Broadscale effects of bleaching
on the Great Barrier Reef

Click on the image for a high resolution view (PDF format 100K) see Note below.

The AIMS Long-term Monitoring Program surveys sites on NE flanks of reefs annually. Reefs are surveyed at about the same time each year. Because of the distances involved, the surveys span a period between October and May. Extensive bleaching was first recorded late in January 1998. The full extent of bleaching and mortality developed over a period of months.

As a simple measure of the broadscale effects of the 1998 bleaching event, the total living hard coral (from video transects (see METHODS)) at the 1997 surveys [well before bleaching] were compared with equivalent data from the 1999 surveys [well after bleaching]. Data were available for 47 reefs. The figure shows the average percent cover values on sites at each survey reef, arranged by latitude and position across the GBR lagoon [inshore to offshore]. For each reef, the darker blue bar represents the mean hard coral cover at the 1997 survey [made before June 1997]. The paler blue bar gives the same information for the 1999 surveys [made after October 1998]. Survey methods are described in a number of documents available on the AIMS web-page; it is important to note in this context that the study sites are generally at 6-9 m depth.

Taking an arbitrary figure of 5% change from pre-existing coral cover as the criterion for substantial change:

• Survey sites on 8 of the reefs lost more than 5% of the initial average coral cover over the two years.
• Survey sites on 32 reefs gained more than 5% of the initial coral cover in that period.
• Sites on 7 reefs showed little average change (-4.9% to 4.9% of initial cover) over the two years.

The reefs in the figure that showed a substantial net loss of hard coral were inshore reefs in the Cairns sector, inshore and midshelf reefs in the Townsville sector and some reefs in the Swains sector. Three of these reefs, Rib Reef (Townsville midshelf) and Gannet Cay and Horseshoe Reef in the Swains sector are known to have had large populations of the crown of thorns starfish, a predator of corals (see COTS ON THE GBR). Aerial surveys in March 1998 did not detect any bleaching in the Swains sector (see MAP of bleaching (GBRMPA))

Seven reefs from four sectors showed little net change in coral cover. Three of these, Hyde Reef and Reef 19-131 in the Whitsundays sector and Turner Cay in the Swains sector are in areas where aerial surveys in March 1998 did not detect any bleaching. Martin Reef and MacGillivray Reef in the Cooktown / Lizards Is sector have had crown of thorns starfish populations. Starfish numbers have also been increasing at Michaelmas Reef in the Cairns sector.

This simple analysis implies that, of the survey reefs, only sites on inner reefs in the Cairns and Townsville sectors were severely affected by the bleaching. Survey sites on the majority of reefs have shown a clear net increase in total coral cover.

For further information contact:
Dr Hugh Sweatman, AIMS
PH: (07) 47534470 International: +61 7 47534470
E-mail: h.sweatman@aims.gov.au 

Note:
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Last updated - December 18, 2008

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