survey- 07-03-2011

Dates: 7^th March - 26^th March 2011

Vessel: MV Centurion

Survey leader:

Ian Miller

Image 1.On Rebe Reef on the outer shelf in the Whitsunday sector there was extensive damage due to the effects of cyclone Hamish a severe category 5 tropical rotating storm that passed some 30km to the east of this reef in March 2009. Here large chunks or reef framework weighing tonnes have broken off and thrown on to the reef crest like so much gravel, a testament to the power of the cyclone.

Photo: AIMS LTMP.

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Image 2.Under water on Rebe Reef the destruction wrought by cyclone Hamish is even starker. Here reef framework has been swept completely clean of nearly every benthic organism and replaced by a light cover of turf algae. Photo: AIMS LTMP.

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Image 3: Ben Reef in the Pompey sector also suffered severe damage from cyclone Hamish. In this photo a once thriving back reef coral community has been broken up and turned into a rubble bed covered in green macroalgae (Cladophora sp.).

Photo: AIMS LTMP.

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Summary

As part of ongoing surveys by the AIMS long term monitoring team, two reefs in the Pompey sector and five reefs in the Whitsunday sector of the Great Barrier Reef were surveyed using manta tows. The same five Whitsunday reefs, along with four others, were also surveyed using scuba to provide more detailed information on benthic organisms, reef fishes and agents of coral mortality. Preliminary results of the manta tow surveys and scuba searches for agents of coral mortality are presented in this report.

No crown-of-thorns starfish (COTS) were observed during manta tow surveys or during scuba searches on the intensive survey sites. Median reef-wide cover of living hard coral (henceforth coral cover) on reefs in the Pompey sector was very low (0-5%). This low coral cover was a result of severe Tropical Cyclone Hamish (category 4) that passed across the region 7-8 March 2009. The exception was Credlin Reef where a COTS outbreak that ended in 2006 had already dramatically reduced coral cover prior to the cyclone. Manta tows of reefs in the Whitsunday sector revealed that the two outer shelf reefs had suffered damage from cyclone Hamish and had very low coral cover, while the mid shelf reefs had suffered less damage with coral cover at two out of three mid-shelf reefs remaining moderate.

Scuba searches on reefs in the Whitsunday sector found no COTS and a relatively low occurrence of "white syndrome" (a disease-like necrosis that particularly affects tabulate Acropora spp.) and "skeletal eroding band disease" on a number of inner and mid shelf reefs. Numbers of Drupella spp. (coral feeding snails) and levels of coral bleaching were within the range of previous estimates at each reef.

A summary of the results is presented in Tables 1, 2 and 3. A full list of survey reefs appears in theLong-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.

Pompey sector reefs

Two reefs were surveyed using manta tows (Table 1). No COTS were recorded on either reef. Coral cover was very low (0-5%) on both reefs. A decrease in coral cover from a high (30-50%) level recorded in 2007 to very low (0-5%) level in 2011 at Ben Reef was almost certainly due to severe tropical cyclone Hamish (category 4) which affected the region 7-8 March 2009 and reportedly produced wind speeds of 100 knots at the nearby Creal Reef weather station before the anemometer broke. Cyclone Hamish entered the reef complex at the northern end of this sector and the eye of the storm passed very close to Ben Reef (<30　kms). Unsurprisingly, there was clear evidence of damage to corals and other benthic organisms from heavy wave action. The clearest evidence included fragmented and rolled corals, exfoliation of the reef matrix, redistribution of old and new coral rubble, extensive stands of green macroalgae and the presence of new underwater ramparts of coral rubble. Coral cover was also very low on Credlin Reef, but this reef suffered a COTS outbreak that ended in 2006 so coral cover was already very low before the cyclone. The likely effect of cyclone Hamish has been to stall recovery of coral cover on this reef.

Table 1.Summary of manta tow survey results for four reefs in the Pompey sector.

Whitsunday sector reefs

Five reefs were surveyed using manta tows (Table 2) and nine core reefs (including the five that were manta towed) were surveyed more intensively using scuba (Table 3). Manta tows surveys were not possible at the other four core reefs because of spring tides and poor underwater visibility (<6m). No COTS were recorded in the manta tow surveys. Coral cover on the outer shelf reefs (Hyde and Rebe) was very low (Table 2). Both these reefs would have borne the brunt of ocean swells produced by cyclone Hamish, which was a category 5 severe tropical rotating storm at this point. Both reefs appeared to have suffered extensive damage similar to that recorded on reefs in the Pompey sector (see above). On Rebe Reef in particular, large boulders weighing many tonnes had been tossed up on the reef crest and new berms had appeared as a result of wave action from cyclone Hamish. In contrast, two of the three mid shelf reefs (19-131 and 19-138) had suffered negligible damage from cyclone Hamish and had moderate coral cover as they did when last surveyed two years ago immediately after cyclone Hamish. The third reef, 20-104, had suffered some minor damage in more exposed areas particularly on the reef front and coral cover was low (5-10%). The disparity in cyclone damage between outer and mid shelf reefs in this sector is presumably because cyclone Hamish tracked outside the GBR out in the Coral Sea along this stretch of the Great Barrier Reef. This meant that major wave forces were dissipated on outer shelf reefs while the mid shelf survey reefs were partially shielded. These reefs remain in the early stages of recovery, particularly reef 20-104 that experienced outbreak populations of COTS as recently as 2007.

No COTS were recorded during SCUBA searches on the intensive survey sites (Table 3). The incidence of coral diseases, "white syndrome" (WS), "skeletal eroding band disease" (SEB), black band disease (BBD) and "brown band" (BrB) was generally low and below levels seen in previous years. Similarly numbers of Drupella spp. (coral feeding snails) and levels of coral bleaching were generally low and within the range of previous levels recorded at each reef.

Table 2.Summary of manta tow survey results for five reefs in the Whitsunday sector.

Table 3.Summary of results of SCUBA searches at nine 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), number of scars (WS, BBD, BrB, SEB) or snails (Drupella) recorded at each reef.

References

English, S., Wilkinson, C. and Baker, V. (1997) Survey Manual for Tropical Marine Resources (2^nd Edition). Australian Institute of Marine Science. Townsville.

This project is partially supported by the Marine and Tropical Sciences Research Facility

For further information contact

Dr. Hugh Sweatman, AIMS

Telephone: +61 7 4753 4470

Fax: +61 7 4753 4288

Email: h.sweatman@aims.gov.au