survey report 22 jan 2011

Dates: 22^nd January - 14^th February 2011

Vessel: MV Floreat

Survey leader:

Mike Emslie

Image 1. Tropical cyclone Yasi was a severe category 5 tropical rotating storm that crossed the coast south of the survey region during the second half of the trip. Resulting damage was apparent at a number of reefs. In particular mechanical damage such as overturned or broken corals, as seen in this photograph, was common on the LTMP survey sites. Photo: AIMS LTMP.

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Image 2.While many corals suffered mechanical damage others that somehow remained intact were totally stressed by the effects of the cyclone often leading to bleaching of the affected colony as can be seen in this photograph. Photo: AIMS LTMP.

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Image 3: On some parts of the reef strong wave action and surge has combined to scrub the substratum clean of any growth forms that protruded into the water column providing abundant new space for turf algae to thrive.Photo: AIMS LTMP.

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Summary

As part of ongoing Long Term Monitoring Program (LTMP), five reefs in the Cairns sector and one reef in the Cooktown-Lizard Island 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 nine reefs in the Cairns sector. Preliminary results of the manta tow surveys and scuba searches are presented in this report.

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 and had remained stable since these reefs were last surveyed. In the Cooktown-Lizard Island sector one crown-of-thorns starfish (COTS) was observed at Rosser Reef during manta tows. Though well below outbreak levels this result is in line with recent surveys that indicate increasing COTS activity on reefs in this sector. All manta tow surveys were conducted prior to cyclone Yasi, a severe category 5 tropical rotating storm that passed to the south of this sector during the period of survey. How these reefs faired after the passage of cyclone Yasi is not known.

Scuba searches found relatively high, though not unprecedented, signs of "white syndrome" (a disease-like necrosis found particularly on tabulate Acropora spp.), brown band disease and "skeletal eroding band" disease on all reefs. Numbers of Drupella spp. (coral feeding snails) were variable with no consistent patterns among reefs and numbers were generally lower than in recent years. One localized outbreak of crown-of-thorns starfish (COTS) was recorded at Green Island Reef. Tropical cyclone Yasi crossed the coast south of the survey region during the second half of the trip. Damage was apparent at a number of reefs after it had passed in the form of many overturned and damaged coral colonies, matrix exfoliation and bleached corals recorded at the majority of survey reefs. Because these reefs were not manta towed (due to poor visibility) it is not known if this damage extended to other parts of the reef.

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.

Cairns sector reefs

Five reefs were surveyed using manta tows (Table 1) and nine reefs were surveyed intensively using scuba (Table 2). Coral cover was moderate (10-30%) on St. Crispin, Opal(2), Pickersgill and Agincourt #1 reefs and high (30-40%) on Mackay Reef. All manta tow reefs were surveyed prior to the passage of cyclone Yasi and the condition of these reefs after the cyclone is not known. However there had been little change in coral cover on these reefs in recent years and all reefs were classified as either recovering or no outbreak. Severe tropical cyclone Yasi made landfall during the second half of this trip and manta tows could not be completed on a number of reefs because of low visibility.

Table 1. Summary of manta tow survey results for 5 reefs in the Cairns sector

Scuba searches on the intensive survey sites (Table 2) showed that signs of 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) were generally higher than the long term average but similar to levels when these reefs were last surveyed (2009). While the levels at each reef were higher than those recorded in most years, they are well below the peak levels recorded in 2003.

Seven COTS were recorded during scuba searches of marked sites on one survey reef, Green Island. This represents an average density of >30 COTS per hectare which is considered above the threshold where densities of COTS generally consume coral faster that the corals grow and thus would be considered a localized outbreak. Because the reef was not manta towed due to poor visibility it is not known how widespread this outbreak is around the reef perimeter. Previous surveys on fixed survey sites indicated localized outbreak levels of COTS on this reef between 1996 and 1998 with lesser numbers of COTS persisting through to 1999. Manta tow surveys indicated that in none of these instances were localized outbreaks reflected in elevated COTS numbers around the entire reef perimeter.

Numbers of Drupella spp. (coral feeding snails) were similar to the long-term average and generally lower than those recorded when these reefs were last surveyed in 2009.

Moderate numbers of bleached hard coral colonies were recorded on the majority of transects surveyed using scuba on the second half of the trip post cyclone Yasi. There were hard and soft coral colonies on many of the transects that appeared pale in colour and this can be attributed to cyclone damage. Furthermore, there was also direct evidence of cyclone damage as many colonies were overturned or broken, large slabs of substratum had been exfoliated and large bommies had been thrown around. The bleached nature of many remaining colonies most probably results from stress experienced by these colonies such as fragmentation and abrasion caused by storm generated swells.

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.

Cooktown-Lizard Island sector reefs.

One reef, Rosser, was surveyed in the Cooktown-Lizard Island sector. Coral cover on this reef was high and at similar levels to those recorded when this reef was last surveyed in 2009. Although one COTS was observed on this reef it remained classified as No Outbreak. The observation of COTS on this reef is not surprising as low numbers of COTS have been recorded on reefs in this sector since the last active outbreak(s) ended in 1999. Furthermore COTS activity has shown a steady increase in this sector since 2006. The last series of COTS outbreaks on the central and northern GBR (1994- 2008) were first seen on reefs in this sector. During recent surveys COTS were recorded on seven of fifteen reefs. Though generally below outbreak levels one reef (Startle East) was reclassified as an Incipient Outbreak this survey year.

Table 3. Summary of manta tow survey results for one reef in the Cooktown-Lizard Island sector.

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