Report on surveys of reefs in the Princess Charlotte Bay and Cape Grenville sectors of the Great Barrier Reef

Dates: 31^st January to 12^th February 2013

Vessel: MV Floreat

Survey leader: Ian Miller

Supported by NERP Emerging Priorities funds, thirteen reefs in the Princess Charlotte Bay sector and eleven reefs in the Cape Grenville sector of the Great Barrier Reef were surveyed using manta tows. Preliminary results of the manta tow surveys are reported here.

Median reef-wide live hard coral cover (henceforth coral cover) was moderate (10-20%) on the majority of reefs manta towed in the Princess Charlotte Bay sector. Five Reefs in this sector were surveyed for the first time. Coral cover on those reefs in this sector with a history of survey had generally increased or remained the same; a decline in coral cover was recorded at only one reef. Crown-of-thorns starfish (COTS) were recorded in low numbers on the majority of reefs. Outbreak levels of COTS were observed on Clack Reef and Corbett Reef. Despite continued COTS activity on Clack Reef, coral cover has increased since this reef was last surveyed. Corbett Reef had not been surveyed previously. Coral cover on reefs in the Cape Grenville sector was generally higher than on reefs in the Princess Charlotte Bay sector. There were no clear trends in coral cover, though cover at a number of reefs had declined since the last survey. These included Ashmore Banks (1), Ashmore Banks (3) and Middle Banks (3). At the time of survey there was evidence of mechanical damage to hard corals suggesting recent storm damage. COTS were present in the Cape Grenville sector at the highest levels recorded since 1996. Outbreak densities of COTS were found on four reefs: Kay, Sir Charles Hardy (1), Sir Charles Hardy (2) and Quoin Island.

Survey results are summarised in Tables 1 and 2. A full list of LTMP survey reefs appears in the Long-term Monitoring Status Report Number 8. Details of the monitoring program design, the sampling methods and a full explanation of the COTS outbreak terminology used in this report can be found on the AIMS reef monitoring website.

Some COTS have been recorded on reefs in the Princess Charlotte Bay sector in the majority of survey years since 1986. Coral cover on reefs in this sector has been generally moderate (20-30%) over the period of surveys (Figure 1). A shallow decline in coral cover during the late eighties was likely due to persistent COTS that exceeded Incipient Outbreak levels for the sector in 1989. Subsequent recovery of coral cover through to 2001 came to a halt as COTS numbers once more built up in the sector, with densities exceeding Active Outbreak levels in 2004. The ensuing decline in sector-wide coral cover was likely driven by COTS though Cyclone Ingrid (category 4) in March 2005 crossed the northern part of this sector and may have contributed to the decline in coral cover seen in 2006. There were then no surveys until 2013. The recent surveys found a general recovery in sector wide coral cover to moderate levels. COTS are present on reefs in this sector but generally below outbreak densities.

Figure 1. Plot showing average coral cover recorded on reefs (scale left line plot) and average number of COTS recorded (scale right bar chart) during manta tow surveys of reefs in the Princess Charlotte Bay sector since 1986. Dot symbols on line plot indicate years when surveys were conducted. Dashed line indicates level at which COTS densities (scale right) may be expected to have an impact on live coral cover.

Thirteen reefs (Table 1) were surveyed in 2013 using manta tows. Coral cover on the majority of reefs was moderate (10-20%); the highest cover was recorded at Grub Reef (40-50%). Reef wide live coral cover on those reefs with a history of survey was similar to the previous survey. The exception was Osborne Reef where coral cover has declined markedly from a high 30-40% in 2005 to a moderate 10-20% in 2013. Evidence of mechanical damage to hard corals suggests that storms may have been responsible, though the long interval between surveys makes any attribution speculative.

For the remaining reefs surveyed in 2013 coral cover on Pelican Island Reef, Clack Island Reef and Reef 13-124 was high (30-40%). Coral cover had increased on all three of these reefs in the seven years since the last survey. In the case of Clack Reef this was despite the presence of a substantial numbers of COTS (above Incipient Outbreak densities). COTS have been recorded at Clack Is Reef in most surveys since 1986; the densities were very high when it was last surveyed in 2006 and this reef remains classified as an Active Outbreak. Reef 13-124 also has a long history of low level COTS activity and COTS were present below outbreak levels in 2013. No COTS were recorded at Pelican Island Reef. Celebration Reef had moderate coral cover and appears to be in the process of recovering from past COTS and storm damage. Coral cover at Morris Reef was similar to that in 1986, but how coral cover changed over the intervening period is unknown. Reef 13-093, Corbett Reef, Hedge Reef, Magpie Reef, Ogilvie Reef were all surveyed for the first time and all had moderate coral cover (10-20%). Corbett Reef had Incipient Outbreak densities and COTS were recorded on all the other reefs except Magpie Reef.

Table 1. Summary of manta tow survey results for 13 reefs in the Princess Charlotte Bay sector. (Click on the reef names for more information.)

* indicates that a reef was surveyed for the first time in 2013. † indicates a reef where only part of the reef perimeter was surveyed.

Eleven reefs in the Cape Grenville sector were surveyed using manta tows (Table 2). COTS densities on reefs in this sector were at the highest recorded levels since 1996 (Figure 2). Low level COTS activity has been observed in this sector in every year of survey with outbreak levels recorded in 1996 and in 2013. A steady increase in sector-wide coral cover during the initial years of survey was followed by an apparent decline from 2003 to 2006. This was likely due to storm damage from the effects of Severe Cyclone Ingrid (category 4) in March 2005. COTS may have contributed to the observed decline; an Incipient Outbreak was recorded at Quoin Island in 2006, even though COTS were below outbreak levels for this sector as a whole. Surveys in 2013 indicate an overall increase in coral cover on reefs in this sector.

The average COTS densities across all the reefs that were surveyed in 2013 were at an Incipient Outbreak level that may be expected to cause a decline in live coral cover. Four reefs had COTS outbreaks; the highest number of COTS was seen at Quoin Island which was reclassified as an Active Outbreak. Kay Reef, Sir Charles Hardy (1) and Sir Charles Hardy (2) were all classified as Incipient Outbreak.

Coral cover had increased at Curd Reef, Kay Reef, Middle Banks (2) and Sir Charles Hardy Island (1) since the last survey. The greatest increase was at Kay Reef from a high level (30-40%) in 2006 to a very high level (50-63%) in 2013. No change in coral cover from previous surveys was observed on Ashmore Banks (2), Pearson Reef and Quoin Island Reef. Coral cover declined at Ashmore Banks (1), Ashmore Banks (3) and Middle Banks (3). In all cases the relatively long interval between surveys means that no specific cause can be identified, though some mechanical damage was evident, suggesting storm damage in recent years may have been a contributing factor.

Figure 2. Plot showing average coral cover on reefs (line plot; left hand axis) and average number of COTS recorded (bar chart; right hand axis) during manta tow surveys of reefs in the Cape Grenville sector since 1986. Symbols on line plot indicate survey years. Dashed line indicates level at which COTS densities may be expected to have an impact on live coral cover.

Table 2. Summary of manta tow survey results for eleven reefs in the Cape Grenville sector. (Click on the reef names for more information.)