The 1997-1998 Mass Bleaching Event Around the World

Introduction

There has been unprecedented bleaching in coral reefs throughout the Indian Ocean, South East Asia, the Caribbean and parts of the far eastern and western Pacific ocean. This bleaching has been in parallel with big swings in the global climate with a severe El Nino event during late-1997 and early-1998, which switched over to a strong La Nina in mid-1998.

The coral bleaching of 1997-1998, is the most geographically widespread ever recorded, and probably the most severe in recorded history. Bleaching means that corals (both hard and soft), as well as giant clams and some other animals like sponges, lose their symbiotic algae (zooxanthellae) and/or the pigments of those algae, such that the coral appears pale to stark white. Some bleaching may be a seasonal event in the Pacific, Indian Ocean, and the Caribbean, and full recovery is the norm. Frequently many corals recover from bleaching, but death may result if the stress is extreme or prolonged. Normally fast-growing, branching corals in the Indo-Pacific are more susceptible than slow-growing boulder corals, which if they are bleached, frequently recover in 1 to 2 months. In this year’s bleaching event, there has been widespread death of the fast growing corals, and also bleaching of the more resistant forms – the boulder and plate-like corals. For example, some 700-year-old Porites corals on inner reefs of the Australian Great Barrier Reef and Vietnam have been extensively bleached, and some have died. Complete recovery of these reefs will be slow.

Mass bleaching can occur when sea-surface temperatures (SST) rise well above the average for summer and this stresses corals. Frequently the warmer temperatures coincide with more solar radiation, particularly during very calm periods. Bleaching also occurs during extreme low tides or heavy freshwater runoff onto reefs. This time, bleaching has been seen both in shallow water down to very deep water, for example down to 50 m deep.

There are some correlations between the widespread coral bleaching in 1997-1998 and one of the strongest El Nino events of this century, but the patterns are unclear with many exceptions. The correlation exists for the east Pacific, but the bleaching in southeast Asia coincides with a strong La Nina (the complete reverse of El Nino), and the bleaching in the Indian Ocean and parts of the Caribbean do correlate with either El Nino or La Nina.

The 1997-1998 episode of worldwide bleaching is a major cause for concern. Although sea temperatures have returned to normal in many tropical areas, the full extent of bleaching-induced mortality may not be fully apparent for several months yet.

This report was only possible through the cooperation of many, many people throughout the world (whose names are listed after the country reports), and through the power of electronic mail, particularly using a list <coral-list@coral.aoml.noaa.gov> run by the National Oceanographic and Atmospheric Administration of the USA. Reports received varied from detailed accounts with accurate measures of bleaching and mortality, to brief anecdotal reports obtained during a rapid site visit. While it is being published for the International Tropical Marine Ecosystems Management Symposium in Townsville in November 1998, it is also being lodged on the internet and hopefully upgraded as more reports come in. Many of the reports were very detailed; I edited them to reduce the length of this report, so any errors are probably mine, not the author’s. The latest reports were added on 1^st November 1998.

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Last updated - 7 December 98

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