The 1997-1998 Mass Bleaching Event Around the World

Abstract

There has been significant bleaching of hard and soft corals in widely separate parts of the world from mid-1997 to the last months of 1998. Much of this bleaching coincided with a large El Nino event, immediately switching over to a strong La Nina. Some of the reports by experienced observers are of unprecedented bleaching in places as widespread as (from west to east) the Middle East, East Africa, the Indian Ocean, South, Southeast and East Asia, far West and far East Pacific, the Caribbean and Atlantic Ocean.

There was a wide spectrum of reports on bleaching ranging from:

• catastrophic bleaching with massive mortality, often near 95% of shallow (and sometimes deep water) corals such as in Bahrain, Maldives, Sri Lanka, Singapore, and parts of Tanzania; through
• severe bleaching over large areas with significant mortality (around 50 to 70%) with recovery of larger, more resistant species (Kenya, Seychelles, Thailand, Vietnam, Japan, and Belize); to
• severe bleaching only in some of the reefs, with a mix of recovery and mortality (around 20 to 50% in places) e.g. Oman, Madagascar, parts of the Great Barrier Reef, parts of Indonesia and the Philippines, Taiwan, Palau, French Polynesia, Galapagos, Bahamas, Cayman Islands, Florida, Bermuda, Brazil; and
• finally, on large areas of the worlds reefs, there was insignificant or no bleaching was observed.

Bleaching was most pronounced in shallow water (less than 15 m) and particularly affected staghorn and plate Acropora and other fast growing species, with a high proportion of coral death. Slower growing massive species, like Porites, also bleached, but many recovered within 1 or 2 months. Some people commented that bleaching like this had not been seen in 40 years of observations.

While this was occurring, there were large areas of the world where bleaching was not observed. Little or insignificant bleaching was seen in the Red Sea, southern Indian Ocean, eastern Andaman Sea, most of Indonesia, large parts of the Great Barrier Reef, most of the central Pacific and many parts of the southern and eastern Caribbean. In some places with no bleaching, severe bleaching similar to that observed above had occurred in past years, with significant recovery since then.

The consensus is that this is probably the most severe bleaching event ever observed, but there were far more observations and observers this year, and a greater degree of interest in reporting bleaching. Many of the bleaching reports e.g. 80% are estimates and may be exaggerated because bleached corals are particularly dramatic. However, amongst the reports there are actual measures which often are close to the estimates. Much of the interest has arisen because regular, real-time reports are available on sea-surface temperatures over the internet and on e-mail lists through the National Oceanographic and Atmospheric Administration of the USA. Now the large questions are whether observed bleaching will result in death or recovery of the corals, and whether there is potential for the damaged reefs to recover from this event. But the most important question is whether this is just a severe, one-off event, as it now appears, or whether events like this will occur more frequently as the world’s atmosphere and waters warm up.

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Last updated - 7 December 98

Copyright ©1996-1998 Australian Institute of Marine Science

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