World / Science & Health

Warm El Nino waters bleaching coral in northwest Hawaii


Warm ocean temperatures have caused large expanses of coral to bleach in the pristine reefs northwest of Hawaii’s main islands, scientists say.

Mass bleaching has occurred at Lisianski atoll, about 1,000 miles (1,600 kilometers) northwest of Honolulu, said Courtney Couch, a researcher at the Hawaii Institute of Marine Biology. Coral also bleached at Midway, Pearl and Hermes atolls, but not as severely.

Couch called the situation “dire,” particularly for Lisianski. In one shallow part of the reef, 90 percent of the coral was bleached, she said. An average of 35 percent of the coral sites observed at the atoll had bleached, she said.

Mass bleaching generally occurs when corals are stressed by warmer-than-normal temperatures. The warm water prompts algae inside the coral to leave, which starves coral and turns it white.

Algae may return to coral, and the coral may recover, depending on how long the bleaching lasts.

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

Even if corals recover after algae returns, they are still significantly weaker and more vulnerable to disease.

This year’s higher-than-normal ocean temperatures are caused by El Nino, which is a warming of the central Pacific that changes climate worldwide, Couch said. “The largest body of warm waters is literally sitting right over Lisianski right now, and it’s moving northward as the summer progresses,” she said.

The Northwestern Hawaiian Islands last had mass-bleaching events in 2004 and 2002. This year’s event is only the third mass coral bleaching event recorded in the remote, mostly uninhabited archipelago that makes up the Papahanaumokuakea Marine National Monument.