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Discussion of the methodological problems associated with estimates of Acanthaster planci (crown-of-thorns starfish) density on the GBR.

A. M. Ayling and Avril L. Ayling

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Contents


List of Tables

Table 1.
Sampling Locations and Times for the Present Study and that of Moran et al. 1988.
Table 2.
Comparison of Acanthaster Abundance Estimates Made From Transect Counts With That Obtained From Manta Tow Techniques.
Table 3.
A Comparison of Crown-of-Thorns Density Estimates Between Transect Counts and Two Types of Manta Tows.
Table 4.
Spatial Distribution of Crown-of-Thorns Populations on the GBR.
Table 5.
Spatial Distribution of 'Outbreak' Crown-of-Thorns Populations on the GBR - Comparison of Transect Count Results with AIMS Manta Tows.

Abstract.

We made comparisons between two techniques for estimating crown-of-thorns density on coral reefs: counts along 50 x 20 m strip transects; and manta tows. On 156 reefs along the length of the Great Barrier Reef (GBR) we made ten haphazard transect counts of crown-of-thorns numbers in a single back reef site between Feb. 1983 and Jan. 1986. On 111 of these reefs a separate team made concurrent manta tows surveys of crown-of-thorns populations and coral communities around the entire reef perimeter, including the transect survey site. On average the manta tows of the entire reef perimeter sampled 10x the area sampled using transect counts. Concurrent manta tows through the transect count site recorded about 10% of the crown-of-thorns counted with transects. In a number of cases high densities of crown-of-thorns were not detected by manta tow surveys. Manta tows of the entire perimeter of each reef recorded on average only 80% of the starfish counted at a single back reef site using replicate transects.

We also compared our transect count data with a GBR-wide manta tow survey of 228 reefs reported by Moran et al. (1988) that employed a different manta tow technique to that used for the concurrent surveys. On the 70 reefs where our transect counts were made during a comparable time period to these manta tows, the manta tows of the entire reef perimeter also recorded similar numbers of crown-of-thorns to transect counts at a single back reef site. As well as underestimating crown-of-thorns density manta tows detected a markedly lower percentage of reefs with low to moderate densities of starfish (23% and 17% of reefs surveyed) than did the transect counts (36% of reefs). This suggests that low to moderate densities of crown-of-thorns are often not detected using manta tow techniques and that these techniques may be of little utility for detected outbreaks in early stages. The three techniques of density estimation all gave comparable patterns for the distribution of reefs within the GBR region that were suffering crown-of-thorns outbreaks, with the majority of outbreaks occurring in the central third of the length of the GBR. Both manta tow and transect count techniques also gave similar estimates for the percentage of reefs affected by outbreaks.

We suggest that replicate transect counts give more accurate and reliable estimates of crown-of-thorns starfish density than either manta tow technique. Although the transect counts reported here were made at a single back reef site about a kilometre long, reef-wide transect based surveys can be made in a comparable time frame to manta tow surveys of the entire reef perimeter if a suitable sampling design is used.

Discussion of the methodological problems associated with estimates of Acanthaster planci (crown-of-thorns starfish) density on the GBR.

Introduction.

There is still considerable dispute over the extent and cause of outbreaks of the crown-of-thorns starfish Acanthaster planci. This is particularly so in the GBR region where both the severity of damage and the role of human activities in initiating outbreaks are subject to constant debate (see review by Moran, 1986). Part of the reason for this debate is the lack of reliable information on the density and distribution of crown-of-thorns populations on the GBR, and the changes in these populations through time. The particular problems of synthesising the history of abundance patterns through the second outbreak are emphasised by Moran et al. (1988).

Reliable estimates of abundance of crown-of-thorns are essential, both to determine the status of populations on a particular reef, and in any study of the ecology and biology of the starfish. The majority of the surveys of crown-of-thorns abundance on the GBR have used various manta tow techniques with free divers (e.g. Endean, 1974; Kenchington and Morton, 1976; Done et al., 1982; Nash and Zell, 1982; Moran et al., 1987). The Great Barrier Reef Marine Park Authority (GBRMPA) incorporated crown-of-thorns abundance estimates into their rapid reef reconnaissance manta tow technique developed by T.J. Done (Done et al., 1982). Moran et al. (1988) report the results of a GBR-wide manta tow based survey of 228 separate reefs carried out by the Australian Institute of Marine Science (AIMS) during a 12 month period in 1985/86. They concluded that 62 of these reefs (27%) were either experiencing a crown-of-thorns outbreak or had experienced one in the previous 5 years (since 1980).

