Coral cover has declined on reefs in this sector due to crown-of-thorns starfish
The number of coral-eating crown-of-thorns starfish continues to increase.
Outbreaks occurred on four of six reefs surveyed by manta tows.
Hard Coral Cover 0-10% 10-30% 30-50% 50-75% 75-100%
Figure 1: Map showing location of reefs in the Swain sector.
Table 1: Overview of results obtained from manta tow surveys of reefs in the Swain sector
|Swains Sector||Summary||Trend since last survey|
|Average Coral Cover (%)||36.14||Increased|
|COTS status:||4 Active Outbreaks, 0 Incipient Outbreaks||Increased|
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* on six reefs in the Swain sector of the Great Barrier Reef (GBR) were completed. Preliminary results of the manta tow surveys are presented in Table 1 and Table 2. Median reef-wide live coral cover (coral cover) had decreased at five reefs and increased on one (Reef 21-529). A seventh reef (East Cay) was surveyed using scuba surveys on fixed transects only and showed no change in coral cover.
Declines in coral cover at four of the reefs were associated with Active Outbreaks of COTS (Table 1). Particularly large numbers of COTS (>4 per tow) at both Horseshoe Reef and Turner Cay Reef had caused catastrophic losses of coral cover from 20-30% and 40-50% respectively to 0-5% in under two years. Even large and presumably very old massive corals were being consumed by COTS on these two reefs. Coral losses at Gannet Cay Reef and Snake Reef were less extreme but are likely to escalate because COTS numbers have only recently increased; just one COTS was recorded on those two reefs in 2017, yet numbers have now increased markedly at Gannet Cay and COTS were recorded for the first time in decades during manta tow surveys at Snake Reef (Table 1).
Scuba surveys of fixed transects recorded coral bleaching on individual colonies (0-1%) on two reefs while cases of coral disease and numbers of the corallivorous snail, Drupella spp., were generally within the range of past values. COTS were recorded on scuba surveys at five reefs, reflecting the increasing reef-wide numbers seen in manta tow surveys.
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 Swain 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||Live Hard Coral Cover %||Soft Coral Cover %||Survey Year||Reef Status|
|TURNER REEF)||Outer||56||2017||246||4.39||5.36 ▼||8.92||2019||AO|
|CHINAMAN REEF(22102))||Mid||29||2018||56||1.93||3.45 ▼||6.89||2019||AO|
|GANNETT CAY REEF)||Mid||20||2017||76||3.8||29.99 ▼||4.46 ▲|
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 Swain sector of the GBR. Orange trend line = Hard coral. Purple bars = Crown-of-thorns (COTS)
Image 1. So few live corals remained at Horseshoe Reef that many COTS were roaming in search of prey; in fact many COTS were recorded on sand in the back reef lagoon.
Image 2. The result of an extreme COTS outbreak at Turner Cay Reef: nearly all live corals have been eaten and the skeletons have taken on a drab brown appearance.
Image 3. A team member surveys the benthic community along transect lines at Snake Reef where coral cover was still very high in places
Image 4. Diverse coral communities were recorded at Gannet Cay but so was an Active Outbreak of COTS. The fate of this reef is hanging in the balance given the devastation COTS have recently wrought on nearby reefs. Here a COTS sits beneath a bright white plate coral skeleton, the result of its recent gorging, with a smorgasbord of preferred coral prey just beyond.