Report on surveys of the Swain sector of the Great Barrier Reef


Summary

  • Strong recovery in hard coral cover on most reefs since storms and Cyclone Hamish in 2008/09 and outbreaks of the coral feeding crown-of-thorns starfish (COTS), Acanthaster planci from the mid-1990s to mid-2000s
  • Very minor bleaching to scattered individual colonies on four reefs with little to no impact
  • Little evidence of significant impact of the 2016 bleaching event
  • Small numbers of COTS on three reefs and one active outbreak

Figure 1 - Map showing location of reefs in the Swain sector. Click on figure to visit AIMS Spatial Maps for information on individual reefs.

 

Swain Sector 

Summary

Trend since last survey

Median Coral Cover:

High (30-50%)

Increased

COTS status:

One Active Outbreak

Increased

Coral disease:

Overall Low

Stable

Coral bleaching:

Low

Increased

As part of the Long Term Monitoring Program (LTMP), manta tow surveys coral cover and abundance of the coral feeding crown-of-thorns starfish (COTS), Acanthaster planci, were completed in January 2017 on seven reefs in the Swain sector of the Great Barrier Reef (GBR; Fig. 1). Median reef-wide live coral cover increased at five of seven reefs (Table 1), driving strong sector-wide recovery (Fig. 2) following storm damage in 2008/09 (Fig. 3) and A. planci outbreaks during the 1990s and early 2000s (Fig. 4). Coral cover remained stable at Horseshoe Reef, while it declined from very high (50 to 63%) to high levels (30 to 40%) at Chinaman Reef (Table 1). Trends at these two reefs likely reflect low numbers of A. planci recorded at Horseshoe Reef and the active outbreak of A. planci recorded during the present survey at Chinaman Reef (Fig. 5). Low numbers of A. planci were observed on two other reefs: Gannet Cay and Turner Cay, but levels were below the Active Outbreak threshold (one A. planci per tow) and have not yet affected coral cover, which has increased since the last survey (Table 1).

The bleaching event of early 2016 appears to have had little to no impact on sector wide coral cover, although its affect is hard to quantify as most reefs are surveyed biennially. Although unlikely, it may have played a role in arresting hard coral recovery at Horseshoe Reef and may have contributed to the decline at Gannet Cay although, as stated above, this is more likely due to the presence of A. planci. Surveys in January 2017 also found low level bleaching of scattered individual colonies on four reefs (Reef 21-529, East Cay, Horseshoe Reef and Gannett Cay). This level is considered minor and is not expected to impact hard coral cover, although sampling probably preceded the peak of heat stress, which is usually around March on the GBR. Low to moderate levels of white syndrome disease and corallivorous snails (Drupella spp) were recorded at most reefs. These were close to the long term average for this sector.

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. planci outbreak terminology can be found on the AIMS website.

 

Table 1. Summary of manta tow surveys for reefs in the Swain sector. Arrows indicate the trend in live coral cover and A. planci since last survey; “ ↑ ”   = increase, “ ↓ ”  = decrease, “  ” = no change.

Reef

Shelf Position

Tows

A. planci

A. planci per tow

Median Live Coral Cover

Median Dead Coral Cover

Median Soft Coral Cover

Reef Status

21-529

Mid

37

0

0

30-40%

0-5%

0-5%

NO

CHINAMAN

Mid

33

99

3.00

30-40%

0-5%

0-5%

AO

GANNET CAY

Mid

20

1

0.05

40-63%

0-5%

0-5%

NO

HORSESHOE

Mid

85

3

0.04

20-30%

0-5%

0-5%

NO

SNAKE

Mid

92

0

0

40-63%

0-0%

0-5%

NO

EAST CAY

Outer

66

0

0

30-40%

0-5%

0-5%

NO

TURNER CAY

Outer

55

2

0.04

40-50%

0-5%

5-10%

NO

Dates: 8th – 28th January 2017

Vessel: RV Cape Ferguson

Survey leader: Mike Emslie  

Figure 2 - Sector wide changes in a. coral cover and b. the numbers of A. planci through time for all survey reefs in the Swain sector of the GBR. Orange points are the mean coral cover estimates and the orange line and envelope are the modelled trends through time (mean ± 1 S.E.) from a Generalized Additive Model (GAM). Blue bars are the number of A. planci (mean ± 1 S.E.). All data are estimates per two-minute manta tow, averaged across all survey reefs in the sector. The dotted horizontal blue line indicates Incipient Outbreak density threshold for A. planci (0.22 A. planci /tow) that may be expected to reduce regional coral cover. The dotted red line indicates Active Outbreak densities of A. planci (1.0 A. planci/tow) that are certain to reduce regional coral cover.

Figure 3 - Recovery from storm damage at Reef 21-529. a. In 2006 this Preservation Zone had a mature, diverse coral community which had endured throughout most of the 2000s b. By 2010, the once luxuriant coral assemblage had been reduced to rubble at the foot of the bare reef slope after Cyclone Hamish tracked ~10 kilometers to the northeast of the reef in 2009 c. The surveys from January 2017 revealed that hard corals have recolonized the reef slope, although recovery on the rubble beds at the foot of the slope remain patchy.

Figure 4 - Signs of life on Gannett Cay. a. High coral cover was encountered on the hard reef matrix of the upper reef slope in January 2017, while b. coral recovery was a lot patchier on the unconsolidated rubble beds at the foot of the slope.

Figure 5 - Plenty of fresh feeding scars and adult corallivorous starfish A. planci are indicative of an Active Outbreak, such as was recorded at Chinaman Reef.