Scientists say warm ocean temperatures are causing large expanses of coral to bleach in the pristine reefs north-west of Hawaii's main islands.

Courtney Couch, a researcher at the Hawaii Institute of Marine Biology, said Tuesday she observed mass bleaching at Lisianski atoll, about 1,000 miles north-west of Honolulu.

Coral also bleached at Midway, and at Pearl and Hermes atolls, but not as severely, the scientists found.

Mass bleaching occurs when corals are stressed by warmer-than-normal temperatures.

The warm water prompts algae inside the coral to leave. This starves coral and turns it white.

Coral start to die after about eight weeks of high temperature-induced stress. This year, Lisianski has had 10 weeks. Midway and Pearl and Hermes atolls have had seven.

The Northwestern Hawaiian Islands last had a mass bleaching event in 2004.

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