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."

Bleaching occurs when water temperatures rise above a certain level, causing coral polyps to expel zooxanthellae, a kind of algae that lives in their tissues. Experts say coral bleaching tends to occur when the water temperature is above 30 degrees Celsius.

Since large-scale coral bleaching was observed in the Sekisei lagoon in summer 2016, the Environment Ministry has conducted a survey on the reef several times a year.

As the sea temperature around the lagoon often fell below 30 degrees Celsius in summer 2017, the latest survey found only 0.1 percent of coral in the area had died as a result of bleaching, significantly below the 70.1 percent from a year earlier. Moreover, healthy coral covered 14.7 percent of the total area of the reef inhabitable by the invertebrate, slightly above the 11.6 percent of a year earlier.

A nationwide Environment Ministry survey conducted last year shows that about 30 percent of the coral off Okinawa and the Amami Islands in Kagoshima Prefecture had bleached -- up more than 10 points from 2016 -- as a result of rising water temperatures.