Recovery of coral populations after the 1998 bleaching on Shiraho Reef, in the southern Ryukyus, NW Pacific

Abstract

The most extensive and severe bleaching of coral ever recorded occurred in 1997 and 1998 as a result of anomalously high sea-surface temperatures. In the Ryukyu Islands, extensive bleaching occurred from late July to early October in 1998. A time-series transect study on a reef flat revealed the different responses of coral populations among species to this event. Coral cover was monitored just before, during, and 6 times after the bleaching occurred, along 5 fixed transects, 710 to 800 m long, across the reef flat of Shiraho Reef, Ishigaki Island, in the Ryukyu Islands of Japan. The dominant corals were Heliopora coerulea, massive Porites, and branching Porites, Montipora, Acropora, and Pavona (these latter 4 genera could not always be distinguished to species level, and are thus grouped by genus and ecoform). H. coerulea was the least susceptible to bleaching and maintained almost constant coverage before and after the bleaching. Massive Porites were suscepti- ble to bleaching, but regained their algae after the bleaching and sustained their coverage. On the other hand, the branching Porites, Montipora, and Acropora were susceptible to bleaching and mortality was high. Coverage by branching Montipora was initially reduced by 66%, but by 2 yr after the bleaching it had recovered to pre-bleaching coverage. However, only large patches of more than 70 m along a transect, with coverage of 10 to 40%, recovered. Small patches of less than 30 m along a transect, with coverage of less than 10%, died and did not recover. The response of corals to bleach- ing differs according to the strategy the coral uses against bleaching; moderately frequent and severe bleaching might permit the coexistence of both types of corals on the same reef flat, as in the case of Shiraho Reef.