Decline in the <i>Acropora</i> population due to repeated moderate disturbances in Okinotorishima Island, Japan

Coral bleaching as a result of increasing sea surface temperatures (SSTs) is the most critical threat to coral reefs globally. However, in addition to this global stress, various local stresses have also resulted in degradation, and the actual effect of these 2 stresses is difficult to distinguish. Okinotorishima Island, an isolated remote table reef in the southernmost territory of Japan, provides an excellent site to examine the effect of only global stress, as there are no local human stresses present. We established 17 fixed quadrats (1×1 meter) on 9 knolls in the shallow lagoon and observed coral coverage every May from 2006 to 2015. The initial coverage of the dominant coral genus Acropora ranged from 4.3 to 20.0%, but decreased significantly during the observation period, with no living Acropora remaining in 12 quadrats at the end of the period. The major cause of the decline was mortality or the disappearance of the colonies. Wide-scale decline in Acropora corals across Okinotorishima did not occur all at once in any 1 year, but due to multiple moderate stress events affecting different quadrats at different times. DHW ranged between 0 to 4.09°C-weeks throughout the survey period, which was below the threshold value of mass coral bleaching. Typhoons alleviated the high SST but caused the physical loss of colonies promoting their decline. The results suggest that even in the absence of direct anthropogenic stress or high thermal stress that causes severe bleaching, recurring moderate disturbances can result in a significant decline in coral populations.