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

Summary

• Seven reefs in the Townsville had coral cover equal to, or greater than, that recorded since systematic manta tows began 31 years ago.
• Coral cover had increased on 11 of the 12 reefs surveyed using manta tows.
• Coral cover ranged from low (5-10%) to very high (63-75%).
• Ongoing crown-of-thorns starfish (COTS) activity but no reef-wide outbreaks recorded.
• Evidence of recent COTS recruitment to some reefs.
• Coral bleaching generally low on midshelf reefs (0-10%) but moderate (10-30%) on outer-shelf reefs
• Low levels of coral disease and Drupella
  
   
  

Map showing location of reefs in the Townsville sector. Click on figure to go to AIMS Spatial Maps and interrogate individual reefs.

Summary of selected survey results from the Townsville sector 2016

As part of the Long Term Monitoring Program (LTMP), surveys of coral cover and abundance of the coral feeding crown-of-thorns starfish (COTS), Acanthaster planci, were completed on twelve reefs in the Townsville sector of the Great Barrier Reef (GBR) using the manta tow technique. In addition, intensive scuba surveys of benthic organisms and fishes, as well as scuba searches for agents of coral mortality, were completed on eleven of these reefs plus one additional reef. Preliminary results are presented in this report.

Manta tow surveys found that median reef-wide live coral cover (coral cover) was variable. Cover was very high (63-75%) at one reef, high (30-50%) at two reefs, moderate (10-30%) at eight reefs and low (0-10%) at one reef. Overall coral cover has improved consistently, increasing at eleven reefs with no reefs showing a decrease. These reefs continue to recover from damage by Severe Tropical Cyclone Yasi that crossed this region in January 2011. COTS were recorded on manta tow at three of twelve reefs. Overall COTS abundance was similar to previous surveys and indicates continuing low level COTS activity on reefs in the region. Rib Reef in particular has a localised Incipient Outbreak and COTS activity had increased compared with previous surveys. In view of the increase in numbers of COTS in the Innisfail sector to the north, COTS numbers on reefs in the Townsville sector can be expected to increase in coming years. Coral bleaching was recorded at all reefs during manta tow surveys. Bleaching was most prevalent on outer shelf reefs where it affected up to 10-30% of total hard coral cover. On midshelf reefs bleaching was generally low, affecting 0-10% of total coral cover, or restricted to scattered individual coral colonies.

Coral bleaching was also recorded on all transects at all uba survey sites. A low level (0-10% of total hard coral cover) of coral bleaching was widespread and individual colonies of a range of coral genera were affected, particularly Montipora spp., Stylophora spp. Seriatopora spp., some Acropora spp. and a variety of corals from the family Faviidae. Mortality from bleaching was low, affecting scattered individual coral colonies. COTS were recorded in the scuba surveys at three reefs. Numbers of individuals and numbers of scars had increased little compared with previous survey in this sector. White syndrome disease, brown-band disease and skeletal eroding band (SEB) were generally at lower levels than have been recorded in previous surveys in the Townsville sector. Black-band disease was not observed at scuba survey sites. The density of the coral-feeding snails Drupella spp. was similar to recent years, ranging from 0 to 293/ha.

Results are summarised in Tables 1 to 3. 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]. Further details of the monitoring program design, sampling methods and a full explanation of the COTS outbreak terminology can be found here.

Townsville Sector

Perimeters of twelve reefs in the Townsville sector were surveyed using manta tow (Table 1). With the exception of Helix Reef, coral cover had increased at all reefs since they were last surveyed, continuing a trend of coral recovery on reefs in this sector since cyclone Yasi in 2011. Grub Reef had the lowest coral cover, increasing from a very low 0-5% in 2014 to a low (5-10%) in 2016 (Table 1). The majority of reefs in this sector had moderate coral cover (10-30%). Coral cover was high (30-50%) at Little Kelso and John Brewer Reef, and very high (50-75%) at Rib Reef. COTS were observed on Grub Reef and Fork Reef but well below outbreak levels. COTS were also observed on Rib Reef that historically has been a good “catch” reef for COTS and starfish numbers on this reef have increased since 2013. Surveys indicated a localised Incipient Outbreak on the second flank of Rib Reef and the prevalence of scars would indicate the presence of many more than were observed, which would not be surprising given the opportunities for concealment provided by very high coral cover on this reef.

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

Coral bleaching was observed around all reef perimeters during manta tow surveys predominately on the reef slope. Bleaching was most prevalent on outer shelf reefs where it affected up to 10-30% of total hard coral cover. On midshelf reefs bleaching was generally low, affecting 0-10% of total coral cover, or restricted to scattered individual coral colonies.

Table 2. Summary of coral bleaching at reefs in the Townsville sector from scuba search surveys. Coral genera are listed from most abundant to least abundant in terms of number of colonies affected. 15 transects were surveyed per reef.

# transects: represents number of transects with bleaching observed from the 15 transects surveys. Median and maximum: refer to values for the bleaching categories.

Three sites at each of twelve reefs were surveyed in detail using scuba. Coral bleaching was observed at all sites on all survey reefs (Table 2). At most reefs coral bleaching was minor, affecting up to 10% of total hard coral cover. However bleaching was variable with some transects recording levels of bleaching much higher than the median value for a given reef (Table 2). This was particularly true on Grub reef where up to 30-50% of total hard coral cover was affected by bleaching. In general outer shelf reefs appeared to have the most bleaching and this reflects the results from manta tow surveys. Hard corals from a wide variety of genera were affected with the genera most commonly bleached varying considerably between reefs (Table 2). Corals of the genera Montipora spp., Stylophora spp. Seriatopora spp., some Acropora spp. and a variety of corals from the family Faviidae were the most affected. Coral mortality from bleaching was observed at all reefs but appeared to be restricted to small numbers of scattered individual colonies.

A single COTS was observed during scuba search surveys at each of three reefs: Rib, Fork and Fore and Aft (Table 3). The COTS on both Rib Reef and Fore and Aft Reef were small adults (15-25cm) suggesting ongoing recruitment to these reefs. Incidence of coral disease and Drupella was generally below those seen in recent years. Signs of Brown Band Disease at Little Kelso Reef were higher than previously recorded but not unusual and may be a reflection of increasing coral cover on this reef. Similarly the highest density of Drupella was recorded on Little Kelso Reef but these numbers were not high compared to past records for reefs in the region and would not be considered a threat. No signs of black-band disease were observed during scuba searches.

Table 3. Summary of SCUBA search surveys for reefs in the Townsville sector.

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