Prior to Cyclone Yasi coral cover at Rib Reef was showing good signs of recovery from an earlier crown-of-thorns starfish outbreak. In the pictured location, large numbers of recovering tabulate corals and other benthic organisms had been removed by wave action during Cyclone Yasi leaving a scoured and relatively featureless substratum, dominated by turf algae.
Myrmidon Reef is located at the outer edge of the continental shelf and so was exposed to the full force of Cyclone Yasi. In this image taken on the north-east flank, a SCUBA diver winds up tape lines used to mark transect locations for coral and fish surveys. Wave action from Cyclone Yasi had removed most corals from the solid reef framework, except for the more robust massive and encrusting forms (seen on the bottom right of the image). Note the extensive beds of rubble at the base of the reef that had been generated by the demolition of more fragile corals.
Despite the widespread damage due to Cyclone Yasi on outer reefs surveyed in the Townsville sector, some pockets of high coral cover were spared, as in this protected lagoon location at Dip Reef. Two diagonal banded sweetlips (Plectorhinchus lineatus) are moving over the branching coral.
The patchy nature of cyclone damage can be astounding. For instance compare this image with the next, both were taken on Pandora Reef (inshore from the Palm Islands). Here on the first survey site at Pandora Reef coral growth was luxuriant and coral cover was consistently high.
Here, on Site 3 at Pandora Reef and less than one kilometre from Site 1, almost all corals had been shattered by wave action (from Cyclone Yasi) and redistributed along the reef face. Most of this rubble was covered in a fine layer of silt.
At Havannah Reef large areas of the reef slope are covered in coral rubble. Much of this rubble existed prior to Cyclone Yasi, but waves from Yasi appear to have redistributed the rubble leaving a benthic community dominated by turf algae that is growing amongst layers of silt. Over the last 15 years the benthic community has changed from one dominated by hard corals, to one dominated by macroalgae, to the current turf and silt based community. It remains to be seen whether hard corals can again dominate at Havannah Reef.
As part of ongoing surveys by the AIMS long term monitoring team, seven reefs in the Townsville sector and two reefs in the Cape Upstart sector of the Great Barrier Reef were surveyed using manta tows. The same seven Townsville sector reefs, along with two 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 are presented in this report.
Only one crown-of-thorns starfish (COTS) was observed during these surveys. Median reef-wide cover of living hard coral (henceforth coral cover) on reefs in both sectors was very low (0-5%) or low (5-10%). These values are below recent average values for the GBR and below long-term average values for mid and outer shelf reefs. Much of this low coral cover can be attributed to severe tropical cyclone Yasi (category 5) that passed to the north of Townsville on 2nd and 3rd of February 2011 and caused exceptionally big seas in the Townsville and Cape Upstart sectors. All benthic communities surveyed in the Townsville sector had been impacted by storm waves from cyclone Yasi. Coral destruction was widespread although damage to corals was patchier on inshore reefs. Evidence for cyclone effects included fragmented and rolled corals, exfoliation of the reef matrix, redistribution of old and new coral rubble and the presence of new underwater ramparts of rubble. In the Townsville sector largest coral cover declines (20-30% to 5-10%) occurred on two mid-shelf reefs. In the Cape Upstart sector, a decline in coral cover from 30-40% to 5-10% on one outer reef since last surveyed in 2005 may also be partly attributable to cyclone Yasi.
Scuba searches on reefs in the Townsville sector found only one COTS and low occurrences of coral diseases. Numbers of Drupella spp. (coral feeding snails) and levels of coral bleaching were within the range of previous counts at each 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.
Townsville sector reefs
Seven reefs were surveyed using manta tows (Table 1). No COTS were recorded during these surveys. Coral cover was very low (0-5%) or low (5-10%) on all seven reefs and had declined on five of these reefs since they were last surveyed in 2009, particularly at Davies Reef and Rib Reef where cover fell from 20-30% to 5-10%. The very low coral cover recorded at two of the seven reefs (John Brewer and Havannah Is. Reefs) did not reflect decreases as cover was similarly low in 2009. Current coral cover values for this sector were below recent average values for the GBR and below long-term average values for mid and outer shelf reefs. These low cover values reflect the effects of recent severe tropical cyclone Yasi (category 5). Cyclone Yasi passed to the north of Townsville during 2nd and 3rd of February 2011, causing exceptionally big seas in the Townsville and Cape Upstart sectors. Unsurprisingly, there was evidence of damage to corals and other benthic organisms from heavy wave action at all nine of the reefs that were surveyed in this sector. The evidence included fragmented and rolled corals, exfoliation of the reef matrix, redistribution of old and new coral rubble and the presence of new underwater ramparts of coral rubble. Damage was most severe on the three outer reefs and at Rib Reef on the mid-shelf where exfoliation of the reef matrix was widespread, indicating extreme wave forces over large areas. Inner shelf reefs were not spared, but damage was patchier. At Havannah Is. Reef, manta tow surveys found large areas of rubble substrate that had been redistributed by cyclone Yasi. As a result of turnover once extensive beds of macroalgae had disappeared from these rubble areas. SCUBA surveys at Pandora Reef found extensive intact coral communities in some places as well as communities that had been largely fragmented and/or removed by wave action in other locations. Patterns were similarly patchy at Middle Reef but damage to corals appeared less than at Pandora Reef.
