World's northernmost coral reef in Japan bleached

Since 2015, all tropical coral reefs have seen above-normal temperatures, and more than 70 per cent experienced prolonged high temperatures that can cause bleaching. (Photo: AFP/Biopixel)

Healthy coral reefs protect shores from storms and offer habitats for fish and other marine life, including ecologically and economically important species.

After coral dies, reefs quickly degrade and the structures that coral build erode. While coral can recover from mild bleaching, severe or long-term episodes are often lethal, experts say.

About 30 per cent of the coral reef off the coast of Tsushima island in Japan, which lies in the temperate zone some 1,000 km southwest of Tokyo, suffered bleaching when Hiroya Yamano's research team observed the area last December.

There was large-scale coral bleaching in Japan's subtropical Okinawan chain of islands last summer, said Yamano, director of the Center for Environmental Biology and Ecosystem Studies at Japan's National Institute for Environmental Studies.

"Recently coral in Okinawa were taking refuge in waters with lower temperatures, expanding their habitat range to (waters off) Kyushu, Shikoku and Honshu," he said, referring to three of Japan's four main islands.

"But now coral in refuges are threatened ... the situation is serious," he told AFP.

Since 2015, all tropical coral reefs have seen above-normal temperatures, and more than 70 per cent experienced prolonged high temperatures that can cause bleaching.

Bleaching of the world's northernmost coral reef in Japan. (Infographic: AFP)

Early in 2017, the rise in water temperature caused significant bleaching in the Great Barrier Reef in Australia for the second consecutive year and also in American Samoa, which was severely affected in 2015.

The US National Oceanic and Atmospheric Administration said last month that coral reef bleaching may be easing after the three years of high ocean temperatures, the longest such period since the 1980s.

Its experts said satellite data and other analysis showed widespread bleaching was no longer occurring in all three ocean basins - Atlantic, Pacific and Indian - "indicating a likely end to the global bleaching event".