May 26, 2006

The long term effect of coral bleaching on coral reefs

The plight of Seychelles’ coral reefs has been taken up by the media around the world following an international scientific study on the state of the islands’ reefs after the great coral bleaching of 1998. The research was conducted by scientists from the UK, Australia and locally from the Centre for Marine Research and the Seychelles Fishing Authority, with additional logistical and in kind support from Nature Seychelles. It was first published in the U.S scientific journal Proceedings of the National Academy of Sciences on Monday 15th May and was immediately taken up by BBC News and The Independent Newspaper and other major European news network. 

The group of international and local scientists, led by Mr. Nicholas Graham from the University of Newcastle upon Tyne, who carried out the surveys last year, said that global warming has had a more devastating effect on Seychelles’ coral reefs than has been previously assumed. They predicted that large sections of coral reefs and much of the marine life they support may be wiped out for good. This came after comparing their survey of 50,000 square metres of corals reefs at 21 sites in the inner islands of the Seychelles carried out in 1994 and 2005. 

Coral bleaching occurs when the seawater temperatures rise above normal limits, which in the Seychelles is in the range of 23 to 30 degrees Celsius. Above this temperature the corals start expelling the algae that live inside their body, which provides them with most of their energy and colour, thus resulting in coral bleaching.

The research showed that, while the 1998 event was devastating in the short term, the main long-term impacts are down to the damaged reefs being largely unable to reseed and recover.  Reseeding of coral reefs with new coral larvae has been one of the main limiting factors that have affected the recovery of reefs in the inner islands. The 1998 bleaching event killed more than 90% of corals in the inner islands. With very little live corals remaining after this event, compounded with the islands’ isolated location, far from other major coral reef systems, the low amount of larvae that are being produced is resulting in the slow recovery that is being presently observed. With such low coral cover many of the reefs are simply collapsing into rubble which becomes covered by unsightly algae.

The current findings follow the publication of an article in 2005 in the Coral Reef Degradation in The Indian Ocean Status Report by scientists from the Seychelles Centre for Marine Research and the Department of Environment in which they showed that more than 60% of the carbonate reefs being monitored in the inner granitic islands are undergoing degradation or recovering very slowly.

The collapse of the reef structure is also impacting the reef fish population. Four fish species (a type of butterfly fish, two types of wrasses and a type of damsel fish) that were found in 1994 were not observed in 2005 and are thought to have become locally extinct. Additional 6 fish species (a type of file fish, three types of butterfly fish and two damsel fish) were found to be at critically low levels, threatening the survival of the population. The 2005 surveys have shown that the diversity of fish at heavily affected sites has reduced by as much as 50%.

General scientific opinion believes that mass coral bleaching events such as that observed in 1998 will become more frequent and that by 2036 it may become an annual event. At such rates many corals will more than likely become locally extinct. The impact of these repeated coral bleaching events will be felt even more so when the height of the reefs reduces and bigger waves make their way to the shore line causing increased coastal erosion and damage to coastal properties. 

Local scientists point out that the world wide coverage of the study is also attracting global scrutiny of the measures Seychelles is taking to protect its coral reefs and minimise the various kinds of stresses and pressures that destroy, weaken and otherwise affect them. Locally these stresses include actions such as coastal over fishing, tourism developments, fishing in marine protected areas and physical destruction of reefs by the opening of channels and trampling over corals as well as coastal pollution, eutrophication and sedimentation.

(Department of Environment & Seychelles Centre for Marine Research and Technology – Marine Parks Authority)