Effects of coral bleaching on the benthos and fish community
in Urasoko Bay, Ishigaki Island

Takuro Shibuno, Osamu Abe, Kazumasa Hashimoto and Yoshitake Takada
(Aquatic Ecology Section, Ishigaki Tropical Station)

ABSTRACT
At Urasoko Bay, Ishigaki Island, the benthos and fish community were monitored after a severe coral bleaching episode in the summer of 1998.  Most of formerly living corals had died by late September 1998 and their dead coral skeletons were coated with filamentous algae, in which gammarids and caprellids crustaceans lived.  At the outer reef, the number of herbivorous fishes increased after the coral bleaching.
INTRODUCTION
Severe coral bleaching occurred throughout the Ryukyus Islands of Japan in the summer of 1998.  Changes in the structure of the benthos and fish community at Urasoko Bay, Ishigaki Island, the Ryukyu Islands were quantified and related to this episode coral bleaching.
RESULTS
At the outer reef, most of the living corals had died by late September 1998, however, at the moat, most of the living corals were not affected by this severe bleaching (Fig. 1).  Their dead coral skeletons were coated with filamentous algae by late October 1998.  These algae succeeded to form mat shaped colonies by April 1999.
  In the benthic community, small crustaceans, mainly caprellids, appeared in the filamentous algae which covered the dead coral skeletons (Table 1).  However, abundances of gammarids and polychaetes increased after the filamentous algae changed to mat shaped algae colonies.  Changes in the benthic community structure appeared to be related to the succession of algae.
  In the fish community at the outer reef, the number of individuals per transect did not change after the coral bleaching, however, the number of species and species diversity (Hf) decreased.  Although the number of individual omnivores and carnivores did b\not differ before and after the coral bleaching, the number of coral-polyp feeders decreased.  In contrast, the number of herbivores, particularly two species of acanthurids, increased after the coral bleaching (Fig. 2).  In the fish community at the moat, both the number of individuals and species per transect did not change before and after the coral bleaching.  At the outer reef, differences in abundances of herbivores and coral-polyp feeders after the coral bleaching appeared to be related to major changes in the benthic habitat.  Results suggested that the two-acanthurids fish respond to the increase in algae biomass by migrating from adjacent habitats to the outer reef.
Table1.  Changes in the abundances of small animals inhabiting in corals which died of bleaching. Data from November 1998 and March 1999 are compared.
Fig. 1.  The outer reef in Urasoko Bay, Ishigaki Island. Left photograph: before the coral bleaching, June 1996. Right photograph: after the coral bleaching, January 1999.
Fig. 2.  Mean number of individuals per transect for each trophic category at the outer reef and the moat.
   C: carnivores, O: omnivores, H: herbivores, CO: coral-polyp feeders.