There has been considerable discussion recently about the methodology used in all these manta tow surveys and a study by Fernandes (1990) using the AIMS personnel has suggested that only 5-20% of the starfish present on a reef are detected using manta tow techniques. There is an obvious need to compare the manta tow method with other routine sampling methods such as the use of strip transects. The central question is, what is the most accurate and effective method of sampling crown-of-thorns density?

Between February 1983 and January 1986 we made quantitative surveys of a variety of reef organisms, including crown-of-thorns starfish, on 156 reefs along the length of the GBR for the Great Barrier Reef Marine Park Authority GBRMPA) (Ayling and Ayling, 1983a, b; Ayling and Ayling, 1984a, b; Ayling and Ayling, 1985; Ayling and Ayling, 1986). These surveys were to be used by the GBRMPA to help make zoning decisions within the GBR Marine Park. Our estimates used replicate strip transect counts, usually run haphazardly within a single back reef site comprising about a kilometre length of reef edge. On 111 of these reefs we supervised concurrent surveys of the reef perimeter made by a separate team using the GBRMPA rapid reconnaissance manta tow technique.

The GBR-wide manta tow surveys of Moran et al. (1988) were made during the same time period as our surveys: January 1985 - February 1986, compared with February 1983 - January 1986. The two surveys had 102 reefs in common.

This paper has two main aims that are summarised by the following questions:

  1. How did the density estimates made using transect counts compare with the concurrent estimates made using the GBRMPA rapid reconnaissance manta tow technique? Were transect counts more reliable and accurate?

  2. Were the patterns of density and patterns of frequency of outbreaks we extracted from our transect counts comparable to those obtained by Moran et al. (1988) using manta tow techniques?

Methods.

Transect Counts

In the quantitative transect surveys carried out by the authors crown-of-thorns numbers were counted in replicate 50 x 20 m transects. Ten haphazard transects were run within a back reef (leeward) survey site comprising approximately a kilometre of reef edge. The site surveyed was chosen haphazardly, usually being the nearest area of back reef to the site selected for an anchorage by the mother vessel we were operating off. For each transect a 50 m fibreglass tape was run from the edge of the reef down the slope approximately at right angles to the edge of the reef and careful searches were made by two experienced observers to locate all crown-of-thorns in 10 m strips each side of the central tape. Densities were converted to numbers per ha. The depth at the end of each transect varied from 10 to 25 m depending on reef topography. The transect counts sampled a total area of a hectare within an approximately 1 km length of reef edge. Each group of ten transects took between 2.5 and 3 hours to complete. All the groups of transect counts were made by the authors, keeping problems of variable observer bias to a minimum. Locations and times for the surveys are shown in table 1 and figure 1

Table 1.  Sampling Locations and Times for the Present Study and that of Moran et al. 1988.

To measure the abundance of corals and dead standing hard corals ten 10 m intersect transects were recorded at each survey site. Total intersections for hard coral and dead standing hard coral were recorded along separate random 10 m segments of the 50 m tape run out to define the crown-of-thorns count area. On early surveys estimates of the abundance of hard and dead standing corals were made.

On all reefs the counts were made at a single back reef (leeward) site. If the back reef was a bommie field rather than a continuous reef slope the site was designated as an approximately 500 m square area of the bommie field and transects were run on a haphazard selection of 4-5 bommies within this area. On a few reefs several different sites were surveyed for comparison with different manta tows (a total of 11 extra sites).

GBRMPA Rapid Reconnaissance Manta Tows

A separate survey team used this technique to survey corals and COT numbers concurrently with the transect counts on 111 of the reefs we surveyed. In all cases the manta tow that encompassed the transect count site was noted.

