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

Summary

  • On average, sector-wide hard coral cover was moderate (10-30%).
  • Of the 12 reefs surveyed by manta tow in this sector, hard coral cover had increased eight reefs and had decreased on four reefs in since last surveyed.
  • There were very low numbers of crown-of-thorns starfish, and no active outbreaks were recorded.
  • In June 2020, bleaching severity was still being recorded as low to moderate on most reefs.
  • Incidence of coral disease was low on all surveyed reefs.

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

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


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

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

Preliminary results of the manta tow surveys for the sector show the overall average hard coral cover was moderate (10-30%) (Table 1; Fig 2) and had increased marginally since January 2019 (Fig 2).

Hard coral cover was high (30 to 50%) on four reefs, moderate (10 to 30%) on seven reefs and low (0 to 10%) on Rib Reef (Table 2). Average coral cover increased on eight reefs but declined on four reefs (Table 2).

Low numbers of COTS were recorded on three reefs. These were below thresholds that define active (1 COTS per tow) or incipient (0.22 COTS per tow) outbreaks, however Kelso Reef has increasing numbers of COTS that could potentially lead to a future outbreak. Active removal of starfish has occurred throughout the Townsville sector in recent years by GBRMPA’s crown-of-thorns starfish control program, which contributed to reduce the number of COTS visible during manta tow surveys. For example, Rib Reef had an active outbreak from 2017 to 2018 and targeted COTS culling began in 2017; no COTS were recorded at this reef in 2020. Hard coral cover at Rib Reef decreased from very high (50 to 75%) in 2016 to low (0 to 10%) in 2020, reflecting the cumulative disturbances affecting this reef (bleaching in 2017 and 2020, outbreak numbers of COTS (>1.0 COTS per tow) from 2017 to 2018).

Despite surveys being conducted in winter following the 2020 mass coral bleaching event, coral bleaching was frequently observed during manta tow surveys. While the overall bleaching severity for the sector was low (Table 1), bleaching severity varied among reefs from a low (0 to 5%) to a high (30 to 50%) proportion of corals bleached.  

Fixed site surveys conducted at fourteen reefs in this sector revealed widespread low to moderate coral bleaching.  Bleaching severity ranged from low (0 to 10% of colonies affected), to moderately bleached (10 to 30% of colonies affected).

The fixed transect surveys also detected increased levels of white-syndrome disease on six of the surveyed reefs, however, the overall incidence of disease and Drupella spp. was low on the surveyed reefs.

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 COTS outbreak terminology can be found on the AIMS website.

 

Table 2: Summary of manta tow surveys of reefs in the Townsville sector. Arrows indicate the trend in live coral cover and A. solaris since last survey; ▲ = increase, ▼ = decrease, " " = no change. Outbreak Status: NO no outbreak, PO potential outbreak >0.1 COTS per tow, 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 Townsville 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 Townsville sector of the GBR. Orange trend line = Hard coral. Purple bars = Crown-of-thorns (COTS)

Image 1. In 2017 and 2018, an active outbreak of COTS was recorded on Rib Reef which dramatically reduced the hard-coral cover. Since then, numbers of COTS have substantially reduced due to active COTS control, however the damage has been done and large areas of this reef are now dominated by algal-covered dead skeletons. Image 1. In 2017 and 2018, an active outbreak of COTS was recorded on Rib Reef which dramatically reduced the hard-coral cover. Since then, numbers of COTS have substantially reduced due to active COTS control, however the damage has been done and large areas of this reef are now dominated by algal-covered dead skeletons.
Image 2. Low to moderate severity coral bleaching was recorded on many reefs during the 2020 surveys. Many like this Platygyra spp. were still partially bleached despite being winter. Image 2. Low to moderate severity coral bleaching was recorded on many reefs during the 2020 surveys. Many like this Platygyra spp. were still partially bleached despite being winter.
Image 3. High levels of Acropora corals were seen on several reefs in this sector, including Helix Reef pictured here. Image 3. High cover of Acropora corals were seen on four of the 12 surveyed reefs in this sector, including Helix Reef pictured here.
Image 4. Coral bleaching severity was moderate to high at Myrmidon Reef, on the outer shelf. Image 4. Coral bleaching severity was moderate to high at Myrmidon Reef, on the outer shelf.