Survey report - Report on reefs in the Swains sector of the Great Barrier Reef

Dates: 10thOctober - 23rdOctober 2009

Vessel:MV Iron Joy

Survey leader:

Michelle Jonker

Summary

The AIMS Long term monitoring program surveyed reefs in the Swain sector in October, as part of the surveys investigating the effects of re-zoning the GBRMP. Eleven reefs were surveyed by manta tow to record broadscale coral cover and coral trout numbers. Nine of the eleven reefs were also surveyed using SCUBA, to provide more detailed information on benthic organisms, reef fishes and agents of coral mortality. A twelfth reef was surveyed by SCUBA but not by manta tow because of dangerous sea conditions. Preliminary results of the manta tow surveys and SCUBA searches for coral mortality are presented in this report.

No Crown-of-thorns starfish (COTS) were recorded during manta tow surveys or SCUBA searches. Incidences of "white syndrome" (a disease-like necrosis found particularly on tabulate Acropora spp.) and numbers of Drupella spp. (coral feeding snails) in the Swain sector were generally low and similar to levels recorded in previous surveys. Only one reef (21-278) had a relatively high incidence of white syndrome and Drupella spp. but this reef also had a moderate coral cover.

Swain reefs fall broadly in two groups: reefs that were affected by Cyclone Hamish (a severe category 4 tropical rotating storm that passed through this sector on the 9thof March 2009) or by COTS and reefs that were not. Coral cover was generally low on reefs in the former group. As a result of Cyclone Hamish, median reef-wide live coral cover had declined on the majority of the survey reefs. Of the less affected reefs surveyed in the Swain sector, coral cover had remained at levels similar to those seen in previous years. A summary of the results is presented in Tables 1 and 2.

Details of the manta tow method can be found in the Standard Operational Procedure No. 1 [ AIMS Research - Crown-of-thorns Starfish and Coral Surveys - Standard Operational Procedure 8 ] or in the Survey Manual for Tropical Marine Resources (English et. al.1997). A full explanation of the COTS outbreak terminology used in this report can be found on the AIMS Reef Monitoring web site (see AIMS Research - Reef Monitoring).

Manta tows in the Swain sector

Eleven reefs were surveyed by manta tow. One reef (21-558) was not manta towed due to dangerous sea conditions. No crown-of-thorns-starfish (COTS) were recorded during manta tow surveys. Median reef-wide live coral cover (henceforth coral cover) in the Swain sector was low (0-10%) for seven reefs surveyed in the Swain sector. Of these five had extremely low (0-5%) coral cover. Two reefs had moderate cover (10-30%), while the remaining two reefs had high coral cover (30-50%). A summary of the results is provided in table 1.

Six of the eleven reefs (Reefs 21-139, 21-187, 21-245, 21-529, Gannett Cay, Jenkins) surveyed in October 2009 showed decreases in coral cover. Three reefs, East Cay, Reef 21-278 and Small lagoon reefs showed no change coral cover, while three reefs, Reefs 21-278, 21-296 and 21-302, showed increases in median coral cover. The decreases in coral cover were attributed to severe tropical Cyclone Hamish. The eye of the Cyclone passed within 30kms of six reefs and within 80kms the other five reefs surveyed.

Other reefs were also affected by Cyclone Hamish, but on a smaller scale, such as Reefs 21-278, 21-302 and Small Lagoon, which did not show a reduction in median coral cover.

In general, decreases in coral cover were not around the entire reef perimeter, but in areas of the reef exposed to the large swell and destructive wave action associated with a cyclone. The exception was Reef 21-529, where coral cover was dramatically reduced from over 50% to less than 5%. In this case cyclone Hamish passed almost directly over the top of the reef and exposed it to the full force of the storm from every direction. On the other reefs affected the decrease in coral cover was most intense on those that were to the south west of the cyclone track, while reefs within 15km to the north east of the cyclone track showed somewhat less reduction in coral cover, mostly on the south west facing parts of the reef that are normally sheltered from prevailing winds and seas. In general the affects of Cyclone Hamish on a reef were heavily dependent on the type of coral community that was present, the position of the reef relative to the cyclones track, the areas that were exposed to rough seas generated by the passage of the cyclone and the proximity to neighbouring reefs that might have provided shelter from the cyclonic winds and seas.

Image 1.The AIMS Long Term Monitoring team recently completed surveys in the Swain sector of the GBR. Reefs in this sector were exposed to the effects of severe tropical cyclone Hamish, in March 2009, which brought damaging winds and "phenomenal" seas to the area. This photograph shows Reef 21-529 that was subject to the full force of the storm. Previously this reef supported some of the highest coral cover recorded on the GBR. Post cyclone it has some of the lowest. The effect of the cyclone varied between reefs and few escaped damage. A full report and results of the survey can be found here. Photo:AIMS LTMP.

Image 2/3.These two photographs were taken on the same reef, the first on the back and the second on the flank. The difference in coral cover is considerable. The flank scene shows a reef slope blasted clean while the back reef scene shows a thriving coral community. This is despite the eye of cyclone Hamish (a severe category 4 tropical rotating storm) passing only 10km from the reef in March 2009. These surveys highlight that the effects of cyclones can vary considerably both within and between reefs. The impact a cyclone has will depend on; the strength of the cyclone, the type of coral community present, the position of the reef relative to the cyclones track, the areas of reef exposed to rough seas generated by the passage of the cyclone and the proximity to neighbouring reefs that might have provided shelter from the cyclonic winds and seas. Photo:AIMS LTMP

Table 1. Summary of manta tow survey results for eleven reefs in the Swain sector.

SCUBA searches

No COTS were observed during SCUBA searches on the intensive surveys sites (Table 2). The incidence of White syndrome and Drupella were similar to that of previous years, except for Reef 21-278 where numbers of both agents of mortality were higher than previously observed for this reef. However, considering some mid shelf reefs (21-139, 21-187, 21-245) were severely damaged by Cyclone Hamish the abundance of Drupella spp. and White Syndrome may be considered relatively higher (relative to absolute coral cover) than previously observed at those reefs.

Table 2. Summary of results of SCUBA searches at ten reefs in the Swain sector.

WS= White syndrome, BBD= Black band disease, BrB= brown band disease, SEB= skeletal eroding band disease. Figures are the number of starfish (COTS), disease infected coral colonies (WS, BBD, BrB and SEB) or snails ( Drupella ssp.) recorded at each reef.

References

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

 

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

Dr. Hugh Sweatman , AIMS

Telephone: +61 7 4753 4470

Fax: +61 7 4753 4288

Email: h.sweatman@aims.gov.au