The rapid reconnaissance manta tow technique used by GBRMPA to survey reef encrusting biota and crown of thorns numbers was developed to gather information to assist reef zoning decisions for managers. The observer is towed along the edge of the reef in a zig-zag path for a period of 20 minutes, or until a marked change in reef communities occurs, and estimates the cover of a wide range of encrusting organisms within the different community types encountered, as well as keeping track of crown-of-thorns numbers. On average the zig-zag path covered reef areas from the crest down to a depth of 10-15 m depending on underwater visibility. This technique uses 4 abundance categories to rank crown-of-thorns numbers seen during the 20 minute tow: 0, 1-9, 10-39 and ≥40. If more than 40 crown-of-thorns are seen in a twenty minute tow the reef is deemed to be suffering a crown-of-thorns outbreak. The average 20 min GBRMPA type manta tow along covered an approximately 1 km length of reef edge and sampled an area of about a hectare, assuming a 10 m wide swath of substratum was searched during the tow.

AIMS Manta Tows.

The AIMS GBR wide survey of 228 reefs reported by Moran et al. (1988) was carried out during the same time period as the surveys reported here (see table 1), and covered 102 of the reefs we used, enabling comparisons to be made with the AIMS technique. In this study observers used 2 minute manta tows around the reef perimeter and recorded the total number of crown-of-thorns seen during each tow, along with an estimate of live and dead hard coral cover. Each 2 minute tow covered a distance of approximately 175 m and, assuming the path scanned by the observer was between 8-10 m wide, searched an area of about 0.15 ha. Using the figures given in Moran et. al. (1988) of a total number of tows of 15,035 it can be calculated that this study searched a mean area per reef of approximately 10 ha. Mean time taken by two teams of observers to survey the entire perimeter of each reef was about 3 hours. This technique is described in detail in the methods volume of the Crown-of Thorns Study publication.(1986) and in Moran et al. (1988).

Results.

Transect Counts versus GBRMPA Manta Tows

A direct comparison of these two sampling techniques is possible using the data collected during this study. For this comparison data from the transect counts recorded as density per ha (equivalent to the total number of starfish seen in the ten counts) was put into the same four abundance categories used for the GBRMPA type manta tow counts. Similar abundance categories were estimated for 64% of the 122 sites (figure 2A), primarily those where no crown-of-thorns were recorded. In 35% of sites the transect counts detected more crown-of-thorns than the manta tow, compared with only a single site where the abundance detected by the manta tow was greater than the transect counts, and that only 1 category greater. At three sites the manta tow recorded zero crown-of-thorns and the transect counts more than 40 per ha. The total number of crown-of-thorns recorded during the transect counts at these 122 sites was 697 compared with an estimate (using the mid-value of each category) of 75 for the manta tow searches of approximately the same area. This indicates that using this manta tow technique, where estimates of crown-of-thorns abundance are only one of a wide range of estimates being made, only about 10% of crown-of-thorns available to transect count observers are seen.

Even if the numbers detected in the transect counts at a single site are compared with the combined abundance estimates from GBRMPA type manta tows of the entire reef perimeter (table 2B) there were still more reefs where transect counts at one site detected greater numbers of crown-of-thorns than the manta tows of the entire reef (26%), than the other way around (9%). The estimate of total crown-of-thorns seen during the manta tows was 565, lower than the number seen in the combined transect count sites (697).

Table 2.  Comparison of Acanthaster Abundance Estimates Made From Transect Counts With That Obtained From Manta Tow Techniques. A.  Transect Count Estimates Compared With GBRMPA Type 20 Minute Manta Tow Through the Transect Count Site. B.  Transect Count Estimates in a Single Back Reef Site Compared With GBRMPA Type Manta Tows of the Entire Reef Perimeter. C.  Transect Count Estimates in a Single Back Reef Site Compared With AIMS Type Manta Tows of the Entire Reef Perimeter.

Transect Counts versus AIMS Manta Tows.

The comparison of these two sampling techniques was limited to those reefs where the surveys were made within about 12 months of each other, and it should be kept in mind when considering these comparisons that the surveys were not concurrent. All the Cairns Section reefs were surveyed using transect counts almost 2 years before the AIMS survey and are excluded from this comparison. Comparisons were possible for 70 of the survey reefs (table 2C). For these comparisons the total number of crown-of-thorns recorded from the perimeter of each reef in the manta tows, and the density per ha from the transect counts, were both converted to the same abundance categories used in the previous two comparisons.

