Summary
- Fourteen reefs were surveyed in this sector.
- Sector-wide hard coral cover was moderate (10-30%).
- Eight reefs were previously surveyed in January 2019; hard coral cover had increased on six reefs and had declined marginally on two.
- Three previously unsurveyed reefs had low coral cover.
- Low levels of crown-of-thorns starfish were recorded at three reefs.
- Coral bleaching was widespread, but at low levels (scattered, individual colonies).
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
| Cape Grenville Sector | Summary | Trend since last survey |
|---|---|---|
| Average Coral Cover (%) | 27.8 | Stable |
| COTS status: | No Outbreaks | Stable |
| Coral bleaching: | Low | Decreased |
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 fourteen reefs 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 moderate (10-30 %) (Table 1) (Fig 2) which was unchanged since the previous surveys in January 2019.
Eight reefs were previously surveyed in January 2019 and hard coral cover had increased on six reefs and had declined marginally on two. Pearson Reef was last surveyed in 2017 and hard coral cover remained low (0 to10%).
Second Small Reef and Reef 12-071 were previously surveyed more than 10 years ago, and now had moderate (10 to 30%) and high (30 to 50%) coral cover, respectively. An additional three mid-shelf reefs (McSweeney Reef, Monsoon Reef and Reef 11-162) were surveyed for the first time in December 2019 and all had low coral cover (0 to 10%), likely reflecting the impacts of the recent mass coral bleaching events. Severe Tropical Cyclone Trevor (category 4) crossed the coast to the south near Lockhart River in March 2019 but had little or no impact on reefs in this sector, although there was still evidence of residual impacts of Tropical Cyclone Penny in January 2019 on some reefs.
More detailed fixed site surveys were conducted on scuba and recorded low levels of coral bleaching on all reefs, restricted to scattered individual colonies. Cases of coral disease were low on all reefs, as were numbers of the corallivorous snail, Drupella spp. Very low numbers of COTS were recorded during fixed site surveys on three reefs, Monsoon Reef, McSweeney Reef, and Reef 11-049. More results on fish and benthic assemblages from fixed site surveys will be available soon.
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 monitoring program design, sampling methods and a full explanation of the A. solaris outbreak terminology 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.
| Reef | Shelf Position | Tows | Previous survey year | Total A. solaris per reef | A. solaris per tow | A. Solaris Outbreak Status | Live Hard Coral Cover % | Soft Coral Cover % | Survey Year |
|---|---|---|---|---|---|---|---|---|---|
| ASHMORE BANKS (2) | Mid | 16 | 2019 | 0 | 0 | NO | 54.4 ▲ | 4.2 ▼ | 2020 |
| ASHMORE BANKS (3) | Mid | 10 | 2019 | 0 | 0 | NO | 50.1 ▲ | 2.5 ▼ | 2020 |
| MIDDLE BANKS (2) | Mid | 10 | 2019 | 0 | 0 | NO | 49.1 ▲ | 5 ▲ | 2020 |
| MIDDLE BANKS B (3) | Mid | 10 | 2019 | 0 | 0 | NO | 61.4 ▲ | 7.5 ▲ | 2020 |
| PEARSON REEF | Mid | 37 | 2017 | 0 | 0 ▼ | RE ▼ | 10.1 ▼ | 2.4 | 2020 |
| LAGOON REEF | Outer | 55 | 2019 | 0 | 0 | NO | 39.2 ▼ | 4.4 ▲ | 2020 |
| SECOND SMALL REEF | Outer | 26 | 2006 | 0 | 0 | NO | 20.9 ▼ | 4.3 ▼ | 2020 |
| ASHMORE BANKS (1) | Mid | 12 | 2019 | 0 | 0 | NO | 43.2 ▲ | 2.5 | 2020 |
| 11049S | Mid | 33 | 2019 | 0 | 0 ▼ | NO | 3.1 ▼ | 2.5 ▲ | 2020 |
| 11162S | Mid | 29 | NA | 0 | 0 | NO | 8.9 | 2.6 | 2020 |
| MCSWEENEY REEF | Mid | 29 | NA | 4 | 0.14 | NO | 2.8 | 2.4 | 2020 |
| MONSOON REEF | Mid | 24 | NA | 1 | 0.04 | NO | 7.2 | 2.5 | 2020 |
| 12071S | Outer | 27 | 1996 | 0 | 0 ▼ | NO | 39.1 ▲ | 4.1 ▲ | 2020 |
| MANTIS REEF | Outer | 36 | 2019 | 0 | 0 | NO | 21.7 ▲ | 7.4 ▲ | 2020 |
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. Stands of dead coral on Mc Sweeney Reef, likely caused by the 2016 bleaching event
Image 2. While there was a very small decrease in hard coral cover recorded at Lagoon Reef, there were still extensive stands of healthy coral in some parts of the reef.
Image 3. There was little evidence of damage on reefs in this sector from Severe Tropical Cycle Trevor, which passed to the south of this sector in March 2019. However, some reefs like Lagoon Reef pictured here, still had damage evident from the passage of Tropical Cyclone Penny in January 2019.
