Hawaii’s reefs are likely to suffer widespread mortality this fall in what’s expected to be the worst episode of coral bleaching ever recorded here, scientists warned Friday.

State officials pledged to step up monitoring as warmer-than-normal ocean water — the trigger for coral bleaching — is expected to linger much longer around the islands due to strong El Nino weather conditions.

National Oceanic and Atmospheric Administration scientists previously predicted the likelihood of coral bleaching this year along with a forecast for warmer ocean temperatures in the Pacific.

Already, up to 50 percent of the corals in Kaneohe Bay are experiencing some degree of bleaching, with ocean water there an average of 1 degree higher than normal seasonal temperatures since June, officials reported Friday.

Reports of bleaching are also coming in from across the state, they said.

“We are all incredibly hot,” said Ruth Gates, director of the Hawai‘i Institute of Marine Biology. “It’s just hotter than normal. And what we feel is what’s happening in the ocean.”

This year’s bleaching will be only Hawaii’s fourth such event on record, and it comes only one year after 2014’s widespread bleaching episode, previously billed as the worst ever in the islands. Other coral die-off events were recorded in 1998 and 2002.

Gates, a veteran University of Hawaii coral biologist, spoke to reporters at the state Department of Land and Natural Resources headquarters in Honolulu. She said that while a majority of corals recovered from last year’s bleaching event, she’s not sure they will survive a second, more severe blow this soon.

“They are really being slapped around the face by an even more stressful set of conditions,” she said. “I don’t feel nearly as hopeful about the implications for what this bleaching event will be. My sense is that you can’t stress an individual or organism for months and hit it again very quickly and hope it will recover as quickly.”

Bleaching occurs when corals are stressed by changes in the environment, especially a rise in temperature. The coral ejects the symbiotic algae living in its tissue, causing the tissue to fade and turn white. Without the algae the coral loses its primary source of food and is left to starve.

Some 30 percent to 40 percent of the world’s reefs have died as a result of bleaching events, officials said.

When coral dies it affects the whole ecosystem, Gates said.

“You go from a vibrant, three-dimensional structure teeming with life and teeming with color to a flat pavement that is really just covered with brown or green algae. That is really a doom-and-gloom outcome, but that is the reality of what we face with this extremely severe bleaching event,” she said.

While Kaneohe Bay has rebounded nicely, the results are mixed in the Northwestern Hawaiian Islands, where widespread bleaching was seen around several atolls last year.

Among the researchers who returned from a NOAA cruise to that region last month was Courtney Couch of the Hawai‘i Institute of Marine Biology.

“Some regions showed positive signs, regaining their pigments and looking happy and healthy,” Couch said Friday. “The bleaching level has really dropped — by three- to fourfold in some cases. But in the area that was bleached the worst last year — along the eastern coast of Lisianski island, where up to 90 percent of the corals were bleached severely — essentially there is a mile and a half of reef dead.”

That section of reef went from 70 percent coral cover down to less than 1 percent, she said.

Couch said the finding was especially disappointing because the main species affected was the purple rice coral, a rare and endemic species to Hawaii that is sensitive to temperature stress.

As depressing as that is, she said, the protection provided by the Papahanau- mokuakea Marine National Monument offers an opportunity to study how these afflicted coral reef systems recover.

“It’s going to be really interesting to see what happens, and I think the monument will teach us a lot about what’s happening here at home,” she said.

Brian Neilson, a state Division of Aquatic Resources biologist, said his field teams will continue to monitor Kaneohe Bay. He said he expects to find even more severe bleaching as high temperatures continue to stress out the corals in Hawaii’s largest sheltered body of water.

With El Nino in the forecast, warmer-than-normal air temperatures should continue into the new year. In the water, warm temperatures could last into November, Neilson said.

Officials are asking citizens to report any coral bleaching they see to the state’s Eyes of the Reef network at eorhawaii.org.

Scientists say corals have a greater chance of recovery if they are left undisturbed. That means minimizing fishing, preventing ocean runoff, refraining from stepping on reefs and more.

These things are important, Gates said, because the reefs are significant to Hawaii tourism.

“Our tourism economy is vibrant, and many people come to see our incredible marine resources. The reef is central to those marine resources,” she said. “So, no reef — a very diminished tourism economy.”