Mass destruction of the hermatypic corals during a bleaching event in Ishigaki Island,southwestern Japan

Abstract

The bleaching of hermatypic corals that occurred in the summer of 1998 throughout the Nansei Islands, southwestern Japan, was the most extensive one in recent years. Field investigations using a large quadrat (10×10m) method distinguished this bleaching from prior episodes as (1) the largest in scale, (2) the most severe, and (3) causing the highest mortality. At two shallow reef flat sites, a total of 85.9 and 92.2% in coverage of hermatypic corals were bleached and mortality reached 60.7 and 78.7% in the most thermally susceptible acroporids. Bleaching damage varied among species and habitats. Dominant coral species were classified into four categories of bleaching; I. both rates of bleaching and mortality were high (e. g. Acropora hyacinthus, A. pulchra), II. rate of bleaching was high, but mortality was low (e. g. A. digitifera, Montipora digitata), III. both rates of bleaching and mortality were low (e. g. Platygyra ryuhyuensis, Porites cylindrica), IV. mortality (due to bleaching) was higher than rate of bleaching (Leptoseris gardineri). Acroporids in Category I were thermally more susceptible than those in Category II as the surface area per volume of skeleton was larger. Species categorized into Category II and III were considered to be best adapted to severe thermal environments such as the shallow reef flat and moat. The mass bleaching during the summer to autumn of 1998 was considered to be caused mainly by extraordinary high thermal stress. Meteorological data showed that average sea water temperatures have risen by 0.7°C in summer and 2.0°C in winter over the past 85 years, resulting in more frequent and serious disturbance by bleaching on coral reefs in recent years.