Although these comparisons are between samples at single limited sites covering a total area of one hectare with the entire reef perimeter (average area searched per reef about 10 ha), the transect counts detected greater numbers of crown-of-thorns than the manta tows on more reefs (23%) than the other way around (16%). There were no cases where manta tow counts of the entire perimeter detected more than 10 crown-of-thorns and the transect counts at the single site indicated there were no crown-of-thorns populations on that reef.

One question we were particularly interested in answering was whether the percentage of reefs sampled as having low, medium and high density crown-of-thorns populations was the same using the different techniques. A comparison of the percentage of survey reefs in the four density categories using transect counts and the two manta tow techniques is shown in table 3. The comparisons are between density per hectare for the transect counts and number recorded on the whole reef from the manta tows (approx 10 ha). The percentage of reefs where the two techniques recorded more than 40 starfish were similar (7% for the AIMS counts; 8% for the transect counts), but the transect counts detected more than twice the number of low density populations (36% of survey reefs) compared with the manta tows (17%). This suggests that as well as underestimating crown-of-thorns numbers manta tow techniques may not detect low density populations of crown-of-thorns at all.

Table 3.  A Comparison of Crown-of-Thorns Density Estimates Between Transect Counts and Two Types of Manta Tows.

Spatial Pattern Comparison.

Transect data indicated that high density populations were confined to reefs in the Cairns and north Central Sections and approximately 20% of the reefs surveyed in these two regions supported crown-of-thorns densities of more than 40 per ha (table 4). Moderate density populations (10-39 per ha) were also concentrated in these two regions, although some occurred in the Far-North Section and the Swain Group at the southern end of the GBR. Low density populations were found throughout the GBR region but were least abundant in the south Central and Capricornia Sections and in the Pompey Complex.

Table 4.  Spatial Distribution of Crown-of-Thorns Populations on the GBR.

Although there were small numbers of crown-of-thorns on some reefs in the Far-North Section in early 1984 (maximum recorded of 16 per ha), no destructive aggregations were encountered in this area.

In early 1983 high density crown-of-thorns aggregations were active in the Innisfail area of the Cairns Section on Peart, Gilbey and Wardle Reefs (densities from 120-568 per ha), with evidence of recent activity on Feather Reef. Elsewhere in the Cairns Section, Green Island and Upolu Reef were devastated (less than 15% hard coral cover), as were 6 reefs in the Cooktown-Lizard Island area. There was also moderate crown-of-thorns activity on No. 4 Ribbon Reef off Cooktown and on Martin Reef off Lizard Island.

In late 1984 crown-of-thorns aggregations had reduced hard coral cover to about 5% on 4 of the northern mid-shelf reefs in the Central Section, while active aggregations (56-108 per ha) were reducing hard coral cover on 4 other mid-shelf reefs and one outer shelf reef in the same region.

There were small localised aggregations of crown-of-thorns on a few reefs in the main body of the Swain Group in late 1985 with a maximum of 26 per ha recorded on the west tip of Horseshoe Reef

We were also interested in seeing whether the transect count data gave comparable indications of the GBR-wide spatial pattern of crown-of-thorns 'outbreaks' to those obtained using manta tow surveys of 228 reefs (Moran et al., 1988). To determine whether a reef had experienced an outbreak using the transect counts we combined the crown-of-thorns density with the measures of coral cover. If more than 40 crown-of-thorns were recorded in the survey site the reef was deemed to be suffering an outbreak. A recent outbreak was indicated if hard coral cover was less than 15% and dead standing coral cover was high, sometimes associated with low to moderate densities of starfish.

Both methods indicated that crown-of-thorns outbreaks in the period 1983-86 were largely confined to the Cairns and north Central Sections of the GBR. Using our criteria, 36% of the reefs surveyed in this central third of the GBR had current or recent outbreaks but no outbreaks were detected on other parts of the GBR. Moran et al. (1988) reported that 55 of the 84 reefs (65%) they surveyed in this central area were experiencing, or had experienced, crown-of-thorns outbreaks, although they also reported 7 outbreaks in other areas, notably the Far-North and the Swain Group. Overall they decided that 27% of the 228 reefs they surveyed were experiencing or had experienced outbreaks, compared to only 16% of the reefs we surveyed using transects counts.

