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


Summary

  • Very high coral mortality on some midshelf reefs as a result of the 2015/16 summer bleaching.
  • Extensive areas of recently dead standing corals around reef perimeters on reefs affected by bleaching. 
  • Patchy mortality of large massive Porites spp. corals on reefs affected by bleaching. Colonies may be hundreds of years old.
  • Live coral cover on five of eleven reefs is now at the lowest levels since surveys began in 1986.
  • Impact of the summer 2015/16 bleaching event varied, with some reefs showing few effects of bleaching while others severely damaged.

Figure 1 - Map showing location of reefs in the Cape Grenville sector. Click on image to go to AIMS Spatial Maps and click on symbols for information on individual reefs. (Note: Pearson Reef is just north of Bowles Reef and Ashmore Reefs just west of the Sir Charles Hardy Islands).

Cape Grenville Sector

Summary

Trend since last survey

Median Coral Cover:

Low (10-20%)

Increased

COTS status:

Not outbreaking

Decreased

Coral disease:

Overall Low: Increased incidence on one reef

Stable

Coral bleaching:

Low: large areas of coral mortaility from 2015/16 summer bleaching were observed.

Increased

 

As part of the Long Term Monitoring Program (LTMP), surveys of coral cover and abundance of the coral feeding crown-of-thorns starfish (COTS), Acanthaster planci, were completed on eleven reefs in the Cape Grenville sector of the Great Barrier Reef (GBR) using the manta tow technique. Results (Table 1) showed that median reef-wide live coral cover (coral cover) had decreased at six reefs Curd, Middle Banks 2, Kay, Pearson, Sir Charles Hardy 1 and Sir Charles Hardy 2. The latter four reefs were characterised by low (5-10%) coral cover and large areas of recently dead standing corals. Given that the majority of coral mortality had occurred recently, the decline on all six reefs was attributed to 2015/16 summer bleaching event. Decline was particularly severe on Kay Reef and the Sir Charles Islands that suffered very high coral mortality (60-70% of total hard coral cover). With the exception of Middle Banks 2, coral cover on those reefs that suffered decline was at the lowest recorded since the beginning of surveys in 1986. Coral cover was already low on Quoin Island Reef as a result of previous COTS activity and it is likely that the summer 2015/16 bleaching hampered recovery. Coral cover was very high (50-63%) and had increased on four reefs, Ashmore 1, Ashmore 2, Ashmore 3 and Middle Banks 3. Despite the very high coral cover there were signs of recent mortality on these reefs in the form of stands of recently dead coral, likely attributable to summer 2015/16 bleaching.

Small numbers of COTS, generally below outbreak levels, were observed on four reefs Kay, Curd, Sir Charles Hardy (2) and Pearson. Curd Reef had a localised Incipient Outbreak (back reef only) while Pearson Reef had a reef-wide Incipient Outbreak that may be expected to impact on coral cover in the future. Overall COTS activity had declined in this sector compared to the last survey in 2013.

Surveys in 2017 also showed that, while coral bleaching was still widespread, it was at a low level and restricted to small numbers of individual scattered coral colonies predominately on reef slopes. Low levels of white syndrome disease were also recorded from reefs in this sector. One reef Middle Banks 3 had elevated levels of white syndrome on the back reef.

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 COTS outbreak terminology can be found on the AIMS website

Table 1. Summary of manta tow surveys for reefs in the Cape Grenville Sector

Reef

Shelf Position

Tows

COTS

COTS per tow

Median Live Coral Cover

Median Dead Coral Cover

Median Soft Coral Cover

Reef Status

CURD

Inner

30

4

0.13

10-20%

10-20%

0-5%

RE

KAY

Inner

58

2

0.03

5-10%

30 to 40

0-5%

RE

ASHMORE BANKS (1)

Mid

11

0

0

50-63%

0-5%

0-5%

NO

ASHMORE BANKS (2)

Mid

14

0

0

50-63%

0-5%

0-5%

NO

ASHMORE BANKS (3)

Mid

11

0

0

50-63%

0-5%

0-5%

NO

MIDDLE BANKS (2)

Mid

10

0

0

40-63%

10-20%

0-5%

NO

MIDDLE BANKS (3)

Mid

12

0

0

50-63%

0-5%

0-5%

NO

PEARSON

Mid

33

9

0.27

5-10%

20-30%

0-5%

IO

QUOIN IS

Mid

6

0

0

5-10%

0-5%

0-5%

RE

SIR CHARLES HARDY (1)

Mid

21

0

0

5-10%

30-40%

0-5%

RE

SIR CHARLES HARDY (2)

Mid

21

1

0.05

5-10%

50 to 63

0-5%

RE

Dates: 12th - 26th September 2016

Vessel: RV Cape Ferguson

Survey Leader: Ian Miller

Graph showing changes in crown-of-thorns (COTS) numbers and in coral cover through time for all survey reefs in the Cape Grenville sector of the GBR. Mean COTS/per tow is the mean value of the average number of COTS counted per two minute manta tow at each survey reef and coral cover is the mean value of the average percent coral cover per two minute manta tow recorded at each survey reef. Gaps in coral cover indicate years when reefs in the sector were not surveyed.

Worryingly, many massive Porites spp. hard corals on reefs in the Cape Grenville sector died as a result of summer bleaching. Some of these colonies may be hundreds of years old suggesting that the event was unprecedented in recent times.

Giant clams also rely on zooxanthellae and bleach as a result of excessive sea water temperatures. Despite the widespread mortality of hard corals, few dead clams were seen on these reefs.

Local oceanographic features including currents and possible upwelling bringing cooler water may have saved some reefs from the massive damage that was evident on nearby reefs. Coral cover was very high on reefs in the Ashmore Banks for instance, but low on reefs 20 km away.

Large stands of dead coral skeletons, mainly Acropora spp. stand testament to the lethal effects of sea water temperatures that exceeded physiological limits of survival for these hard corals during the summer in 2015/16. This video was taken at on the reef slope Sir Charles Hardy Island (1) in the Cape Grenville sector.