Mass coral settlement on the artificial reefs in Ishigaki Island, Okinawa, Japan

The 1998 bleaching event was the most extensive and severe one ever observed in the Ryukyu Islands, Japan. From late August to September, mass mortality of hermatypic corals, especially the genus Acropora, occurred in Urasoko Bay of Ishigaki Island (Fujioka 1999, 2002). On the outer reef flat, where the present study was conducted, the live coral coverage drastically decreased from 80.4 % (estimated) at the highest before bleaching to only 6.6 % at four months after the bleaching event (Fujioka 2002). It was reported that in the breeding season following the bleaching event of 1998 the reproduction of corals was severely affected by the high thermal stress that had caused the mass bleaching in Okinawa (Omori et al. 2000 Hirose and Hidaka 2001) and the Great Barrier Reef (Hoegh-Guldberg 1999 Baird and Marshall 2002), and a drastic reduction in recruitment was predicted. Contrary to these predictions, an unexpectedly rapid recovery was reported to have occurred in some coral reefs (Normile 2000). However, it is not been clear whether the recovery can be attributed to new recruits or to the survival of juveniles. In October 1998, we deployed several artificial reefs in Urasoko Bay, immediately after the bleaching event. At present, many juvenile coral colonies can be observed on these artificial reefs. This provides evidence that the sexual recruitment resulted in recovery from the mass mortality by the bleaching event of 1998.