Discussion.

Density estimates of crown-of-thorns populations are needed for a number of purposes. Reef managers need to know whether a reef is experiencing a crown-of-thorns 'outbreak' and the number of reefs that are being affected. The GBR-wide manta tow based survey reported by Moran et al. (1988) was designed to give reef managers a 'snapshot' view of the percentage of reefs that were suffering, or had recently suffered, crown-of-thorns outbreaks. Studies of the origin and causes of these outbreaks need to detect the 'normal' low to moderate density populations of starfish that must initially be responsible for the development of outbreaks and to follow the progress of such populations through time. The primary aim of this study was to look at the optimum method for estimating crown-of-thorns density, both high density outbreak populations and the low to moderate density populations that must be monitored if the causes and progress of further GBR-wide outbreaks are to be determined.

In our comparison of the density estimates made with replicate transect counts within a single back reef site with two different manta tow techniques a number of conclusions were reached. Concurrent estimates made using the GBRMPA rapid reconnaissance type of manta tow through the transect count site detected approximately 10% of the crown-of-thorns density recorded using the transect counts. Of particular concern was the failure of the manta tows to detect any starfish in three instances where the transect counts recorded densities of over 40 per ha. Several factors may contribute to this discrepancy. In these manta tow surveys the estimates of crown-of-thorns numbers are not of primary concern being only part of a large group of estimates of hard coral communities and other encrusting organisms. Also important are the problems of detecting often secretive starfish while being towed over the reef community at between 1.0 and 1.5 knots, often in less than ideal conditions of underwater visibility.

The transect counts were not designed to give reef-wide estimates of crown-of-thorns populations as the counts of starfish were an add-on to a separate study of reef fish populations. In spite of this they gave a good indication of reef-wide crown-of-thorns population density when compared with both the GBRMPA and AIMS manta tow techniques (table 2B, 2C). On average the GBRMPA technique gave a slightly lower estimate of starfish numbers on each reef as a whole than the transect counts recorded at single kilometre long back reef site (on average this site was about 10% of the reef perimeter).

For the AIMS type of manta tow the estimates of crown-of-thorns numbers were the primary aim of the technique, with the only other estimates made being total hard coral cover and the cover of dead standing hard coral. However, this technique also appeared to markedly underestimate crown-of-thorns numbers, especially in low density populations. Although no direct comparisons with our transect counts were possible as the surveys were made at different times and in many cases in different parts of the reef (the AIMS survey did not include tows through back reef bommie fields and many of the transect counts were made in such areas),.various comparisons suggested that this type of manta tow detected between 10-30% of the numbers recorded using transect counts.

In a study of the biases and variability associated with the AIMS manta tow technique Fernandes (1990) also came to the conclusion that only high density populations could be confidently identified. She used the AIMS personnel for her experiments and found that average sightability of starfish over the reef slope was only 5.6%, and that even if the width of search was rigidly confined to 10 m (the width of search had previously been undefined) sightability only increased to 22.7%. This confirms the above estimates of 10-30%.

Transect counts clearly give a more accurate estimate of crown-of-thorns density than either manta tow technique. Of more serious concern is the evidence that manta tow techniques often failed to detect low to moderate density populations (<40 per ha) at all (table 3). Transect counts were more reliable at detecting these populations as well as giving more accurate density estimates.

As the transect counts were only made at a single back reef site incorporating an average of 10% of the reef perimeter there may be concern that this application of the technique could miss detecting high density outbreak populations if they were localised within the other 90% of the reef at the time of the surveys. To some extent the same argument could be levelled at the manta tow technique. Neither technique is covering the entire reef, both are only sampling part of the potential population. The transect counts sampled about 20% of the back reef perimeter with accurate counts that ranged from the reef crest down to about 20-25 m depth. The manta tows sampled a narrow zig-zag strip around the main reef perimeter to a maximum depth of about 20 m, including back and front reef but not lagoons or back reef bommie fields and recording only 10-20% of the starfish within the sampled area. The manta tows sampled an average of 10x the area of the transect counts but with only 10-20% efficiency - both techniques were effectively sampling similar areas of each reef.

