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

Dates: 3^rd January to 19^th January 2013

Vessel: MV Cape Ferguson

Survey leader: Ian Miller

As part of the Long Term Monitoring Program (LTMP), eleven reefs in the Cairns sector and two reefs in the Innisfail sector of the Great Barrier Reef were surveyed using manta tows. Detailed scuba surveys for benthic organisms, reef fishes, and agents of coral mortality (scuba searches) were also completed on ten reefs in the Cairns sector. Preliminary results of the manta tow surveys and scuba searches are reported here.

Median reef-wide live hard coral cover (henceforth coral cover) was moderate (10-30%) on the majority of reefs manta towed in the Cairns sector. Coral cover on the two reefs surveyed in the Innisfail sector was low (0-10%). In all cases coral cover was similar to when these reefs were last surveyed using manta tows. On reefs in the Cairns sector there has been a large increase in crown-of-thorns starfish (COTS) activity with COTS recorded from 7 of the 11 reefs surveyed including Incipient Outbreaks on Green Island and Reef 16-017. This is the first time that outbreak densities of COTS have been recorded during manta tow surveys of reefs in this sector since 2002. In addition, scuba surveys recorded outbreak levels of COTS on a further three reefs: Mackay Reef, Low Isles and Fitzroy Island. This buildup of COTS numbers was expected following increasing COTS activity on reefs in the Cooktown-Lizard Is sector since 2006. The last wave of outbreaks (beginning in 1994) greatly reduced coral cover on reefs in the northern and central GBR. During the last wave of outbreaks, COTS numbers built up on reefs in the Cooktown-Lizard Is region over a number of years. These populations subsequently reached densities that produce very large numbers of larvae that, travelling on prevailing currents, colonized and reached outbreak densities on reefs immediately to the south. In this way, outbreaks cascaded through the central GBR. Recent observations in the Cairns sector are consistent with this pattern being repeated. The high proportion of relatively small starfish seen during scuba searches and the large and increasing number of COTS recorded on reefs immediately to the north suggests that starfish populations on reefs in the Cairns sector will continue to increase in the next few years. Substantial loss of coral cover will follow, and a wave of outbreaks is likely to pass southwards through the central GBR over the next decade.

Scuba searches found cases of "white syndrome" (a disease-like necrosis found particularly on tabulate Acropora spp.), brown band disease, and "skeletal eroding band" disease at or near background levels on all reefs surveyed. Numbers of Drupella spp. (coral feeding snails) varied among reefs. The largest numbers were seen at Green Island and Thetford Reef but these numbers were not exceptional when compared to previous surveys of these reefs or of other reefs in the sector. Three localized outbreaks of crown-of-thorns starfish (COTS) were recorded on the intensive survey sites at Low Isles, Fitzroy Island Reef and Mackay Reef, with the majority of individuals recorded belonging to the smaller size classes indicating relatively recent recruitment.

A summary of the results is presented in Tables 1, 2 and 3. 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.

Eleven reefs were surveyed using manta tows (Table 1) and ten reefs were surveyed intensively using scuba (Table 2). Coral cover on the majority of reefs was moderate (10-30%); the highest cover was recorded at Mackay Reef. A substantial proportion of reefs had low coral cover (0-10%) with the lowest coral cover recorded from Green Island and Michaelmas Reef. Crown-of-thorns starfish were recorded on seven of the eleven reefs surveyed using manta tows. Two reefs (Green Island and Reef 16-017) had Incipient Outbreak levels of COTS. Coral cover on Green Island Reef has declined from 5-10% in 2012 to 0-5% in 2013, most likely due to COTS feeding activity. There is a concentrated population of small (15-25cm) starfish on the SW end of Green Island Reef that has a high potential to cause further damage to coral cover in the future. Coral cover on Reef 16-017 has also declined, from a high 30-40% in 2010 to the current 10-20%. While COTS certainly contributed, the decline is also likely to be partially due to Severe Tropical Cyclone Yasi (category 4) that passed some 200km to the south in February 2011.

Coral cover had also declined at three other reefs since the last surveys: at Reef 16-024 cover declined from 20-30% in 2010 to 10-20% in 2013, at Opal(2) Reef cover declined from 20-30% in 2011 to 10-20% in 2013, and at Michaelmas Reef cover declined from 10-20% in 2009 to 0-5% in 2013. The most recent past survey of each of these reefs was prior to STC Yasi, so this is likely to have contributed to the decline. No recent changes in coral were recorded on the remaining reefs; these were last surveyed by manta tow in 2012 (except for Middle Cay(B) Reef that was last surveyed in 2007).

COTS were recorded during scuba searches of fixed sites on four of the ten survey reefs. At three of these (Low Islets, Fitzroy Island and Mackay Reef), starfish were present at outbreak levels and their feeding is likely to reduce coral cover. Fitzroy Island and Mackay Reef had particularly high numbers of COTS. In each case, many of the starfish were small, suggesting recruitment in the last two to three years.

Scuba searches on the intensive survey sites (Table 2) found coral disease including "white syndrome" (WS, a disease-like necrosis found particularly on tabulate Acropora spp.), brown band disease (BrB) and "skeletal eroding band" (SEB) was generally more common than the long term average, but levels had been similar in the previous survey (2011). Numbers of Drupella spp. (coral feeding snails) were generally below the long-term average and lower than those recorded when these reefs were last surveyed in 2011. The exception was Green Island Reef, where density was above the long term average though similar to 2011 levels.

Table 1. Summary of manta tow survey results for 11 reefs in the Cairns sector.

Table 2. Summary of results of SCUBA searches at 4 reefs in the Cairns 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.

Two inshore reefs, Normanby and Mabel Islands and Jessie and Kent Islands, were surveyed in the Innisfail sector (Table 3). No crown-of-thorns starfish were recorded at either reef. Coral cover on these reefs was low (0-10%) and similar to those recorded in the last survey (2007 and 2002 respectively). STC Yasi (category 4) passed just 20km to the south of Jessie and Kent Islands in February 2011 and most likely caused loss of coral cover there (currently low (0-5%)). Similarly STC Yasi passed 70km to the south of Normanby and Mabel Islands and probably also reduced coral cover at that reef. In both cases the long interval between surveys means that any coral recovery prior to the cyclone cannot be assessed.

Table 3. Summary of manta tow survey results for two reefs in the Innisfail sector.

Dr. Hugh Sweatman , AIMS

Telephone: +61 7 4753 4470

Fax: +61 7 4753 4288

Email: h.sweatman@aims.gov.au