Table 1. Summary of manta tow survey results for seven reefs in the Townsville sector.
|
Reef |
Shelf Position |
Tows |
COTS |
COTS per tow |
Median % Live Coral Cover |
Median % Dead Coral Cover |
Median % Soft Coral Cover |
Reef Status |
| HAVANNAH IS |
Inner |
31 |
0 |
0 |
0 to 5 |
0 to 0 |
0 to 5 |
NO |
| RIB |
Mid |
36 |
0 |
0 |
5 to 10 |
0 to 5 |
0 to 5 |
RE |
| JOHN BREWER |
Mid |
71 |
0 |
0 |
0 to 5 |
0 to 0 |
0 to 5 |
RE |
| DAVIES |
Mid |
47 |
0 |
0 |
5 to 10 |
0 to 5 |
0 to 5 |
RE |
| MYRMIDON |
Outer |
45 |
0 |
0 |
0 to 5 |
0 to 0 |
0 to 5 |
NO |
| DIP |
Outer |
52 |
0 |
0 |
5 to 10 |
0 to 5 |
0 to 5 |
RE |
| CHICKEN |
Outer |
41 |
0 |
0 |
0 to 5 |
0 to 0 |
0 to 5 |
RE |
| Overall GBR average for last survey (2009) |
All |
0.01 |
10 to 20 |
0 to 5 |
0 to 5 |
- |
||
| Long-term average value |
Inner |
0 |
5 to 10 |
0 to 5 |
5 to 10 |
- |
||
| Long-term average value |
Mid |
1.62 |
10 to 20 |
0 to 5 |
0 to 5 |
- |
||
| Long-term average value |
Outer |
1.03 |
10 to 20 |
0 to 5 |
0 to 5 |
- |
||
| Average last survey (2009) |
Inner |
0 |
0 to 5 |
0 |
0 to 5 |
- |
||
| Average last survey (2009) |
Mid |
0 |
10 to 20 |
0 |
0 to 5 |
- |
||
| Average last survey (2009) |
Outer |
0 |
10 to 20 |
0 to 5 |
0 to 5 |
- |
SCUBA surveys were conducted on nine reefs including the seven that were also surveyed by manta tow. Only one COTS was observed during scuba surveys (Table 2). 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 ofDrupella 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 results of SCUBA searches at seven reefs in the Townsville sector.
|
Reef |
Shelf Position |
COTS (<5cm) |
COTS (>5cm) |
COTS (>15cm) |
COTS (>25cm) |
WS |
BBD |
BrB |
SEB |
Drupella |
| PANDORA |
I |
0 |
0 |
0 |
0 |
8 |
0 |
3 |
0 |
3 |
| HAVANNAH IS |
I |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
| MIDDLE |
I |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
| RIB |
M |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
0 |
37 |
| JOHN BREWER |
M |
0 |
0 |
0 |
0 |
2 |
0 |
1 |
0 |
5 |
| DAVIES |
M |
0 |
0 |
0 |
1 |
0 |
0 |
0 |
5 |
39 |
| MYRMIDON |
O |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
43 |
| DIP |
O |
0 |
0 |
0 |
0 |
0 |
0 |
1 |
0 |
30 |
| CHICKEN |
O |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
13 |
| Overall GBR Average for last survey (2009) |
All |
0 |
0.021 |
0.11 |
0.043 |
14 |
0.19 |
4.9 |
4.8 |
15 |
| Long term average |
I |
0.028 |
0.11 |
0 |
0 |
1.1 |
0.08 |
0.31 |
0.4 |
2.5 |
| Long term average |
M |
0.014 |
0.5 |
4.8 |
1.1 |
3.1 |
0.018 |
0.49 |
2 |
17 |
| Long term average |
O |
0.097 |
0.64 |
1.7 |
0 |
2.7 |
0 |
0.38 |
4.7 |
23 |
| Average for last survey (2009) |
I |
0 |
0 |
0 |
0 |
1 |
0 |
0 |
0.67 |
1.3 |
| Average for last survey (2009) |
M |
0 |
0 |
0 |
0.33 |
2 |
0 |
1 |
1.7 |
7.3 |
| Average for last survey (2009) |
O |
0 |
0 |
0 |
0 |
2 |
0 |
0.33 |
3.7 |
12 |
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.
Cape Upstart sector reefs
Two reefs were surveyed using manta tows (Table 1). No COTS were recorded during these surveys. Coral cover was low (5-10%) at both reefs. Coral cover at Viper Reef had declined substantially since last surveyed in 2005 (from 30-40% to 5-10%). This decline may be partly attributable to cyclone Yasi given the obvious signs of wave damage to the benthos as observed in the Townsville sector. Coral cover at Bowden Reef had remained relatively stable since the last survey in 2007.
Table 3. Summary of manta tow survey results for two reefs in the Cape Upstart sector.
|
Reef |
Shelf Position |
Tows |
COTS |
COTS per tow |
Median % Live Coral Cover |
Median % Dead Coral Cover |
Median % Soft Coral Cover |
Reef Status |
| BOWDEN |
Mid |
77 |
0 |
0 |
5 to 10 |
0 to 5 |
0 to 5 |
RE |
| VIPER |
Outer |
27 |
0 |
0 |
5 to 10 |
0 to 0 |
0 to 5 |
NO |
| Overall GBR average for last survey (2009) |
All |
0.01 |
10 to 20 |
0 to 5 |
0 to 5 |
- |
||
| Long-term average value |
Inner |
3.19 |
0 to 5 |
10 to 20 |
5 to 10 |
- |
||
| Long-term average value |
Mid |
0.68 |
10 to 20 |
0 to 5 |
0 to 5 |
- |
||
| Long-term average value |
Outer |
0.02 |
20 to 30 |
0 to 5 |
0 to 5 |
- |
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 (2nd Edition). Australian Institute of Marine Science. Townsville.
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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
