Some 49.9 percent of Sekisei coral reef — Japan's largest — had bleached by the end of 2017, the Environment Ministry has revealed. The figure is substantially less than the bleaching ratio of 91.4 percent on the reef between Okinawa Prefecture's Ishigaki and Iriomote islands at the end of 2016. However, "the water temperature remains high and the bleaching ratio is still high. We can't be optimistic," said an Environment Ministry official. "Coral in the area hasn't shown signs of real recovery, and remains in critical condition."