The above concern that transect counts may miss detecting high density populations can be addressed by comparing the percentage of reefs that were recorded as having over 40 starfish. The concurrent manta tows recorded only half the percentage of high density populations as did the transect counts, whereas the AIMS manta tow surveys detected an approximately equal percentage to the transect counts. This indicates that both techniques have an approximately equal chance of detecting high density populations.

Both techniques gave similar GBR-wide patterns of the distribution of crown-of-thorns outbreaks but produced different estimates of the overall percentage of reefs that had experienced or were experiencing an outbreak: 27% of reefs surveyed by Moran et al. (1988) compared to 16% of our transect count survey reefs (table 5). A large part of this difference can be accounted for by differences in the interpretation of what constitutes an outbreak.

Table 5.  Spatial Distribution of 'Outbreak' Crown-of-Thorns Populations on the GBR - Comparison of Transect Count Results with AIMS Manta Tows.

There appears to be no biological reason that can be used to separate reefs into outbreak and normal on the basis of crown-of-thorns density. An examination of the site density data from the 156 reefs presented here shows that density follows a log distribution (see figure 2a), with only 27 reefs supporting more than 10 starfish per ha, and only 5 of these with densities over 100 per ha. There is no obvious cut-off point above which outbreaks can be said to be occurring (figure 2b), although it is obvious that at densities approaching or above 100 per ha starfish will have a profound effect on coral communities in the short term. We have chosen 40 per ha as the lower limit for an active outbreak, and reference to coral cover estimates confirmed this. We consider that an outbreak should markedly reduce coral cover over at least half of the reef perimeter. A reef was considered to have suffered a recent outbreak if hard coral cover was low and dead standing coral high. This was often combined with the presence of moderate populations of starfish.

Examination of the data presented by Moran et al. (1988) suggested that in many cases the decision to place a reef in the category of recent infestation was based on previous surveys or anecdotal reports and was often not supported by the reported figures on median coral cover. Specific examples are Tydeman and 13-055 in the Far-North Section with coral cover ratings of 3 (30-50%) and 2 ( 11-30%) respectively, and Carter (2) and Thetford (3) in the Lizard Island and Cairns areas. Our observations on these reefs in 1984-1986 suggest that none of them had received anything more than minor damage. On six other Cairns Section reefs they classed as recent infestations, all with median coral cover ratings of 2, our observations suggested that only slight crown-of-thorns damage had occurred.

Although Moran et al. (1988) stated in the methods that reefs with more than 66 starfish recorded per perimeter were considered to have an active outbreak their actual classification is sometimes doubtful, with 8 of the 21 reefs classified as active outbreak having between 5 and 40 starfish per perimeter. For instance five starfish were reported on Stanley Reef, and a median coral cover in the 30-50% range, but the reef was deemed to be suffering an outbreak. Similarly, Sanctuary Reef where 40 starfish were recorded over less than 10% of the reef perimeter of about 10 km, and median coral cover was rated at 3, is a doubtful example of an outbreak. In all, we believe 19 of their reported 62 outbreak reefs are not strongly supported by the data presented, and if these are omitted the percentage of outbreak reefs in their sample is reduced to less than 19%, comparable to our estimate of 16%.

Moran et al. (1988) also give figures for the total number of reefs within each of their 11 survey sectors and these can be used to give a GBR-wide perspective to the percentage of reefs badly affected by crown-of-thorns grazing. Their survey covered a much greater percentage of the reefs present in the sectors within the central third of the GBR region that was affected by starfish outbreaks than it did in the areas to the north and south where outbreaks were not experienced. Using their figures for the percentage of reefs in each sector that have experienced outbreaks, combined with the number of reefs in each sector, indicates that about 14% of the reefs in the GBR region had been affected during the current crown-of-thorns outbreak by early 1986 when activity was starting to decline. If the doubtful outbreak reefs are excluded this figure for the overall percentage of damaged reefs drops to 8.5%.

However, whatever interpretation of the data in Moran et al (1988) is made, the results of that survey and the one reported here indicate that the coral communities on between 36-65% of the reefs in the central third of the GBR have been badly damaged by crown-of-thorns since 1980, an estimated total of between 130 and 240 reefs.

