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Coral Bleaching in Seychelles 

A brief review of the current status of coral reefs around Seychelles and what you can expect if you are visiting the area by Udo Engelhardt, consultant for the coral monitoring component of the Seychelles Marine GEF. 

	Porites coral head, Aldabra December 2001

During the period from autumn 1997 through to spring 1998 many of the world’s tropical oceans, including the Indian Ocean, suffered from a dramatic increase of seawater temperature. In the Indian Ocean this event was attributed to the ‘El Niño - Southern Oscillation’ phenomenon or ENSO. The primary cause for this event is still somewhat unclear but there seems to be general agreement that the overall pattern of events is linked to global warming.

In the western Indian Ocean the warming event had drastic effects on shallow water corals, in particular the branching stony corals. Warm water causes the corals to discharge the normally abundant symbiotic algae from within their tissues rendering the corals translucent and colourless, hence the term ‘bleached corals’. If the warm water period is only short-lived then the coral colonies may soon regain their symbiotic algae and continue growth as normal. However, if the exposure to excessively high water temperatures persists over a longer period the chances of full coral recovery are much reduced and many colonies will die off. 

The corals’ skeletons soon become subject to a cycle of colonisation by fleshy algae, sponges and other marine organisms. Many of these organisms are known to erode the old coral skeletons to a point where they eventually break down to form coral rubble. However, this coral rubble may become an ideal substrate for the settlement of new corals. A purple, encrusting layer of coralline algae is all that is needed to cement these rubble fragments together thereby forming a new clean platform for coral regrowth.

New Acropora coral recruit, Mahe September 2001

In terms of coral bleaching the reefs on Seychelles’ inner islands suffered major coral death to the branching coral species, mainly the members of the Genus Acropora, which includes the well-known staghorn, elkhorn and table corals. In some locations, the massive boulder and brain corals were not as badly affected. As many of the inner islands reefs are actually growing on very solid granitic rock structures much of the complexity of these special marine habitats has been retained with most reef creatures still able to find sufficient food and shelter.

On a more optimistic note it has been very encouraging to see the emergence of newly settled juvenile corals on many of the reefs in the inner islands. These are the first signs of coral re-colonisation and growth following the bleaching event. Recent scientific surveys of the inner island reefs have already shown an increase of both the overall numbers of small corals as well as a corresponding increase in local hard coral diversity. 

On the mostly steep-walled reefs of the outer islands, such as the Amirantes and Southern Islands groups, coral bleaching effects were generally not as severe with a much higher proportion of corals surviving. It is likely that the frequent cold-water upwellings that characterize this area were responsible for the much-reduced impact. The big gorgonian fan formations also seem to have survived the onslaught and remain as vibrant as ever.

	Acropora table coral regrowth, Aldabra December 2001

On the outer islands a massive re-growth of branching coral forms has also been noted. Much of this regrowth seems to stem from remnant coral colonies that partially survived the bleaching event. Small remnant patches of live coral tissue, particularly when found on branching staghorn corals, may often be sufficient to promote the onset of the reef recovery process. Branching corals are particularly fast growing and so providing that there isn't another warming episode in the near future a strong reef recovery is hopefully heralded!!