ltm200811-12

Dates: 18thNovember– 4thDecember 2008

Vessel: MV Elizabeth E II

Survey leaders:

AlistairCheal

Capricorn-Bunker sector

Swain sector

Summary

Four reefs in the Capricorn-Bunker sector and seven reefs in the Swain 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 completed on all reefs. Preliminary results of the manta tow surveys and SCUBA searches are presented in this report.

COTS were recorded at Lady Musgrave Island in the Capricorn-Bunker sector during SCUBA searches and at Chinaman Reef in the Swain sector during manta tows. In both cases COTS were in low numbers. No Active Outbreaks have been recorded in the Capricorn-Bunker sector since surveys began in 1985 or in the Swain sector since 2006. However, four of the seven Swain reefs remain listed as Recovering from past outbreaks.

Hard coral cover was moderate (10-30%) on all reefs in the Capricorn-Bunker sector and had remained stable on two reefs and decreased on the other two reefs since last surveyed one to two years previous. Major storm damage (as evidenced by overturned corals and rubble banks) had occurred on the exposed reef fronts on all four reefs in the Capricorn-Bunker sector. Hard coral cover in the Swain sector varied from very low (0-5%) to very high (>50%) but in all cases cover had remained stable since surveyed one to two years previous. The Swain reefs where hard coral cover was lowest were those recovering from COTS outbreaks that had persisted for up to 14 years prior to the mid 2000's.

SCUBA searches found relatively high incidences of "white syndrome" (a disease-like necrosis found particularly on tabulate Acropora spp.) on four reefs in the Swain sector. Incidences of "skeletal eroding band" coral disease had also increased on a few reefs in the Swain sector and one reef in the Capricorn-Bunker sector. Incidences of "brown band" coral disease and numbers of Drupella spp. (coral feeding snails) had increased on one reef in the Capricorn-Bunker sector since 2006.

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

Capricorn-Bunker Sector

Four reefs were surveyed by both manta tow (Table 1) and SCUBA (Table 2). No COTS were recorded during manta tows and all reefs remain classified NO Outbreak. No active outbreaks have been recorded on these reefs since surveys began in the mid 1980's. Four adult COTS were recorded during intensive SCUBA searches at Lady Musgrave Reef, but low numbers of COTS have been observed at this reef on a number of occasions since surveys began.

Median reef-wide live coral cover on the four reefs was moderate (10-30%) based on manta tow surveys (Table 1). Coral cover had remained stable at One Tree Reef and Wreck Island Reef since last surveyed one to two years previous but had fallen considerably at Broomfield Reef (40-50% to 10-20%) and Lady Musgrave Island Reef (50-60% to 20-30%). These declines in coral cover are largely due to the effects of major storms (at all reefs, areas exposed to prevailing south-east swells were littered with overturned and broken corals). Based on the extent of algal overgrowth, observations from staff at One Tree Island Research Station and the fact that storm damage was not evident at Broomfield Reef when last manta towed in November 2007, we estimate that major storm swells damaged these coral communities sometime between December 2007 and June 2008. A similar event occurred in this sector in the late 1980's. Such cycles of disturbance and recovery are likely to be the norm on exposed reef faces in this sector as coral communities are dominated by relatively fragile plating and branching Acropora species making them particularly vulnerable to storm damage.

Image 1:

On all four reefs surveyed in the Capricorn-Bunker sector wave action due to major storms had broken, overturned and redistributed hard corals in exposed areas. Here on a reef slope at Wreck Island Reef coral skeletons had mostly been reduced to rubble. Contrast this plate with plate 2, photographed only 10m away but protected enough to be safe from storm damage.

Photo:AIMS LTM

Table 1. Summary of manta tow survey results for the Capricorn-Bunker sector.

