- Nine reefs were were surveyed using manta tows in 2017, hard coral cover had increased on three, remained the same on five and had declined on an one.
- Sector-wide, hard coral cover had increased since 2017 but was still low (0-10%).
- There were no outbreaks of crown-of-thorns-starfish (COTS), although two animals were observed on one reef.
- Coral bleaching was observed at low levels (scattered, individual colonies) on all reefs.
Hard Coral Cover 0-10% 10-30% 30-50% 50-75% 75-100%
Figure 1: Map showing location of reefs in the Cooktown-Lizard sector.
Table 1: Overview of results obtained from manta tow surveys of reefs in the Cooktown-Lizard sector
|Cooktown / Lizard Island Sector||Summary||Trend since last survey|
|Median Coral Cover||Low (0-10%)||Increased|
|COTS status:||No Outbreaks||Stable|
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 nine reefs in the Cooktown-Lizard Island sector of the Great Barrier Reef (GBR). Preliminary results of the manta tow surveys are presented in Tables 1 and 2. The overall median hard coral cover for the sector was low (0-10 %) but had increased since last surveyed in 2017 (Table 1). Median reef-wide hard coral cover had increased on three reefs, but was still low. Reefs in the Cooktown-Lizard Island sector were impacted by Cyclone Ita and COTS before surveys in 2015, then impacted by Cyclone Nathan and bleaching in summer of 2015/2016 after surveys in 2015, which reduced hard coral cover to low levels (0-10%) by 2017. The small increase in hard coral cover recorded in 2019 indicated some reefs were in the early stages of recovery. Hard coral cover had declined on Sandbank No. 1 Reef since the previous survey in 2005 to moderate levels (10-30%), yet it had the highest coral cover for the sector in 2019 surveys (Table 2). Due to the long period between surveys the cause of this decline was not clear, but may have been due to coral bleaching between 2015 and 2017 and/or storm damage. No COTS were observed on manta tow surveys, but very low numbers (two individuals) were recorded on Lizard Island Reef.
Fixed site surveys and broad-scale manta tow surveys recorded low levels (0-1%) of coral bleaching on all reefs. Coral disease was low on one reef and rare on the other six reefs. Disease of coralline algae was present on four outer shelf reefs and was notably higher than on previous surveys. Low counts of the corallivorous snail, Drupella spp. were recorded. Coral juveniles (small corals less than 5cm diameter) were observed on broad-scale manta tow surveys and on fixed site surveys at densities likely to support future reef recovery.
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 Cooktown-Lizard 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|
|NORTH DIRECTION IS||Mid||16||2017||0||0||0-10%||0-5%||RE|
|NO NAME||Outer||60||2017||0||0||5-10% ▲||0-5%||NO|
|SANDBANK NO.1||Outer||42||2005||0||0||10-20% ▼||0-5% ▼||NO|
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 Cooktown-Lizard sector of the GBR. Orange trend line = Hard coral. Purple bars = Crown-of-thorns (COTS)
Image 1. A view of the reef slope showing corals from a number of size classes. The largest corals most likely survived the severe bleaching events in 2015/16 and 2016/17, while the smaller corals are those which resulted from coral spawning events after the bleaching. The regeneration of coral populations has led to small increases in coral on some reefs.
Image 2. A view of the reef crest showing ridges, which are the dead standing skeletons of former living corals (Isopora spp.), that probably died as a result of the severe bleaching event in 2015/16 and 2016/17. There appears to be some regrowth of small colonies in the valleys between ridges.
Image 3. Porites corals dominate this scene from a mid-shelf reef. These corals are typically most resistant to coral bleaching and the colonies seen here have survived recent disturbances.