Report on surveys of the Cape Grenville sector of the Great Barrier Reef

Summary

  • 17 reefs were surveyed in the Cape Grenville sector.
  • Sector-wide hard coral cover was high (30-50 %) having increased from moderate levels in 2020.
  • Hard coral cover had increased on 13 reefs and had declined on four reefs.
  • Levels of crown-of-thorns starfish (COTS) had increased at two reefs and decreased at another two reefs.
  • An Incipient Outbreak (IO) of COTS was recorded at one reef.
  • Low levels of hard coral bleaching were observed.

Hard Coral Cover   0-10%  10-30%  30-50%  50-75%  75-100%

Figure 1: Map showing location of reefs in the Cape Grenville sector. Click the points for more information.


Table 1: Overview of results obtained from manta tow surveys of reefs in the Cape Grenville sector

As part of the Long-Term Monitoring Program (LTMP), manta tow surveys of hard coral cover and the abundance of the coral feeding crown-of-thorns starfish (COTS), Acanthaster cf. solaris were completed on 17 reefs were surveyed in the Cape Grenville sector of the Great Barrier Reef (GBR) (Fig 1).

Preliminary results of the manta tow surveys are presented in Tables 1 and 2. The overall median hard coral cover for the sector was high (30-50%) (Table 1) (Fig 2) and had increased since previous surveys in 2020 (Fig 2).

Hard coral cover was very high (50 to 75%) on four reefs, high (30 to 50%) on five reefs, moderate (10-30%) on five reefs and low (0 to 10%) on three reefs (Table 2). Since the previous surveys hard coral cover had increased on 13 reefs and declined on four reefs (Table 2).

There were no Active Outbreaks of COTS. One Incipient Outbreak was recorded at Reef 11049S.  Low numbers of COTS were also recorded at Middle Banks during manta tow surveys (Table 2). Sector-wide numbers of COTS were low and had increased since previous surveys (Table 1, Fig 2).

Low levels of coral bleaching were recorded at 11 reefs, restricted to scattered individual colonies.

Details of the manta tow method can be found in the Standard Operational Procedure No. 9 [AIMS Research - Crown-of-thorns Starfish and Coral Surveys - Standard Operational Procedure 9]. Further details of the Long-Term Monitoring Program can be found on the AIMS website.

Table 2: Summary of manta tow surveys of reefs in the Cape Grenville sector. Arrows indicate the trend in live coral cover and A. solaris since last survey; ▲ = increase, ▼ = decrease, " " = no change. Outbreak Status: NO no outbreak, PO potential outbreak >0.1 COTS per tow, IO incipient outbreak >.22 COTS per tow, RE recovering, AO = Active Outbreak>1 COTS per tow.

Figure 2: - Sector-wide changes in coral cover and the numbers of A. solaris for survey reefs in the Cape Grenville sector of the GBR. Orange trend line = Hard coral. Purple bars = Crown-of-thorns (COTS)

Figure 2: - Sector-wide changes in coral cover and the numbers of A. solaris for survey reefs in the Cape Grenville sector of the GBR. Orange trend line = Hard coral. Purple bars = Crown-of-thorns (COTS)

Image 1. The reef flat at Mantis Reef, an offshore reef where coral and fish communities are thriving. Image 1. The reef flat at Mantis Reef, an offshore reef where coral and fish communities are thriving.
Image 2. Complex assemblage of corals on the back of Pearson Reef, showing signs of past mortality and survival of Turbinaria and Porites colonies. Image 2. Complex assemblage of corals on the back of Pearson Reef, showing signs of past mortality and survival of Turbinaria and Porites colonies.
Image 3. One of the few surviving tabulate Acropora on the sheltered western side of Monsoon Reef. Image 3. One of the few surviving tabulate Acropora on the sheltered western side of Monsoon Reef.
Image 4. The AIMS monitoring team were at Lagoon Reef during coral spawning and observed some dense slicks of coral eggs and larvae. Image 4. The AIMS monitoring team were at Lagoon Reef during coral spawning and observed some dense slicks of coral eggs and larvae.