13:10-15:10 Session IIa Dynamics of Coral Ecosystem
Condition of reef-building corals in Akajima Island;
effect of the heavy coral bleaching in 1998
H. Taniguchi1), K. Iwao1) and M. Omori2)
1) Akajima Marine Science Laboratory, 179 Aka, Zamami-son, Shimajirigun, Okinawa 901-3311, Japan. 2) Tokyo University of Fisheries, 4-5-7, Konan Minato-ku, Tokyo 108-8477, Japan
An unprecedented episode of coral bleaching hit the Okinawa waters including Akajima Island in summer 1998. In Akajima Island coral bleaching occurred in a few coral colonies first time in late July, and then it expanded widely by the end of August. Percentage bleaching relative to total coverage of corals in a 2m x 2m quadrate ranged from 56 to 97% at 15 sites (1.3-7.2 m in depth) in late September. Three out of 4 sites that showed over 90% bleaching were located in either a most inside of a bay where the water exchange rates were low. Another heavily damaged site was characterized by dominant occurrence of branched corals of the genus Acropora. This type of corals seems to be more easily affected by bleaching episode than other types. The highest surface sea temperature is normally 28℃ around the Island, but it exceeded 30℃ for a month in August 1998. This high temperature may have been a primary factor of the bleaching. Geographical features, rate of water exchange, and composition of coral species seem to be responsible for the differences in extent of bleaching.
We continued observation of reef-building corals at Maenohama of the Island where percentage bleaching was high in September 1998. Number of colonies, genus, morphologic type and bleaching level were recorded along a 30 m line sect. There were 298 colonies of 17 genea of which the genea Acropore and Montipora dominated. In September 1998, 33% of total colonies were completely bleached, 58% were partly bleached, and 2% died. Whereas, in March 1999, completely bleached colony was none and 15% were still partly bleached. In total, 25% dead and 49% recovered after 6 months. This recovery seems to be much better than other area of the Okinawa waters.
