Scientists record biggest ever coral die-off on Australia's Great Barrier Reef

A man snorkels in an area called the 'Coral Gardens' near Lady Elliot Island, on the Great Barrier Reef, northeast of Bundaberg town in Queensland, Australia, June 11, 2015. REUTERS/David Gray/File Photo

Warm seas around Australia's Great Barrier Reef have killed two-thirds of a 700-km (435 miles) stretch of coral in the past nine months, the worst die-off ever recorded on the World Heritage site, scientists who surveyed the reef said on Tuesday.

Their finding of the die-off in the reef's north is a major blow for tourism at reef which, according to a 2013 Deloitte Access Economics report, attracts about A$5.2 billion ($3.9 billion) in spending each year.