Status of Coral Reefs of the World 2000

The 1997-98 Mass Coral Bleaching and Mortality Event
Mitch hit some areas. Most of the bleaching was associated with the La Niña event of the
latter half of 1998. This contrasts with the Pacific coasts which were bleached in late 1997
associated with El Niño conditions. For example, early reports in Belize and Puerto Rico
were of massive bleaching, but mortality was eventually low in most places (Case Studies 6
and 7).
In August 1998, more than 60% of all head corals to a depth of 15m bleached around
Walker’s Cay, New Providence Island, Little Inagua, Sweetings Cay, Chubb Cay, Little San
Salvador, San Salvador and Egg Is, Bahamas and up to 80% between 15-20m depth.
Samana Cay was less effected with Montastrea cavernosa not bleached, and Acropora
palmata bleached on only the upper sides. A large area of the Carribee bank, Barbados
apparently bleached in September 1998, whereas on Bermuda bleaching started in August
1998 and continued into October, when surface temperatures rose to 30 o C. There was 2-
3% bleaching of the 25% coral cover at 8m on rim reefs, 5-10% bleaching of the 40% coral
cover at 15m on offshore terrace reefs, and 10-15% of the 15-20% lagoon coral cover.
Mortality was low, perhaps 1-2% of affected colonies.
There was mass bleaching on reefs off Bahia State (12 o S; 38 o W) Brazil in April 1998, with
60% of Mussismilia hispida (endemic coral), 80% of Agaricia agaricites, and 79% of
Siderastrea stellata (endemic) bleached. Similar bleaching was reported on the Abrolhos
Reefs (18 o S; 40 o W). By October 1998, all colonies have recovered.
CONCLUSIONS
The bleaching in 1997-98 was the most intense on record with damage recorded in all
oceans. The major causal factor was increased seawater temperatures during the extreme El
Niño and La Niña events. Bleaching predominantly occurred on southern reefs when the El
Niño event coincided with the summer, whereas severe bleaching occurred in the northern
hemisphere when the strong La Niña coincided with summer.
Coral bleaching started with the build up of El Niño associated warm waters in the far
eastern Pacific between May and December 1997 when the Southern Oscillation Index
(SOI) averaged around minus 16, in: Colombia starting May; Mexico from July-September;
Panama in September; and the Galapagos Islands in December. Most of this bleaching was
relatively mild, with little resulting mortality.
The most severe impacts occurred in the southern Indian Ocean between March and June,
1998 and during the El Niño when the SOI averaged below minus 23 for 4 months: in Kenya
and Tanzania in March–May; Maldives and Sri Lanka in April to May; Western Australian reefs
from April to June; India from May to June; Oman and Socotra, Yemen in May.
There was also some bleaching in Southeast Asia, Australia and the southern Atlantic Ocean
between January and May, 1998, in: Indonesia from January to April; Cambodia, Thailand
and East Malaysia during April–May; Eastern Australia and the Great Barrier Reef in January
and February, 1998; and the Southern Atlantic Ocean off Brazil in April, 1998. These events
were of modest severity, with some incidences of high mortality.
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