Report on surveys of reefs in the Whitsunday sector of the Great Barrier Reef

Dates: 13th April to 27^th April 2013

Vessel: MV Cape Ferguson

Survey leader: Kate Osborne

As part of the Long Term Monitoring Program (LTMP), manta tows and detailed scuba surveys for benthic organisms, reef fishes, and scuba searches for agents of coral mortality were completed on nine reefs in the Whitsundays sector. Preliminary results of the manta tow surveys and scuba searches are reported here.

Median reef-wide live hard coral cover (henceforth coral cover) ranged from very low (0-5%) to moderate (10-30%) on reefs surveyed by manta tow in the Whitsundays sector. Coral cover was similar (within half a category) to when these reefs were last surveyed using manta tows. Coral cover on all the inshore reefs had declined, though the reason for this decline is uncertain because of the long period between surveys (eight years). Cyclone activity in recent years is likely to have played a major role and Cyclone Ului in 2010 is a likely candidate. No crown-of-thorns starfish (COTS) outbreaks were recorded. COTS were seen on two reefs (Reef 20-104 and Langford and Bird Islets) but there was no evidence of widespread scarring that would indicate high densities of COTS.

Occurrence of coral diseases was generally at or below background levels. Numbers of Drupella spp. (coral feeding snails) were also low, although large individuals of Drupella spp. were causing noticeable mortality on Hayman Is Reef.

A summary of the results is presented in Tables 1 and 2. A full list of survey reefs appears in the Long-term Monitoring Status Report Number 8. Details of the monitoring program design, the sampling methods and a full explanation of the COTS outbreak terminology used in this report can be found on the AIMS reef monitoring website.

Nine reefs were surveyed using manta tow (Table 1) and intensive scuba searches (Table 2). All outer shelf reefs had very low coral cover (0-5 %). These reefs are in the early stages of recovery after being stripped of most of their live coral cover by the full impact of Severe Cyclone Hamish (category 5) that passed within 30km of these reefs in March 2009. Inner and midshelf reefs were less damaged by Cyclone Hamish than the outer shelf reefs were. All reefs in this sector were also affected by Cyclone Ului, a category 2 storm that passed through this sector in March 2010. Coral cover on mid-shelf reefs was moderate in 2013 (10-20%) and similar to the value recorded in the last survey in 2011. Coral cover had increased at one reef (20-104), the first signs of recovery in coral cover on a reef that had previously been affected by COTS (2005-2007), Cyclone Hamish (2009) and Cyclone Ului (2010).

The waters around the inshore reefs were unusually clear, which allowed three reefs to be surveyed by manta tow for the first time since 2005. Coral cover on all three of these inshore reefs had declined since they were last surveyed. Coral cover at Border Island and Langford and Bird Island has declined from 10-20% in 2005 to 5-10% in 2013. Coral cover at Hayman Island declined similarly from 20-30% in 2005 to 10-20% in 2013. Given the eight year break between surveys, the reason for the decline is not known with any certainty. However the major declines in coral cover at both Border Island and Hayman Island and to a lesser extent at Langford and Bird Islands, occurred on exposed parts of the reef, which suggests storm damage from the cyclones that have affected this region in recent years (particularly Cyclone Ului that passed through the Whitsunday islands in March 2010) and these have likely played a major role in the observed decline.

Crown-of-thorns starfish were seen on two of the nine reefs that were surveyed by manta tow. One COTS was observed on the back of Reef 20-104. Though this constituted a localised incipient outbreak, a lack of feeding scars suggested that starfish densities were unlikely to be a threat to reef-wide live coral cover in the immediate future. Similarly three large COTS were seen on the back reef at Langford and Bird Island, this was also enough to constitute a localised incipient outbreak. In this case the large size and number of COTS suggest that they may have some localised impact on coral cover on this part of the reef, however this density of COTS would not be considered to be a threat to coral cover at the reef level.

One COTS was recorded during scuba searches of the marked survey sites on Reef 20-104. Scuba searches on the nine intensive survey reefs (Table 2) found coral diseases including "white syndrome" (WS, a disease-like necrosis found particularly on tabulate Acropora spp.), brown band disease (BrB) and "skeletal eroding band" (SEB). Virtually no diseases were seen on outer reefs, but coral cover was low. Disease was rare elsewhere except at two reefs: Hayman Island and Reef 19-131, where there were low levels of brown band disease and white syndrome. Drupella spp. (coral feeding snails) were generally found in numbers similar to those recorded when these reefs were last surveyed in 2011. The exception was Hayman Island Reef, where density of snails was higher than both the long term average and the density recorded at the last survey in 2011.

Table 1. Summary of results of manta tow surveys of reefs in the Whitsunday sector (Click on reef names for more information).

Table 2. Summary of results of Scuba searches of reefs in the Whitsunday sector.

WS = White Syndrome, BBD = Black Band Disease, BrB = Brown Band Disease, SEB = Skeletal Eroding Band Disease. Figures are the number of starfish (COTS), infected coral colonies (WS, BBD, BrB, SEB) or snails (Drupella) recorded at each reef.