Coral bleaching occurred at various places around the Ryukyu Islands in summer 1998. Several researchers have suggested links to high sea surface temperature. However, the observation of coral bleaching around Ishigaki-jima of the Yaeyama Island Group where the bleaching has been extensive since July shows that there was already a sign of the event during the Baiu rainy season. The study of corals on Ishigaki-jima presents the following: 1) Coral bleaching was probably caused by the continued high sea surface temperature from July through September 1998. 2) The corals had already been severely damaged by the sedimentation of red soil from the disturbed land surface during the Baiu season of the year which had nearly twice as much rainfall as normal year. The subsequent high sea surface temperature in summer contributed to accelerated bleaching. The observation of bleaching in moats around Ishigaki-jima in summer 1998 suggests that the progress of bleaching be divided into the following three time stages: 1) Early stage: from around June 20 through early August 2) Mature stage: from middle August through early September 3) Final stage: from middle September through middle October The bleaching process divided by these stages is synchronous, with a time lag, with the seasonal change in sea surface temperature and the prevailing weather conditions in Okinawa.
1998年夏、南西諸島において造礁サンゴの白化現象が発生した。石垣島において大コドラート法 (10×10m) によるフィールド調査を行った結果、(1) 白化の範囲が広い、(2) 白化の程度が強い、(3) 白化に伴う死亡が顕著という、従来にはない際立った特徴が認められた。 水深1～2mの浅い礁原上では、平均85.9～92.2%の造礁サンゴが白化し、影響の大きいミドリイシ類の死亡率は60.7～78.7%にも達した。白化被害は種や生息場所によって大きく異なっており、以下の4つのグループに分けられた。 I. 白化率、死亡率とも高い (クシハダミドリイシ、オトメミドリイシなど) II. 白化率は高いが、死亡率は低い (コユビミドリイシ、エダコモンサンゴなど) III. 白化率、死亡率とも低い (リュウキュウノウサンゴ、ユビエダハマサンゴなど) IV. 白化率よりも死亡率の方が高い (エダセンベイサンゴ) 今回の白化の主な原因は、夏季の高水温である。石垣港の月平均海水温は、過去85年間に、夏季0.7℃、冬季2.0℃上昇しており、このことが近年の白化被害の増大に影響しているという可能性が示唆された。
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.
Changes in living coral coverage along five transects on Shiraho reef and one transect on Kabira reef, Ishigaki Island, Ryukyu Islands were investigated before and after the 1998 bleaching event. The coverage of living corals after the bleaching decreased by 50% of their original values. Branching types of Montipora, Acropora and Porites were most severely bleached and their mortality was also high. Bleaching was intermediate to high for massive Porites whose recovery was better than branching corals. Heliopora coerulea was less susceptible to bleaching and mortality was low. Higher than 30°C sea surface temperature prevailed over the south sea of Japan, which induced higher than 30°C reef water temperature on the coral reef flat and resulted in coral bleaching.