Preliminary report on surveys of the Cairns sector of the Great Barrier Reef


Summary

  • Hard coral cover remained low (0-10%) in 2019,

  • Low cover was a result of declines to historical lows in 2017 due to coral bleaching

  • No coral feeding crown-of-thorns starfish Acanthaster solaris* were observed

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

Figure 1: Map showing location of reefs in the Cairns 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 fourteen reefs in the Cairns 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 remained low (0-10 %), as per the previous survey report in 2018 (Table 1). Hard coral cover declined on nine reefs and remained stable on five reefs. Declines were mainly due to coral bleaching events in the summers of 2016 and 2017. Outbreaks of COTS may also have contributed to declines (Table 2). However, because  COTS control has been active the contribution of COTS to coral decline is difficult to measure accurately. No COTS were observed on manta tow surveys in 2019.

Scuba surveys at fixed sites (ten reefs) and broad-scale manta tow surveys recorded low levels (0-1%) of coral bleaching on all reefs. Coral diseases including white syndrome, black-band disease, brown-band disease and skeletal eroding band disease were rare at all reefs. The density of the corallivorous snail, Drupella spp., was moderate at Hastings reef (287 per hectare) and very low on the other reefs where scuba surveys were conducted.

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).

Figure 3: Estimates for benthic data and fish abundance from fixed site surveys. Data are mean estimates ± 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 <15cm long, that are counted along a 1m wide swathe. The trends in abundance for two specific groups of large bodied fish that are particularly important for different reasons are also shown; large herbivorous fish aid reef resilience by removing algae that may otherwise out-compete corals and harvested fish have great economic and social value.