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.