- Hard coral cover remained stable at moderate values (10-30%).
- Crown-of-thorns starfish activity increased with one Active Outbreak recorded.
- Low levels of coral disease were recorded.
Hard Coral Cover 0-10% 10-30% 30-50% 50-75% 75-100%
Figure 1: Map showing location of reefs in the Townsville sector.
Table 1: Overview of results obtained from manta tow surveys of reefs in the Townsville sector
|Townsville Sector||Summary||Trend since last survey|
|Median Coral Cover||Moderate (10-30%)||Stable|
|COTS status:||1 Active Outbreaks, 0 Incipient Outbreaks||Increased|
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 five reefs in the Townsville sector of the Great Barrier Reef (GBR). Preliminary results of the manta tow surveys are presented in Tables 1 and 2.
Five reefs were surveyed for reef-wide median coral cover. Overall median coral cover was stable at the sector level. Median coral cover increased on two reefs, declined on two reefs and remained stable on one reef. Two reefs had COTS. Active removal of COTS has occurred in the Townsville sector over the last few years and may have reduced the number of COTS visible when manta tows were done. No COTS were observed on Rib reef, which had an active outbreak in 2017. No signs of coral bleaching were observed during manta tow surveys. Manta tow surveys were not conducted on the inshore reefs due to poor visibility. Results from scuba surveys on fixed sites on two inshore reefs (and) indicated a three-fold increase in hard coral cover on Havannah Island to 18% whereas Pandora reef was similar to the previous survey at 42% hard coral cover.
Fixed transect surveys on scuba were conducted on eight reefs in this sector and indicated low levels of bleaching (0-1%) on Rib reef and all other reefs had no bleaching. Scuba surveys also indicated signs of white-syndrome disease were elevated on Davies Reef. For other reefs in the sector, the incidence of disease and Drupella spp. was low and generally below levels seen in previous years.
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), a 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 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, IO incipient outbreak >.22 COTS per tow, RE recovering, AO = Active Outbreak>1 COTS per tow.
|Reef||Shelf Position||Tows||Previous survey year||A. solaris||A. solaris per tow||Median Live Coral Cover||Median Soft Coral Cover||Reef Status|
|DAVIES||Mid||46||2017||1 ▲||0.02||30-40% ▲||0-5%||NO|
|JOHN BREWER||Mid||86||2017||3 ▲||0.03||10-20% ▼||0-5%||AO|
Figure 3: Estimates for benthic data and fish abundance from fixed site surveys. Data are mean estimates Â± 95% credible intervals derived from Bayesian hierarchical linear models. Total fish abundance is the combined counts of two groups of fishes that have different maximum body sizes and are surveyed differently. Large bodied fish are highly mobile, generally have maximum sizes > 15cm and are counted by a trained diver in a 5m wide swathe along each transect. Small bodied fish are all site dependent damselfishes, mostly
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. View looking down the reef slope showing abundant coral and fishes on Davies Reef.
Image 2. Some live coral colonies interspersed among the casualties from an Active outbreak of crown-of-thorns starfish at John Brewer Reef.
Image 3. View looking along the slope at Dip Reef with plenty of pink coralline algae as well as sand and rubble at the reef base.