The time taken to complete the surveys is also a consideration in any comparison. Each group of ten transect counts took a total time for two observers of between 2.5 and 3 hours. For comparison GBRMPA manta tows took a team of 4 people 2-10 hours depending on the size of the reef. The AIMS manta tows were limited to small to medium sized reefs and used two teams of observers, taking an average of about 3 hours per reef.

Although the strip transect searches for crown-of-thorns as used for this study have proved to be a reliable and efficient method for detecting starfish populations on a reef, they have a number of problems. The most obvious is the limited back reef site covered by the transects. This could be solved by surveying a smaller number of replicates at more sites around the reef, eg five transects in six sites, three on the front reef and three on the back reef. Recent work looking at the optimum methodology for transect counts (Mapstone B.D. and Ayling A.M., unpublished data) has suggested that a design such as this with six sites per reef but using five 50 x 5 m replicate transects at each site gives the most cost-effective estimate of reef-wide density. Using two teams of observers this design would take about 4 hours in the field for each reef.

Another problem with using the transect counts to detect density differences between reefs and changes through time is the very high variances in all groups of counts. As can be seen from the data in appendix 1 the standard deviation was never less than half of the mean and in most cases was greater than the mean, in extreme cases more than twice the mean. As a result the precision of the groups of ten counts ranged from 0.08-0.95 with a grand mean of 0.6, a serious obstacle to testing the significance of changes of less than an order of magnitude. However, data presented by Ayling and Ayling (1989) on the pattern of density changes on three Central Section reefs over a period of 6 years shows that changes through time can span almost three orders of magnitude, with similar differences among reefs in any area at one time. Hence, in spite of the high variance, significant patterns can still be detected using the transect count technique.

In general it is clear that manta tow techniques markedly underestimate crown-of-thorns densities on a reef and are not suitable for detecting any starfish population of less density than an active outbreak (>40 per ha). This will be of particular concern when attempting to forecast outbreaks by monitoring the presence of low to medium density source populations of crown-of-thorns. If time is an important consideration in the decision on which technique to use it should be noted that in the average time taken to survey a reef using the AIMS manta tow technique (half a day) the same two groups of observers could survey six separate sites (3 each on the front and back reef) using five 50 x 5 m strip transect counts. This study supports our contention that the data obtained would be of at least comparable use in detecting whether or not a reef is suffering an active starfish outbreak but would have the advantage of enabling the recording of low density populations that need to be monitored if the sources of any future outbreaks are to be detected.

Acknowledgements.

A large number of people provided assistance in the various sections of this study; spending 600 hours underwater on 156 reefs surveying over 2,000 transects requires a lot of help. The Great Barrier Reef Marine Park Authority funded the entire project as part of its research and monitoring program. Wendy Craik of the GBRMPA provided the initial suggestion to start counting crown-of-thorns, helped organise most of the field trips and provided constant encouragement. Many other staff members at the GBRMPA have provided help over the years. The crews of the charter boats 'Jessie Rae', 'Hero', 'Takaroa' and 'Tropic Rover', notably Lee Lafferty, Cocky Watkins, Harry Johnson, Robyn Springett and Ron Isbel, were unfailingly helpful. The GBRMPA type manta tows were carried out by a separate party of four observers on each trip: our thanks to Chrissy Maguire, Murray Bain, Cathy Bone, Randy Lovell, Mick Haywood and Penny Butcher. Assistance was provided for some of the transect counts by Warren Nott, Melita Samoylis, Rich Braley and Ian Douglas. Discussions with Howard Choat have helped us to develop our survey strategies over the years and are gratefully acknowledged here.


References.

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Ayling AM, Ayling AL (1983b) Distribution and abundance of coral trout species (Plectropomus spp.) in the Townsville and Whitsunday areas of the Great Barrier Reef Marine Park. Unpublished report to GBRMPA.

Ayling AM, Ayling AL (1984a) Distribution and abundance of coral trout species (Plectropomus spp.) in the Swain Group of reefs. Unpublished report to GBRMPA.

Ayling AM, Ayling AL (1984b) A biological survey of selected reefs in the Far-North Section of the Great Barrier Reef Marine Park. Unpublished report to GBRMPA.

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