Status of Coral Reefs of the World 2000

The 1997-98 Mass Coral Bleaching and Mortality Event
Arabian Sea
There was major bleaching around Mirbat, southern Oman, in May 1998 with 75 to 95% of
abundant Stylophora bleached, and 50% of large Porites colonies were partially bleached.
No bleaching was observed at Sudh, 40km east of Mirbat, nor in the Muscat Area, Gulf of
Oman at temperatures around 30.5°C. Upwelling during the southeast monsoon normally
drops temperatures to 19ºC. No recovery of bleached colonies was seen in mid-October
when temperatures increased to 25°C after the summer upwelling. There was also extensive
coral bleaching on the Socotra Archipelago (see chapter 2).
Indian Ocean
Probably the most extensive coral bleaching ever witnessed occurred in the central to
northern Indian Ocean during the first 6 months of 1998, with devastation of coral reefs
along Eastern Africa, in the Seychelles, Chagos Archipelago, the Maldives, Sri Lanka and
India and on reefs off Western Australia. The normal monsoon and trade winds stopped
and surface waters warmed during this severe El Niño, and the warm water ‘followed’ the
sun from south to north between January and June. When the El Niño switched to an equally
strong La Niña in June 1998, winds and currents started again and bleaching stopped.
Eastern Africa
Coral bleaching and mortality were severe on the coasts of Kenya and Tanzania from March
to May, 1998. In Kenya, bleaching varied from 50% to 90% on most reefs, but less on reefs
below 10 m depth (about 50%). Coral cover in many sites has dropped from a mean of
30% to 5–11%, irrespective of whether or not the reefs were protected in a protected area.
New coral recruitment is very low, so recovery will be prolonged. Bleaching was similar or
worse in Tanzania, particularly in the northern half with 60-90% losses of corals at Tutia
(Mafia Island Marine Park) and Misali (west coast of Pemba). There was less bleaching on
some reefs e.g. 10% or less on Unguja Island, Zanzibar. Reefs in the northern part of
Mozambique with up to 99% mortality e.g. on some patch reefs at Inhaca Island. Inshore
areas, which experience greater variations in temperatures experienced less bleaching.
Only a few corals in South Africa were affected, probably because most coral growth is
deeper than 10 m.
Southern Indian Ocean
Bleaching impacts were far more severe in the north (Comoros and Seychelles) than in the
south (Madagascar, Mauritius and Reunion) where it was minimal. Mortality on the Comoros
reef flats and slopes was approximately 40% to 50%. Most reefs of the inner Seychelles
have less than 10% coral cover and on some it is as below 1%. On Madagascar, there was
30% bleaching of corals at Belo sur Mer (mid-west coast) and similar bleaching at
Antananbe, Toliara, Nosy Bé, and Mitsio archipelago. At Mananara-Nord (northeast coast,
15 o S), 40-80% of corals bleached with high mortality, and 10-40% of mixed species corals
bleached in deeper water. A similar pattern was reported on Mayotte, except on the
exposed southern end and the lagoon, which receives cooler water from the north.
South Asia
The most severely impacted reefs in the world during the 1998 bleaching event were
probably those of Chagos, Sri Lanka, India and the Maldives. In this region, and on nearby
reefs off the Seychelles and Western Australia (Case Study 3), there were losses of 80% of
23