An initial assessment of coral disease on Ningaloo Reef

Coral disease is often defined as an abnormal condition of a coral colony, which harms the coral’s function and is often associated with specific signs/symptoms.  While background levels of disease exist in all healthy reef ecosystems, coral disease outbreaks are having a negative impact on the structure and appearance of coral reefs, and have contributed to declines in live coral cover and productivity of coral reefs worldwide.  The number and distribution of tropical coral diseases appear to be on the rise, with much of the information being reported from widely studied regions such as the Caribbean and parts of the Pacific (e.g. Great Barrier Reef).  That said however, very little is known about coral disease prevalence and status in the Indian Ocean, with only two recent published accounts of coral disease in the eastern Indian Ocean.

Coral disease monitoring surveys were undertaken by the Western Australia Department of Environment and Conservation (DEC) between May 2009 and February 2010 to identify and map the distribution of coral disease on Ningaloo Reef for the first time.  Twenty-eight sites (Figure 1C) were surveyed covering ~200 km of Ningaloo Reef from the Muiron Islands in the north to Pelican Sanctuary in the south.  At each site, three 30 m x 2 m belt transects were surveyed using standardised protocols that identified each individual coral colony (identified to the lowest taxonomic level possible) as ‘healthy’, ‘bleached/traumatised’, ‘diseased’.  The ‘diseased’ category was further divided into specific disease states (Figure 1A).  Disease prevalence was calculated as the mean percentage of coral colonies affected by disease per site.  (see Onton et al. 2011 for more detailed methodologies, results, discussion and references).

The aims of the monitoring surveys were to:

1)      Identify the disease states present and the coral taxa affected on Ningaloo Reef.

2)      Compare coral disease distribution patterns across different spatial scales on Ningaloo Reef.

3)      Identify possible environmental drivers of coral disease prevalence on Ningaloo Reef.

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Figure 1.  Coral disease monitoring on Ningaloo Reef – May 2009 to February 2010.  A. Six of the seven disease states identified in the surveys.  Examples of Other Cyanobacteria are not shown.  B. Presence of the seven coral disease states across coral family groups.  Disease prevalence decreases from right to left across family groups.  C. Prevalence of coral disease on Ningaloo Reef (mean ± SE).  Disease prevalence is expressed as the mean percentage of diseased coral colonies per site.  Data extracted from Onton et al. 2011.

Coral disease types and coral taxa affected

Seven disease states were identified and recorded on Ningaloo Reef (Figure 1A; Table 1), with Skeletal Eroding Band and Other Cyanobacteria being the most prevalent.  On the whole, coral disease prevalence on Ningaloo Reef was low, with only 2.3 % of coral colonies affected by a disease state.  The disease states recorded and the disease prevalence numbers obtained are consistent with other Indo-Pacific locations (e.g. Great Barrier Reef: 0.5 – 2.3 %; Guam: 1.3 %), but substantially lower than the Caribbean and Red Sea.

There was a significant difference of coral family susceptibility to coral disease, with the Acroporidae showing the highest prevalence of disease and the Faviidae the lowest (Figure 1B, Table 1).  The mean number of disease states per coral family was 2.8, which was inflated by all seven disease states recorded within the Acroporidae (Figure 1B).  On average, Skeletal Eroding Band and Other Cyanobacteria were the most prevalent across the coral families (with the exception of the Pectiniidae).  Once again these results are consistent with other reef locations where the prevalence of disease is greatest within the Acroporidae, which is potentially related to the fact that the Acroporidae often dominate Indo-Pacific reefs, and that they tend to invest in rapid growth and expansion rather than in resistance against disease or other threats.

Table 1.  Coral disease prevalence of disease states and coral families affected on Ningaloo Reef. Data extracted from Onton et al. 2011.

Coral disease distributions on Ningaloo Reef

At a reef-wide (~200 km) scale, coral disease prevalence during January 2010 varied sevenfold on Ningaloo Reef.  The percentage of diseased coral colonies was greatest at Pelican Sanctuary (7.0 %), Oyster Stacks (6.1 %), Bundegi (5.7 %), and Bundara (5.5 %), and lowest at Turquoise Bay (1.5 %), South Muiron Island (1.3 %), Winderabandi (1.1 %), and Lefroy Bay (1.1 %) (Figure 1C).  As such, there was no direct latitudinal trend in disease prevalence on Ningaloo Reef.  There was also a change in the temporal prevalence of disease at Coral Bay with a fivefold increase in disease prevalence between May 2009 (0.8 %) and January 2010 (4.1 %), which could possibly reflect a seasonal signal.

Potential drivers of coral disease at Ningaloo Reef

Unlike other studies which have shown a strong correlation between human use and coral disease presence, human use proved to be a poor predictor of disease prevalence within management zones on Ningaloo Reef.  Rather, it appeared that the presence of Drupella sp. snail feeding scars was the best single variable predictor of total disease prevalence on Ningaloo Reef (and especially for Skeletal Eroding Band).  A similar result between corallivores (such as Drupella) and disease prevalence has been previously reported from the Red Sea, highlighting three possible explanations for the correlations observed:

a)      Corallivores may play a role as vectors that transmit disease.

b)      Injury caused by corallivore feeding may compromise the coral host’s resistance, which in turn makes the coral more susceptible to disease.

c)       Corallivores may be attracted to feed on corals that are already health compromised.

While the prevalence of coral disease at Ningaloo Reef between May 2009 and February 2010 was low, predictions of sea temperature increases as a function of a changing climate has the potential to enable some diseases to expand their geographic ranges into previously unaffected reef areas, and hence potentially increase the prevalence of coral disease on Ningaloo Reef.  Recent evidence also suggests an increase in coral disease prevalence in the months following a coral bleaching event such as the bleaching experienced on Ningaloo Reef in February to April 2011.  As such, there is a need for more studies and continued monitoring of coral disease on Ningaloo Reef.

For more information and access to resources on coral disease, visit the Global Coral Disease Database.

For more detailed information on this study, see Onton K, Page CA, Wilson SK, Neale S and Armstrong S (2011), or contact the corresponding author, Dr Shaun Wilson.

Dataset details