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

Summary

  • Surveys were completed on eight reefs in the Swain sector of the Great Barrier Reef.
  • Four reefs in this sector were not surveyed in this reporting year due to Covid restrictions.
  • Sector-wide hard coral cover was moderate (10-30%).
  • Five reefs were previously surveyed in 2018; hard coral cover increased on one and decreased on the remaining four.
  • Two reefs were last surveyed in 2016 and hard coral cover increased on both
  • Very high numbers of crown-of-thorns starfish were recorded on three reefs, with low numbers on another three reefs.
  • Coral bleaching was observed on scattered individual colonies at one reef.

Hard Coral Cover   0-10%  10-30%  30-50%  50-75%  75-100%

Figure 1: Map showing location of reefs in the Swains sector. Click the points for more information.


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

As part of the Long-Term Monitoring Program (LTMP), manta tow surveys of coral cover and the abundance of the coral feeding crown-of-thorns starfish (COTS), Acanthaster cf. solaris were completed on eight reefs in the Swain sector of the Great Barrier Reef.

The preliminary results of the manta tow surveys are presented in Tables 1 and 2. Average sector-wide live coral cover was moderate (10-30%) and had increased slightly since previous surveys in October 2018.

Hard coral cover was high (30-50%) on three reefs, moderate (10-30%) on two reefs and low (0-10%) on three reefs (Table 2). Five reefs were previously surveyed in 2018; hard coral cover increased on one and decreased on the remaining four.

Two reefs were last surveyed in 2016 and both had increased hard coral cover while the remaining reef was last surveyed in 2019 and coral cover was stable (Table 2).

Very high numbers of COTS were recorded during manta tow surveys at three reefs, and numbers had increased from prior surveys to be now above active outbreak levels (Table 2). There were low numbers of COTS recorded at another three reefs. At two of these reefs, numbers of COTS had decreased since previous surveys and these reefs were classified as Recovering (Table 2). At the remaining reef with low numbers of COTS, numbers had increased since previous surveys but remained below outbreak levels (Table 2). No COTS were recorded on two reefs (Table 2).

Scuba surveys of fixed transects were conducted at eight reefs and recorded high numbers of COTS on two reefs. Low levels of coral bleaching were recorded at one reef but was restricted to scattered individual colonies. For all reefs, cases of disease and numbers of the corallivorous snail, Drupella spp. were within the range of past values recorded from reefs within this region.

More detailed results on fish and benthic assemblages from fixed site surveys will be available soon.

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.

Table 2: Summary of manta tow surveys of reefs in the Swains sector. Arrows indicate the trend in live coral cover and A. solaris since last survey; ▲ = increase, ▼ = decrease, " " = no change. Outbreak Status: NO no outbreak, IO incipient outbreak >.22 COTS per tow, RE recovering, AO = Active Outbreak>1 COTS per tow.

Figure 2: - Sector-wide changes in coral cover and the numbers of A. solaris for survey reefs in the Swains sector of the GBR. Orange trend line = Hard coral. Purple bars = Crown-of-thorns (COTS)

Figure 2: - Sector-wide changes in coral cover and the numbers of A. solaris for survey reefs in the Swains sector of the GBR. Orange trend line = Hard coral. Purple bars = Crown-of-thorns (COTS)

Image 1. Coral cover on Reef 21-245 was very good in places showing a remarkable recovery since Severe Cyclone Hamish in March 2009, despite the presence of crown-of-thorns starfish on the reef. Image 1. Coral cover on Reef 21-245 was very good in places showing a remarkable recovery since Severe Cyclone Hamish in March 2009, despite the presence of crown-of-thorns starfish on the reef.
Image 2. Sub-massive hard corals growing on the crest of Reef 21-245 were a refreshing sight as these corals are particularly susceptible to the depredations resulting from coral bleaching.                    Image 2. Sub-massive hard corals growing on the crest of Reef 21-245 were a refreshing sight as these corals are particularly susceptible to the depredations resulting from coral bleaching.
Image 3. Active outbreaks of COTS were recorded on three reefs in the Swain sector. Here a COTS lies adjacent to the transect tape, with an obvious white scar of dead coral from recent feeding activities. Image 3. Active outbreaks of COTS were recorded on three reefs in the Swain sector. Here a COTS lies adjacent to the transect tape, with an obvious white scar of dead coral from recent feeding activities.
Image 4. Divers record the number of COTS and their feeding scars during fixed site surveys along permanent transects. Image 4. Divers record the number of COTS and their feeding scars during fixed site surveys along permanent transects.
Fish abundance from fixed site surveys Fish abundance from fixed site surveys
Fish abundance from fixed site surveys Fish abundance from fixed site surveys