Report on surveys of the Capricorn-Bunker sector of the Great Barrier Reef

Summary

• Hard coral cover stable since recovering from storms and Cyclone Hamish in 2008/09
• Little evidence of significant impact of the 2016 or 2017 bleaching event
• Low numbers of coral feeding crown-of-thorns starfish, Acanthaster solaris* observed at two reefs

Figure 1: Map showing location of reefs in the Capricorn-Bunker sector. Click on figure to go to AIMS Spatial Maps page for access to information on individual reefs.

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

As part of the Long Term Monitoring Program (LTMP), manta tow surveys of coral cover and abundance of the coral feeding crown-of-thorns starfish (COTS), Acanthaster solaris, were completed on seven reefs in the Capricorn-Bunker sector of the Great Barrier Reef (GBR). Detailed surveys for benthic organisms, visual census of fishes and surveys for agents of coral mortality (scuba search) were made on these reefs and an eighth reef. Preliminary results of the manta tow and scuba search surveys are presented in this report. Manta tow surveys (Table 2) showed that median reef-wide live coral cover (coral cover) had remained stable at four reefs, increased at two reefs and declined on one. Coral cover on these reefs has recovered rapidly from storm damage in 2008/09.

Low levels of coral disease were observed on all reefs during manta tow surveys. A single COTS was observed on Fairfax Island where numbers continue to decline from Active Outbreak levels recorded in 2014. Two COTS were observed at Lady Musgrave Island. Small numbers of COTS have been observed along the back reef at Lady Musgrave Island in previous surveys, but these have always been well below outbreak densities. Overall COTS activity remains low in this sector.

Scuba surveys in 2018 recorded very low levels of coral bleaching, which was limited to scattered individual colonies on one reef (Hoskyn Island). There was no evidence of mortality from coral bleaching the previous summer. Incidence of white syndrome coral disease had increased on Boult Reef, Broomfield Reef, Fairfax Island, Masthead Reef and North Reef since the previous survey. Disease levels were low to moderate and were below those recorded in previous years. This most probably reflects the high cover of table and branching Acropora corals, which are among the most susceptible corals to this disease. Incidence of brown-band coral disease had increased on three reefs (Boult, Fairfax Island and Lady Musgrave Island) but, as was the case for white syndrome, these levels were unexceptional.

Summaries of the effects of zoning on study reefs in this sector are shown in Figure 3.

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

Table 2: Summary of manta tow surveys of reefs in the Capricorn-Bunker 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: 11^th – 27^th October 2017

Vessel: RV Cape Ferguson

Survey leader: Ian Miller

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. solaris outbreak terminology.

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

Video of very high coral cover on one of the LTMP’s permanent monitoring sites at North Reef. Reefs in the Capricorn-Bunker Group have high cover of tabulate and branching Acropora spp. corals on exposed reef faces.

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

Image 1: Very high coral cover on Broomfield Reef that has shown remarkable recovery from storm damage. Reduced to bare framework in 2008/09 after storms and Cyclone Hamish surveys in 2018 found that hard corals, particularly table Acropora spp., have recolonized the reef slope.

Image 2: Exposed parts of reefs in this sector are subject to strong wave action from oceanic swell creating distinctive “spur and groove” formations in the reef slope.

Image 3: Chevron Butterflyfish (Chaetodon trifascialis) are coral specialists that feed on polyps and mucus; they thrive in areas of high coral cover.