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

Summary

• Hard coral cover was high (30-50%) but had declined overall since 2017
• Recent coral losses are due to increased crown-of-thorns starfish, Acanthaster cf. solaris* activity with outbreak densities observed on several reefs
• There were no obvious signs of major coral mortality from bleaching in the past year on the perimeters of surveyed reefs.
  
   

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

Table 1: Overview of results obtained from manta tow surveys of reefs in the Swain sector.

As part of the Long Term Monitoring Program (LTMP), surveys of coral cover and of the abundance of the coral feeding crown-of-thorns starfish (COTS), Acanthaster solaris* on twelve reefs in the Swain sector of the Great Barrier Reef (GBR) have been completed using the manta tow technique. Preliminary results of the surveys are presented in Table 1 and 2. These show that median reef-wide live coral cover (hereafter: coral cover) had increased at six reefs, declined on five reefs and remained stable on one (Reef 21-139). Coral cover ranged from very low (0-5%) to very high (63-75%).

Outbreak densities of COTS were observed on Chinaman Reef, Jenkins Reef, Wade Reef and Reef 21-187. Coral cover was very low to moderate at each of the first three reefs, with COTS having already caused large declines in coral cover, particularly on Wade Reef and Jenkins Reef. Conversely, overall coral cover had increased on Reef 21-187 despite outbreak levels of COTS occurring in some areas of the reef. Coral cover had increased overall on the other reefs that were surveyed with the exception of Reef 21-296 and 21-139, where coral cover remained stable. The reason for the decline in coral cover at East Cay is unknown.

There were no signs of major coral mortality from bleaching in the past year on the perimeters of surveyed reefs. Low levels of white syndrome disease were recorded during scuba search surveys and small numbers of colonies with minor bleaching were seen on each reef. Overall there was no indication of widespread bleaching but there was some evidence of stress in the form of a slight colour change, especially on parts of Jenkins Reef where some corals on the reef flat were “fluorescent” in appearance.

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.

*Note: genetic studies show that there are at least four species of COTS. These are the North and South Indian Ocean species (A. planci and A. mauritiensis), Red Sea species (not yet named) and a Pacific species. The range of the Pacific includes the Great Barrier Reef and it has been provisionally named Acanthaster solaris (Haszprunar et. al. 2017).

Table 2: Summary of manta tow surveys of reefs in the Swain sector. Arrows indicate the trend in live coral cover and A. solaris since last survey; ▲ = increase, ▼ = decrease, “ ” = no change.

Hard coral cover from fixed site surveys

Total fish density from fixed site surveys

Coral trout density from fixed site surveys

Soft coral cover from fixed site surveys

Large fish density from fixed site surveys

Secondary target density from fixed site surveys

Algal cover from fixed site surveys

Small fish density from fixed site surveys

Herbivore density from fixed site surveys

Figure 3: Effects of marine park zoning. Comparisons of the median values of nine variables through time, based on surveys of fixed sites on matched pairs of reefs in this sector. One reef in each pair was rezoned so fishing was prohibited (Green zone) in 2004, the other reef in each pair remains open to fishing (Blue zone). Error bars are 95% credible intervals. Please click on each panel for an enlarged view.

Dates: 15^th January– 4^th February 2018

Vessel: RV Cape Ferguson

Survey leader: Kate Osborne and Mike Emslie

Details of the manta tow method and results can be found here.

Click here for further details of the monitoring program design, sampling methods and a full explanation of the A. planci outbreak terminology.

For enquiries, please contact monitoring@aims.gov.au

Figure 2: Sector-wide changes in coral cover and the numbers of A. solaris for survey reefs in the Swain sector of the GBR.

Image 1: There have been A. solaris outbreaks on reefs in the south east of the Swains since they were last surveyed in 2016. On Jenkins Reef coral cover was 0-5%.

Image 2: Crown-of-thorns starfish attack a surviving coral colony on Jenkins Reef.

Image 3: Panoramic view of the reef slope on Reef 21-558. The reef crest had high coral cover, an indication that coral bleaching has been less severe than on the northern Great Barrier Reef

Image 4: AIMS volunteer diver snorkels on Reef 21-558, where coral cover was still high. There was no evidence of large scale mortality from coral bleaching since the last surveys in 2016/2017, but some scattered colonies were pale in January 2018