Summary
- Fourteen reefs were surveyed in this sector, one inshore, eight mid-shelf and five outer-shelf reefs.
- Sector-wide hard coral cover was moderate (10-30%) and had increased overall since the previous survey.
- Among the seven reefs surveyed within the last two years, coral cover had increased at five reefs and decreased at two reefs.
- No crown-of-thorns starfish (COTS) were observed during manta tow surveys or at fixed site surveys.
- Coral bleaching was observed at low levels at fixed site surveys at three of five reefs. Bleaching was most notable at Thetford Reef.
Hard Coral Cover 0-10% 10-30% 30-50% 50-75% 75-100%
Figure 1: Map showing location of reefs in the Cairns sector. Click the points for more information.
Table 1: Overview of results obtained from manta tow surveys of reefs in the Cairns sector
| Cairns Sector | Summary | Trend since last survey |
|---|---|---|
| Average Coral Cover (%) | 12.4 | Increased |
| COTS status: | No Outbreaks | Stable |
| Coral bleaching: | Low | Stable |
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 Cairns 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 (12.4%) (Table 1) (Fig 2) and had increased marginally from a low level since the previous surveys in February 2019 (Fig 2).
Hard coral cover was moderate (10 to 30%) on eight reefs and low (0 to 10%) on six reefs (Table 2). Among the seven reefs surveyed within the last two years, hard coral cover increased at five reefs and decreased at two reefs. Another seven reefs did not have recent survey history. Moderate hard coral cover was recorded at four of these reefs and low hard coral cover at the remaining three reefs. Generally low coral cover in the recent survey history of this sector reflects the cumulative disturbances, such as COTS outbreaks and the 2016/2017 mass coral bleaching events, that have afflicted these reefs in recent years.
No COTS were recorded during manta tow surveys (Table 2), nor were they observed during scuba surveys at fixed sites. Two reefs were classified as ‘recovering’ from incipient outbreaks reported in 2015 (Table 1, Fig 2).
Coral bleaching was evident during manta tow surveys at Thetford Reef. Bleaching was restricted to scattered, individual colonies in all habitats, but was most notable in shallow, back reef habitat. More detailed fixed site surveys on scuba recorded low levels of coral bleaching at three of five reefs. Bleaching was most notable at Thetford reef where all transects were affected. Bleaching remained restricted to scattered individual colonies at the time of survey. Bleaching particularly affected colonies in the genera Goniastrea, Montastrea, Leptoria, Platygyra and Favites. Thetford Reef was the last reef surveyed in the sector and as such was exposed to the greatest accumulation of temperature stress (degree heating days) at the time of survey. Surveys after the 13th February 2020, including surveys at Thetford Reef, coincided with a bleaching warning issued by NOAA Coral Reef Watch for most of the GBR Marine Park.
Abundance of coral disease, particularly white syndrome and skeletal eroding band disease, was moderate at Hastings Reef and Arlington Reef, and low at other reefs in this sector. The densities of the corallivorous snail, Drupella spp. varied between 27/ha and 227/ha, and were within the range reported in previous years.
Table 2: Summary of manta tow surveys of reefs in the Cairns 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 |
|---|---|---|---|---|---|---|---|---|---|
| ARLINGTON REEF | Mid | 60 | 2018 | 0 | 0 | NO | 8.7 ▲ | 14 ▲ | 2020 |
| LOW ISLANDS REEF | Inner | 27 | 2019 | 0 | 0 | NO | 16.2 ▼ | 4.6 ▼ | 2020 |
| 16013A | Mid | 9 | 2007 | 0 | 0 | NO | 21.4 ▲ | 8.3 ▲ | 2020 |
| 16013B | Mid | 7 | 2007 | 0 | 0 | NO | 25.3 ▼ | 6 ▲ | 2020 |
| 16013C | Mid | 4 | 2007 | 0 | 0 | NO | 3.8 ▼ | 2.4 | 2020 |
| EVENING REEF | Mid | 51 | 2015 | 0 | 0 ▼ | RE ▼ | 9.4 ▲ | 3.2 ▲ | 2020 |
| PICKERSGILL REEF | Mid | 75 | 2019 | 0 | 0 | RE | 17.7 ▲ | 5.5 ▲ | 2020 |
| FLYNN REEF | Outer | 24 | 2009 | 0 | 0 | NO | 9.3 ▼ | 2.5 ▼ | 2020 |
| NORMAN REEF | Outer | 36 | 2015 | 0 | 0 | NO | 18.8 ▲ | 6.1 ▼ | 2020 |
| RACHEL CARSON REEF | Outer | 41 | 2004 | 0 | 0 | NO | 20.6 ▼ | 2.7 ▼ | 2020 |
| HASTINGS REEF | Mid | 71 | 2019 | 0 | 0 | NO | 4.3 ▼ | 5.2 ▼ | 2020 |
| THETFORD REEF | Mid | 43 | 2019 | 0 | 0 | NO | 14.4 ▲ | 12.5 ▲ | 2020 |
| AGINCOURT REEFS (NO 1) | Outer | 32 | 2019 | 0 | 0 | NO | 25.1 ▲ | 3.6 ▲ | 2020 |
| ST CRISPIN REEF | Outer | 89 | 2019 | 0 | 0 | NO | 8.2 ▲ | 3.4 ▲ | 2020 |
Figure 2: - Sector-wide changes in coral cover and the numbers of A. solaris for survey reefs in the Cairns sector of the GBR. Orange trend line = Hard coral. Purple bars = Crown-of-thorns (COTS)
Image 1. Branching Acropora sp. thrives in the back reef habitat at Agincourt Reef No. 1
Image 2. A juvenile Acropora sp. coral shows signs of heat stress (bleaching) at Thetford Reef
Image 3. A tabulate Acropora sp. at Arlington Reef regrows, perhaps from a surviving remnant from bleaching in 2016/2017
Image 4. A diver searches for causes of coral mortality along a transect at Agincourt Reef No. 1
