ReefBase :: Global Database : Threats

Coral bleaching refers to a process in which corals expel the algal cells (zooxanthellae) that normally live within their tissue. These algae give corals their characteristic brownish colour and once they have been expelled, the white skeleton shows through a coral’s transparent tissue, giving it a bleached white appearance. Bleached coral looks very similar to coral that has recently died, but can be distinguished (on close inspection) by the presence of small polyps and tentacles on the coral surface.

Coral bleaching can be caused by stressful environmental conditions such as extreme temperature, low salinity, extreme light and various toxins. However, large scale bleaching episodes are usually associated with unusually high sea temperatures. This relationship has led to the suggestions that coral reefs are showing early signs of stress due to global warming caused by green house gas emissions.

Bleached corals are still alive and can recover fully if the stressful conditions are not too severe or prolonged. However massive coral mortality is a feature of many severe bleaching episodes. The events of 1982/83 and 1998 in particular resulted in wide spread mortality of some coral in some areas. Following milder bleaching events, most coral recover their health, but this may take several months, and can result in lower growth and abnormal reproduction for some time after normal colour has returned.

ReefBase has the most up-to-date global database on coral bleaching events available, as well as a comprehensive collection of images and reports related to bleaching. If you have any information on bleaching events, please submit a bleaching report. A guide to monitoring coral bleaching has been developed by the WorldFish Center and WWF and can be downloaded here. One new method for assessing the level of bleaching is to use specially prepared colour cards. Information on this method, developed by the University of Queensland can be obtained here.

We also welcome any documents or photographs on coral bleaching.

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