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Loss of symbiotic zooxanthellae, or “bleaching” is one of the first visible signs of thermal stress. Bleaching can be induced by short-term exposure (i.e. 1-2 days) at temperature elevations of 3 degree C to 4 degree C above normal summer ambient or by long term exposure (i.e. several weeks) at elevations of 1 degree C to 2 degree C. Corals in both tropical and subtropical locations live at temperatures close
to their lethal limits during the summer months. Temperature elevations above summer ambient, but still below the bleaching threshold, can impair growth and reproduction. A warming trend in Hawaiian waters has been observed over the past decade. In 1986, 1987 and 1988 Hawaiian corals were perilously close to their bleaching threshold during the
summer months, and localized bleachings did occur. In some cases, local warming of surface water on shallow reef flats exceed this threshold temperature and caused localized coral bleaching. A continuation of the warming trend in Hawaii would lead to mass bleachings similar to those observed recently in other geographic locations.