Report on surveys in the Pompey and Townsville
sectors of the Great Barrier Reef
 

Manta tow surveys were completed on 6 reefs in the Pompey sector and 3 reefs in the Townsville sector of the Great Barrier Reef (GBR). More intensive surveys including video transects to survey benthic organisms, visual census of fishes and surveys for agents of coral mortality (SCUBA search) were completed on 6 reefs in the Pompey sector and 5 reefs in the Townsville sector. These intensive surveys continue to build a baseline to assess the effectiveness of the new Zoning Plan on coral reefs of the Great Barrier Reef Marine Park.

The protocol involves sampling matched pairs of similar reefs. Each pair includes one reef that was rezoned from being open to fishing to being a no-take area in July 2004 and a similar nearby reef that remained open to fishing. To achieve this, new intensive survey sites have been established on reefs that have only been surveyed by manta tow or have not been surveyed previously. Preliminary results of the manta tow and SCUBA search surveys are presented in this report.

Crown-of-thorns starfish (COTS) were recorded on four of the six reefs surveyed in the Pompey Sector. While only one COTS was recorded during manta tows in the Pompey sector (at Reef 21-060) elevated numbers of COTS were observed during SCUBA searches (<8 individuals per reef) on four reefs. Only one reef in the Pompey sector (Pompey Reef) had been surveyed previously. Hard and soft coral cover at this reef was comparable to that when last surveyed in 1990. Hard coral cover was generally moderate to high in the Pompey sector and all reefs surveyed have been classified as No Outbreak. In the Townsville sector COTS were only recorded at Grub Reef where numbers were low. Hard coral cover at the three reefs surveyed by manta tow in the Townsville sector remains very low since major COTS outbreaks in 2002/03 that were particularly large at Little Kelso Reef and Slashers Reef No.2. Coral bleaching was rarely recorded during this cruise and incidence of disease was generally low and restricted to small numbers of scattered coral colonies on all reefs. However, elevated levels of Brown Band Disease were recorded on one reef in the Pompey sector (Reef 21-060).

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. 8 or in the Survey Manual for Tropical Marine Resources (English et. al.1997). For a full explanation of the terminology concerning COTS outbreaks used in this report, refer to the Reef issues page.

Six reefs were surveyed in the Pompey sector; four were true mid-shelf reefs and two (Penrith Island and Tern Island, arbitrarily labelled as mid-shelf reefs) were located in the shipping channel that generally separates near shore and mid-shelf reefs. Median reef-wide coral cover (henceforth coral cover) was moderate (10-30%) to high (30-50%) at all reefs surveyed in this sector. Only one reef, Pompey, had been surveyed previously by manta tow. Coral cover at this reef (20-30%) was comparable to that recorded when last surveyed in 1990.

Only one crown-of-thorns starfish (COTS) was recorded during manta tow surveys (at Reef 21-060). COTS were recorded during SCUBA search surveys at four of the six reefs. All size classes of COTS were represented in the SCUBA search counts including juveniles (<5cm) indicating ongoing recruitment to reefs in this sector. In all four cases (Tern Island, Pompey Reef #1, Reef 21-060 and 21-064) COTS populations were in densities that could be considered locally high (>1500/km², see Moran and De’ath 1992). However in the majority of cases these COTS were less than 15cm in diameter and only one reef (21-060) had large adults (>25cm diameter) in elevated numbers.

Because of the localised nature of these populations, the spread of COTS size classes and the lack of adult COTS and associated scarring seen by manta tow all these reefs have been classified as No Outbreak. Future surveys will determine whether current recruitment translates into elevated adult COTS numbers in future years. Coral disease was mostly restricted to small numbers of scattered colonies on reefs in the Pompey sector. However, at Reef 21-060, incidence of Brown Band Disease was relatively high (23 infected colonies from SCUBA searches). Numbers of the corallivorous gastropod Drupella spp. recorded during SCUBA search surveys were below levels that should affect coral cover. Coral bleaching was only recorded on one transect at Pompey Reef.

Click here for a larger view  Image 1: Steep sided reef walls were common on the back reef of Reef 21-060 in the Pompey sector. Photo: AIMS LTM

Click here for a larger view  Image 2: Swarms of ctenophores or comb jellies (Phylum Ctenophora) were observed in the Pompey sector. Ctenophores resemble jellyfish but do not sting. These animals move through the water by beating tiny hairs called cilia that can be seen on the eight comb-like plates (red coloured in this case) running along the length of the body. Ctenophores feed on small fishes and crustaceans that are captured using sticky tentacles. Photo: AIMS LTM

Click here for a larger view  Image 3: Extensive colonies of branching Acropora sp. corals grow on a steep reef slope at Reef 21-064 in the Pompey sector, while planktivorous fusiliers (mainly Caesio cuning) feed in the water column above. Photo: AIMS LTM

Table 1a. Summary of manta tow survey results for the Capricorn Bunker sector of the Great Barrier Reef Marine Park.

Table 1b. Summary SCUBA search survey results for the Pompey sector of the Great Barrier Reef Marine Park.

Table 1b. Continued

Three reefs in the Townsville sector were surveyed using manta tow. SCUBA surveys were conducted at these reefs and a further two reefs (Kelso and Fork). Rough weather prevented manta tow surveys on Fork Reef and Kelso Reef. Coral cover was very low (0-5%) at the three manta-towed reefs and reflects the presence of large numbers of COTS from 2000 to 2003, particularly on Little Kelso Reef and Slashers Reef No.2. Although listed as Recovering, these reefs are yet to show increases in coral cover. The presence of numerous coral recruits on some reefs (eg. Little Kelso), and previous history of recovering from past outbreaks suggest that the chances of recovery in future for these reefs are good. Small numbers of COTS were still present on Grub Reef though COTS activity in this sector is now declining from whenit peaked in 2003. COTS were not recorded elsewhere in this sector.

Corallivorous gastropods, Drupella spp., were found in SCUBA searches but numbers were too low to cause widespread coral mortality. Coral disease was at a low level and restricted to a few scattered colonies on two of the five reefs. Coral bleaching was not recorded.

Table 2a. Summary of manta tow survey results for the Townsville sector of the Great Barrier Reef Marine Park.

Table 2b. Summary of SCUBA search survey results for the Townsville sector of the Great Barrier Reef Marine Park.

Table 2b. Continued.