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.

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.