sub-tidal areas of high starfish (Acanthaster or Linckia) abundance. One specimen of Acanthaster planci was located intertidally and although many Linckia laevigata were located intertidally, not one specimen was located subtidally.

The manta tow data are listed in Appendix 1. The belt transect data are listed in Appendix 2. The abundances of each species of starfish encountered during the survey are summarised in Tables 1 and 2.

TABLE 1.
Summary of species located on Hastings Reef during survey.
(spot area=3000 sq m.; manta tows=62 X 2 mins; transect
area=4200 sq m.)

FREQUENCY OF OCCURRENCE ON:   Manta     Spot      Transect
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SPECIES OF STARFISH

Acanthaster planci                                        1
Ophidiaster lorioli                                                                            1
Gomophia watsoni                                                                           2
Nardoa novaecaledoniae                                                                3
Neoferdina cumingi                                                                         4
Fromia monilis                                                                                  1
F. milleporella                                                                                 16
F. elegans                                                                                          15
Linckia laevigata                                     207              50             48
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TABLE 2.
Summary of species located on Norman Reef during survey.
(spot  area=3000 sq m.; manta tows=41 X 2 mins; transect
area=3800 sq m.)

FREQUENCY OF OCCURRENCE ON:   Manta     Spot      Transect
___________________________________________________________

SPECIES OF STARFISH

Asterina burtoni                                                                                 1
Ophidiaster cribrarius                                                                      2
Fromia monilis                                                                                   1
F. milleporella                                                                                    1
F. elegans                                                                                             9
Linckia laevigata                                           68             32            48
L. multifora                                                                                          1
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4. DISCUSSION

Although manta tows were conducted along back-reef and fore-reef slopes, no suitable reef slope areas could be selected for belt transect sampling on the basis of high starfish (Acanthaster or Linckia) abundance. The manta tow data (Appendix 1) showed that all reef slope habitat supported low starfish abundance. By necessity, all back-reef and fore-reef slope belt transect sites were selected solely on the basis of high physical relief (habitat complexity). The belt transect data (Appendix 2) confirmed this low starfish abundance on the reef slope.

These data provide base-line (average) estimates of triton and starfish abundance and may assist future testing of the null hypothesis that tritons are distributed randomly on reefs. These data can be compared with preliminary data from other reefs (e.g. John Brewer Reef – Appendix 5) where it has been suggested that tritons were aggregated in the area of sub-tidal sampling due to the presence of a residual population of starfish. In this survey, the failure to locate any tritons would seem to be a result of the low sub-tidal starfish abundance. The absence of localised areas of high subtidal starfish abundance (area of proposed high triton abundance) resulted in the sub-tidal belt transects being laid at random (unstratified) with respect to starfish abundance.

A comparison of the abundances of each starfish species on different transects (Appendix 2) suggests within and between zone variation in the distribution and abundance of species but further sampling would be required to establish any significant pattern beyond that apparent for Linckia laevigata.

It should be noted that less than one percent of the reef slope (sub-tidal area) was surveyed by belt transect and that the sub-tidal belt transects were not sited on areas of high starfish abundance, that is, the a-priori categories of proposed highest triton abundance did not occur inclusively. The low subtidal abundance of Linckia laevigata is in contrast to observations from John Brewer Reef (Appendix 5) and can be compared with the results of Laxton (1974) that suggest that the abundances of other species of starfish also decline during the period between outbreaks of Acanthaster planci. The reason for this decline in the abundance of other species in addition to A. planci is not understood, but if this decline is a result of triton predation then the residual abundance of all starfish species at the end of the coral recovery cycle may be an indicator of the abundance of tritons and may provide a means of predicting primary outbreak conditions. If reduced triton abundance was thought to be a possible cause of outbreak then an examination of the abundance of all starfish species on a selection of reefs would be of management significance to the Authority.

Use of this survey methodology in regions of projected primary outbreak will identify areas of locally high subtidal starfish abundance with a view to establishing a correlate or cause of this high abundance (e.g. reduced triton abundance). Further survey, in and adjacent to outbreaks, should identify areas of locally high triton abundance with a view to demonstrating the functional response (or attraction) of tritons to aggregations of their prey, and to the subsequent demonstration of the effect of tritons on the abundance and aggregation pattern of starfish.

ACKNOWLEDGMENTS

Thanks to Lyndon DeVantier, Brian Lassig, Ann Poulsen and Leon Zann for collaboration; Ray Williams for logistic support and John Day, Martin Drury, Meryl Sherrah, Mary Speedy and Jarrod Williams for field assistance during this survey. Further thanks to the AUSLIG survey staff and the crew of MV Febrina. This survey was funded by COTSREC.

REFERENCES

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Feeding in some Australian Cymatiidae (Gastropoda: Prosobranchia). Zool. J. Linn. Soc. 50: 1-9.

Laxton, J.H. 1974.
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Ormond,R. et al. 1990.
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Pennington,J.T. 1985.
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Percharde,P.L. 1972.
Observations on the gastropod Charonia variegata, in Trinadad and Tobago. Nautilus, Philad. 85: 84-92.

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