Recovery of Maui colonies after coral bleaching a ‘mixed bag’

While coral colonies in Kaneohe Bay and other shallow waters off Oahu seem to be on the rebound from a severe bleaching event last fall, experts say the status of colonies around Maui are more of a “mixed bag.”

Coral surrounding Molokini and in the shallows offshore of Olowalu experienced significant bleaching in October and November, when sea temperatures spiked at 86 degrees. Officials say some colonies have begun to show signs of recovery with color returning, but other colonies are still chalk white.

“Some colonies are getting their color back, some colonies have partial mortality and others just didn’t make it,” state Division of Aquatic Resources biologist Darla White said in a phone interview Thursday. “We’re still trying to learn . . . how come some make it and some don’t.”

White said the recent bleaching observed near Molokini and Olowalu appeared to be “very patchy, with one colony here and there.”

The genetic makeup and species of a coral could play a role in determining which coral are more susceptible to bleaching, White said. For example, the cauliflower coral and blue rice coral were “hit very hard” in the recent bleaching, while the lobe coral seemed to be able to withstand the increased ocean temperatures, she said.

White added that even colonies that retain color still can be very stressed and in danger of bleaching.

“It’s well known that bleaching is a big stress point for corals, and they do tend to be more susceptible to disease thereafter,” White said in a video circulated by the state Department of Land and Natural Resources. “Bleaching around Maui wasn’t as much as it was around places like Kaneohe or Lanikai, but it was considerably more than we get on a normal summer.”

She estimated that less than 10 percent of coral colonies around Maui were affected by the latest bleaching, which state officials described as Hawaii’s worst on record.

No one knows for certain why coral colonies on Oahu were more severely impacted than on Maui, but White suggested that at Kaneohe Bay, at least, coral colonies are closer to the surface, which exposes them to more sunlight and warmer temperatures. But Olowalu has similarly shallow reefs but did not experience the same severe bleaching.

White hypothesizes that the difference could have something to do with wave activity – there is a lot of wave activity around Olowalu that keeps the water turbid to block some of the sunshine and that brings cooler water from greater depths. Parts of Kaneohe Bay don’t experience much water circulation, which could have caused increased water temperatures and greater bleaching, White said.

Bleaching is a stress response to unusually warm water. It causes coral to lose algae and color from their tissue, making them appear snow white and more vulnerable to disease. A bleached coral reef can take anywhere from a couple weeks to decades to recover.

“Our reefs are really important, it’s vital that we start to work proactively to remove land-based stresses so corals can be as healthy as they possibly can be to deal with these larger stressors (of) warming ocean temperatures and ocean acidification,” White said.

White and other reef advocates called on the public to reduce its carbon footprint and land-based pollution to boost the odds of coral colonies surviving a bleaching event, which is only expected to become more frequent as global warming takes hold.

“Coral are able to recover quickly (if) the reef (is) already healthy and resilient, whereas a place like Oahu or Maui or Kauai, where you already have these stressors caused by us (people), a big bleaching event . . . can be really devastating for the reef,” said Wes Crile, Hawaii field manager for the Coral Reef Alliance, a San Francisco-based nonprofit group.

The state Department of Land and Natural Resources predicts temperatures will become dangerously warm for coral again this year. Bleaching is forecast to occur during the hottest months from July to October. The National Oceanic and Atmospheric Administration estimates that by 2050, severe bleaching events will occur on a regular basis annually.

“Some of these things we can’t do too much about, climate change is happening,” Crile said. “But we can control those sources of land-based pollution from getting into the environment.”

During the 2014 bleaching season, the state received more than 100 reports and 60 photographs of bleached coral on Oahu, Maui and Kauai. To report bleaching, visit www.eorhawaii.org or send photos to rrcpcoordinator@gmail.com.

* Eileen Chao can be reached at echao@mauinews.com.