Tokyo ? Large swaths of coral off islands around Okinawa Prefecture have been turning white due to a phenomenon known as coral bleaching – a sign that the coral is dying.

This phenomenon has been sighted for the first time in four years in locations in the prefecture such as the coast around Ishigakijima island, which boasts the country’s largest coral reef, and is thought to have been brought on by high water temperatures in July.

Should the water temperature stay high in August, it is feared that more coral will be destroyed than ever before – surpassing the case that occurred in 1998, when about 40 percent of the coral around Ishigakijima died.

The Environment Ministry has begun investigating the situation.

According to information supplied by the ministry and experts, the bleaching has been confirmed around Ishigakijima and in locations such as the sea off Sesokojima island near Okinawa.

Kenji Ohori, an Ishigakijima coral reef tour guide said he had confirmed Tuesday that most of a reef stretching from Yonehara beach on the north of the island to a point far out into the ocean had turned completely white. He added that part of the reef had already died.

Coral bleaching happens when the sea temperature stays above 30 C for days at a time. It has occurred three times in Okinawa Prefecture before – in 1998, 2001 and 2003.

Average daily air temperatures of more than 30 C were recorded for a record 32 days in June and July on the island, according to Mineo Okamoto, an associate professor at Tokyo University of Marine Science and Technology.