NHK has learned that researchers in Japan's southern prefecture of Okinawa have found widespread bleaching of coral at a location where the sea is calm and warm.

A team led by Associate Professor Nakamura Takashi of the University of the Ryukyus surveyed three locations on Monday in Motobu Town, in the north of Okinawa's main island.

In an area where the sea is calm, the water temperature was 30.3 degrees Celsius. Bleaching was confirmed on most of the coral in the shallows 2 to 4 meters deep.

Bleaching is a phenomenon in which a type of phytoplankton that cohabits with and supplies nutrients to coral dies as the seawater is warmed.

It is believed to occur when the water temperature remains above 30 degrees.

Waters around Okinawa were the hottest last month since 1982, when data with analytical value first became available.

Nakamura says if temperatures stay high and no typhoon approaches, more bleached coral will likely die and the reef ecosystem could collapse.

He says the scale of bleaching could compare to the phenomenon in 1998, when coral around Okinawa's main island was devastated.