Coral Bleaching in Seychelles

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Coral Bleaching

SEYCHELLES CORAL BLEACHING

Not really a visitor's article but a brief review of the current status of coral reefs around Seychelles and what you can expect if you are visiting the area.

During autumn 1997 to spring 1998 the Indian Ocean in particular, but many of the worlds tropical oceans in general, suffered from an increase of sea water temperature; in the Indian Ocean this was attributed to an "El Nino Southern Oscillation" event or ENSO. The primary cause for this event is still unclear but seems to be a result of the overall pattern of global warming.


	Massive Porites boulder coral, Aldabra November 2001

In the western Indian Ocean this had drastic affects on shallow water coral forms, in particular the branching stony corals. Warm water causes the corals to discharge their symbiotic algae from their tissues rendering them translucent and colourless, hence the term "bleached coral". If the warm water period is not too long then the coral colonies can later regain their algae and continue growth as normal. However, if the exposure is for a long period of time then chances of coral recovery are not good and they will generally die off. The corals will then become subject to a cycle of colonisation by algae and sponges that may ultimately revert back to hard coral formations if good water conditions persist.

In terms of Coral Bleaching the reefs on Seychelles inner islands suffered major coral death to the branching coral species, mainly the Acropora forms such as Stag Horn, Elk Horn and Table corals. The massive boulder and brain corals were not so badly affected. The Inner islands reefs are however based on granite rock substrates and the granite reefs still provide a huge resource in terms of marine habitats and shelter and as such there has not been a marked change in the fish population. There have been changes in species densities and herbivores such as Parrot and Surgeon fish that were able to effectively graze on the algae covering the dead corals have greatly increased in numbers.


	New Acropora coral plaque, Mahe September 2001

On reefs in the outer islands the bleaching affects were not so severe on islands with steep wall formations, such as in the Amirantes and Southern Islands groups, probably due to the frequent cold water upwellings in this area. The big gorgonian fan formations also seem to have survived the onslaught and remain as vibrant as ever.

On an optimistic note it has been reassuring to see the emergence of new "Coral Plaques" on many reefs on the inner islands. These are the first signs of new branching coral re-colonisation and growth after the bleaching event.


	Acropora table coral regrowth, Aldabra November 2001

On the outer islands a massive re-growth of branching coral forms has been noted which seems to stem from latent coral polyps of the original colonies becoming awakened and re-colonising the original coral skeletons. Branching corals are particularly fast growing and so providing there isn't another warming episode a strong recovery is hopefully heralded!!

David Rowat

Last updated February 01, 2004

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