Report on surveys of the Princess Charlotte sector of the Great Barrier Reef


Summary

  • Nine reefs were surveyed in this sector.
  • Five reefs were previously surveyed in 2017, after the major bleaching event in 2016. Between 2017 and 2019, hard coral cover declined on two and was unchanged on three reefs.
  • The overall median hard coral cover for the sector was moderate.
  • Low numbers of COTS were recorded on two reefs, but below outbreak levels.
  • Coral bleaching was widespread at low levels (scattered, individual colonies).
  • Significant coral bleaching (up to 50% of hard coral) was observed on localised parts of the reef flat of Rodda Reef and Reef 13-124.
  • Recent storm damage observed on the three additionally surveyed outer shelf reefs was likely due to Cyclone Penny in January 2019.

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

Figure 1: Map showing location of reefs in the Princess Charlotte 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 nine reefs in the Princess Charlotte Bay 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 moderate (10 to 30%) and was unchanged since last surveyed in 2017 (Table 1). Median reef-wide hard coral cover was moderate (10 to 30%) on eight reefs and low (0 to 10%) on the remaining reef (Table 2). Five reefs were previously surveyed in 2017, and hard coral cover remained stable on three reefs and declined on two reefs (Table 2). Low numbers of COTS were recorded at two reefs (Table 2). Declines in hard coral cover were likely due to low numbers of COTS, cyclone activity or exposure to high sea surface temperatures since the last survey.

Scuba surveys of fixed transects recorded low levels of coral bleaching, likely to be caused by exposure to above-average sea temperatures in December 2018.

Cases of coral disease were low on all reefs as were numbers of the corallivorous snail, Drupella spp. Coral juveniles were observed in multiple size classes at densities likely to support 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).

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.