SCUBA searches on the intensive survey sites showed that incidences of "white syndrome" (a disease-like necrosis found particularly on tabulate Acropora spp.), were relatively high compared to the GBR average (Table 2) but levels were generally within the range recorded over the past few surveys. Incidences of "skeletal eroding band" coral disease and "brown band" coral disease were generally low except on Lady Musgrave Island Reef and One Tree Island Reef, respectively. Numbers of Drupella spp. (coral feeding snails) were low to moderate except on One Tree Island Reef where numbers had increased considerably since last surveyed in 2006.

Table 2. Summary of results of SCUBA searches on reefs in the Capricorn-Bunker sector.

WS = White Syndrome, BBD = Black Band Disease, BrB = Brown Band Disease, SEB = Skeletal Eroding Band Disease.

Swain Sector

Seven reefs were surveyed by both manta tow (Table 3) and SCUBA (Table 4). COTS were recorded from one reef (Chinaman) in very low numbers. Three reefs (Reef 21-529, Snake and East Cay) were classified as No Outbreak and the other four remained classified as Recovering from long-term COTS outbreaks that ended in the mid 2000's. The outbreak at Gannet Cay lasted for 14 years (1989 to 2003) and this is the first time that COTS have not been recorded from Horseshoe Reef since surveys began in 1986.

Median reef-wide live coral cover in the Swain sector varied from very low (0-5%) to very high (>50%) but in all cases remained stable since surveyed one to two years previous. However, on those four reefs that suffered major coral losses due to past COTS outbreaks there are encouraging signs of coral recovery that is yet to be reflected in the broad manta tow coral cover categories.

SCUBA searches on the intensive survey sites showed that incidences of "white syndrome" (a disease-like necrosis found particularly on tabulate Acropora spp.), were relatively high compared to the GBR average and the Swain sector average (Table 4). Levels of "white syndrome" at Gannet Cay Reef, Horseshoe Reef and Turner Reef were clearly the highest recorded at each reef since surveys began. This result cannot be explained by seasonal differences in time of sampling or major changes in coral cover (incidence of disease increases with coral cover). Incidences of "skeletal eroding band" coral disease were slightly elevated at a few reefs and may partly reflect increased awareness of this particular disease by observers. Numbers of Drupella spp. (coral feeding snails) were low at all reefs.

Image 3: On Chinaman Reef in the Swain sector a transect line (lower right) snakes it's way over the reef slope. Each year a series of 50m transect lines are laid between star pickets that had previously been hammered into the reef. Photographic and visual surveys of the benthos along with counts of reef fishes are then conducted along these transect lines. This technique allows valid comparisons of changes over time because it ensures that annual surveys are always conducted in the same area. Transect lines are retrieved after each dive. Photo:AIMS LTM Click here for a larger view

Image 4: The yellow boxfish (Ostracion cubicus) lives in close association with the reef matrix and feeds on a range of benthic mollusks, crustaceans, worms and fishes. The species occurs in tropical and temperate marine waters, extending south of Sydney on the east coast of Australia. This specimen was photographed in the Swain sector. Photo:AIMS LTM

Image 5: Here at Gannet Cay Reef in the Swain sector new coral growth is evident on the skeletons of branching corals. BranchingAcroporacorals dominated the reefscape at Gannet Cay prior to an outbreak of corallivorous crown-of-thorns starfishes (COTS,Acanthaster planci) that ran for 14 years and only finished in the mid 2000's. The COTS outbreak caused massive mortality of hard corals at this reef. The appearance of new coral colonies is the first sign of coral recovery on this reef for a long time although growth on the relatively fragile branching skeletons may turn out to be a risky strategy if major storm waves occur. Photo:AIMS LTM

Image 6: A juvenile brown booby (Sula leucogaster) stands on a coral cay in the Swain sector. These birds are commonly encountered on the southern GBR and are often very inquisitive towards humans. They feed with spectacular plunges into the sea to capture surface fishes. Photo:AIMS LTM

Table 3. Summary of manta tow survey results for the Swain sector.

Table 4. Summary of results of SCUBA searches on reefs in the Swain sector

WS = White Syndrome, BBD = Black Band Disease, BrB = Brown Band Disease, SEB = Skeletal Eroding Band Disease.

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