Status of the Worlds Coral Reefs
Executive Summary

THE CORAL BLEACHING EVENT OF 1997–1998

There has been unprecedented bleaching of hard and soft corals throughout the coral reefs of the world from mid-1997 to late-1998. Information is coming in daily via the internet and from GCRMN and Reef Check teams. Much of the bleaching coincided with a large El Niño event, followed by a strong La Niña, but bleaching in other areas appears uncorrelated. Four overlapping levels of bleaching are apparent:

• ‘catastrophic’, with massive mortality (often near 95% of shallow corals) in Bahrain, the Maldives, Sri Lanka, Singapore, and in large areas of Tanzania;
• ‘severe’ bleaching with around 50–70% mortality, and also coral recovery, in Kenya, Seychelles, Japan, Thailand, Vietnam, and Belize;
• ‘moderate and patchy’ bleaching on some reefs in large areas, with a mix of coral recovery and around 20–50% mortality, but no effects in other parts, such as in Oman, Madagascar, the inner Great Barrier Reef, parts of Indonesia and the Philippines, Taiwan, Palau, French Polynesia, the Galapagos, the Bahamas, Florida, the Cayman Islands, Bermuda, and Brazil;
• ‘insignificant’ or no bleaching in large areas of the world’s reefs such as the Red Sea, the southern Indian Ocean, the Andaman Sea, most of Indonesia, large parts of the Great Barrier Reef, most of the central Pacific, and parts of the southern and eastern Caribbean.

Bleaching and mortality were most pronounced in shallow water (less than 15 m) and particularly affected staghorn and plate Acropora and other fast growing corals. Many of the massive, slow-growing species bleached, but many recovered within 1 or 2 months. The consensus is that this is the most severe bleaching event ever observed, although in this case there were also more people looking specifically for bleaching following internet advice of the location of above average sea-surface temperatures. More observations and monitoring are required to determine whether bleached corals will recover (or die), and whether damaged reefs have the potential to ‘bounce back’. More importantly, there is a need for continued observations to determine whether this is a rare, severe event, or part of a pattern of increasing disturbance associated with global climate change.

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Last updated - 7 December 98

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