for National Geographic News
Eight years after warming seas caused the worst coral die-off on record, coral reefs in the Indian Ocean are still unable to recover, biologists say.
Many reefs have been reduced to rubble, a collapse that has deprived fish of food and shelter.
As a result, fish diversity has tumbled by half in some areas, say authors of the first long-term study of the effects of warming-caused bleaching on coral reefs and fish.
The study focused on reefs near Africa's Seychelles islands, north of Madagascar (see Seychelles map), which sustained heavy losses from bleaching in 1998.
"The outlook for recovery is quite bleak for the Seychelles," said lead study author Nicholas Graham, a tropical marine biologist at England's University of Newcastle Upon Tyne.
The study, in today's Proceedings of the National Academy of Sciences, predicts that isolated reef ecosystems like that around the Seychelles will suffer the most from global warming-caused bleaching events.
Small but prolonged rises in sea temperature force coral colonies to expel their symbiotic, food-producing algae, a process known as bleaching.
While the dying reefs, which turn ghostly white, can recover from such events, many do not.
In 1998 an El Niño weather pattern sparked the worst coral-bleaching event ever observed.
"Over 16 percent of the world's reefs were lost in that one year," said Graham, part of a team that recently received an unrelated research grant from the National Geographic Society Committee for Research and Exploration. (National Geographic News is a part of the National Geographic Society.)
"It was a huge event."
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