Marine researchers said an El Nino weather pattern had been developing in recent months, raising ocean temperatures and stressing delicate coral reefs.
"The worst coral bleaching event ever recorded for the Marshall Islands has been occurring since mid-September," Karl Fellenius, a Majuro-based marine scientist with the University of Hawaii told AFP.
C. Mark Eakin, manager of the US National Oceanic and Atmospheric Administration's Coral Reef Watch programme, said recent observations showed the problem was widespread across the vast waters of the northern Pacific.
"Major bleaching was seen in Guam and the Commonwealth of the Northern Marianas Islands, the northwestern Hawaiian Islands (NWHI), the Marshall Islands, and Kiribati," he said.
"Thermal stress levels set new record highs in CNMI and the NWHI and we saw the first widespread bleaching event in the main Hawaiian Islands."
Fellenius said coral bleaching was a naturally occurring phenomenon but not on the scale currently being seen.
"While bleaching can occur on very hot days in pools of water with little circulation (such as) very low tides on reef flats, it has become a global problem due to greenhouse gas emissions causing elevated temperatures under climate change."
He said sea surface temperatures had been on average half to a full degree Celsius higher than normal for months, adding: "This does not seem like a lot but it makes a big difference to corals."
Fellenius said the last major bleaching event was in 1997, when an exceptionally strong El Nino system affected about a quarter of the world's coral reefs.
He said indications were that the latest episode had affected up to 75 percent of smaller corals and 25 percent of the larger varieties at some sites in the Marshalls.
He said the bleached coral was becoming covered with algae, hindering its chances of recovery.
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