Coral bleaching around Akajima, Okinawa

Extensive bleaching of corals occurred around Akajima, Okinawa, in the summer of 1998. Bleaching was observed in a few corals in late July 1998, but by the end of August many pocilloporiid and acroporiid corals were bleached. The bleaching frequency, relative to total coverage of corals, at 15 sites (1.3-7.2m in depth) around Akajima ranged from 55.9 to 97.4% in late September 1998. We measured 2m x 2m quadrats at each site. Three out of 4 sites that showed over 90% bleaching were located in either a moat or inside of a bay, where water exchange was slow. One site was characterized by a dominant occurrence of branched corals of the genus Acropora. This group was more easily bleached than other morphologic groups around Akajima. Sea surface temperature over 30°C (normally about 28°C) was recorded for a month in August. This high temperature may have been a primary factor of the coral bleaching. Coral diseases caused by viruses or bacterial infection after bleaching were not observed. Geographical features, rate of water exchange, and composition of coral species seem to be responsible for the differences in extent of bleaching.