Report on surveys of the Swain sector of the Great Barrier Reef

Dates: 4^th to 24^th January 2016

Vessel: RV Cape Ferguson

Survey leader: Alistair Cheal

Manta tow surveys for the 2016 survey year* were completed on 12 reefs in the Swain sector of the Great Barrier Reef (GBR). Detailed surveys for benthic organisms, visual census of fishes and surveys for agents of coral mortality (scuba search) were also made on those 12 reefs and on one other reef in the Swain sector. Preliminary results of the manta tow and scuba search surveys are presented in this report.

Median reef-wide live coral cover (coral cover) on 11 reefs in the Swain sector had increased or remained at levels similar to those recorded in the last survey. Coral cover had declined slightly on one reef. Good coral cover was widespread, ranging from moderate (20-30%) to very high (50-65%) on 11 reefs. Five of these reefs had the highest coral cover since manta surveys began 10 years previous. Only one crown-of-thorns starfish (COTS) was recorded in manta tows and all reefs were classified as non-outbreaking. Only one very big COTS (>25cm diameter) was observed during detailed scuba surveys, while four juvenile COTS (<5cm diameter) were observed among three reefs. All five COTS were observed on mid-shelf reefs.

Signs of coral disease on reefs surveyed using scuba were generally similar too or at lower levels to those recorded previously, with only one reef having high incidence of white syndrome disease. Counts of Drupella spp. were also high on this reef and on one other reef, though counts were relatively low on the other reefs. Two reefs had considerably more transects with bleaching than in past surveys, but incidences were few and restricted to isolated colonies.

Details of the manta tow method can be found in the Standard Operational Procedure No. 9 [AIMS Research - Crown-of-thorns Starfish and Coral Surveys - Standard Operational Procedure 9]. For a full explanation of COTS outbreak terminology refer to the following web-page: Crown-of-thorns on the Great Barrier Reef.

* Survey year reflects financial year of survey, for instance the 2016 survey year represents reefs surveyed between July 2015 and June 2016

Perimeters of 12 reefs were surveyed using manta tow (Table 1). Three sites were also surveyed in detail using scuba on 13 reefs (Table 2). No reefs were classified as having incipient or active outbreaks of COTS. Manta tow surveys recorded only one COTS (at Jenkins Reef) and all survey reefs were classified as No Outbreak. Five COTS were recorded in scuba search surveys, all on mid-shelf reefs. One very big COTS (>25cm diameter) was recorded at Wade Reef along with one juvenile COTS (<5cm diameter). Another juvenile COTS was recorded at nearby Jenkins reef and two more at Reef 22-084 further to the west. These sightings of juvenile COTS suggest that recruitment is occurring in this southern region of the GBR, but the significance for future COTS numbers is unclear. Major COTS outbreaks in the Swain sector subsided over 10 years ago, but small numbers of COTS have been observed regularly, though not as part of any Active Outbreaks. Coral cover had increased or remained steady at the majority of reefs in the Swain sector that were surveyed by manta tow. Coral cover in 2016 had increased on four reefs (21-550, 22-084, Jenkins and Wade Reefs), reaching high (30-50%) to very high (50-65%) cover in 2016. Coral cover on seven of the other eight reefs remained unchanged. The lack of increase in coral cover on some reefs (notably Reef 21-296 and 21-558) may have been due to the effects of Tropical Cyclone Edna in 2014. However, the decline in coral cover on Chinaman Reef from very high cover in 2015 is more difficult to explain because in 2015, Chinaman Reef was surveyed after TC Edna. Despite the decrease, coral cover at Chinaman Reef was still high. Overall, coral cover was encouragingly high (ranging from 20-63%) on 11 of the 12 Swain reefs surveyed by manta tow. Five reefs had the highest coral cover since manta tow surveys of these reefs began in 2006. The one exception was Reef 21-296, an outer shelf reef that had relatively low coral cover (10-20%).

Table 1. Summary of manta tow surveys for reefs in the Swain sector.

Scuba searches on the intensive survey sites (Table 2) found that incidence of coral diseases and counts of Drupella spp. were mostly unexceptional and similar to levels reported in previous years. There were two exceptions: incidences of white syndrome on Small Lagoon Reef had risen markedly along with counts of Drupella spp. There were similar increases in numbers of Drupella spp. on Wade Reef. Reasons for patterns in white syndrome disease and Drupella spp. at these two reefs are unclear, though white syndrome is often associated with high coral cover as was the case on the reef front where scuba searches were conducted. Two mid-shelf reefs not far apart (21-550 and 22-084) had considerably more transects with bleaching than in past surveys, but incidence was still low and restricted to isolated colonies. For more information on coral disease refer to the AIMS web site here: Coral disease.

Table 2. Summary of SCUBA search surveys for reefs in the Swain sector.

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