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Hawaii braces for potential mass-coral bleaching event

A Hawaiian coral which sustained bleaching in 2015. Image by Greg Asner.

  • Current sea surface temperatures are warmer than normal for this time of year and have exceeded the temperatures preceding the catastrophic 2015 bleaching event.
  • Bleached coral is not dead, but because the vast majority of the energy for the coral is coming from the algae’s activities, the vacated coral is severely weakened.
  • People can act to alleviate coral stress by not touching, standing or anchoring on the reef; keeping chemicals such as sunscreens with oxybenzone or octinoxate out of the water; and suspending fishing for herbivorous fish.
  • Visitors to Hawaiian reefs are being urged to participate in the real time monitoring of the reefs’ health using the newly launched Hawaiicoral.org website

Coral reefs in Hawaiian waters are facing the prospect of another major bleaching event as ocean temperatures rise above the norm. “We found the first signs of [coral] bleaching off the coast of Maui [Hawaii] about 10 days ago,” says Greg Asner, Director of Arizona State University’s Center for Global Discovery and Conservation Science. “And more signs are popping up everywhere. Usually when this happens, certain species are like the canaries in the coal mine, and we now have those species showing severe stress first.”

For Asner and his partners, this situation is eerily similar to the massive coral bleaching event which devastated Hawaii’s coral reefs in 2015. At that time, an unprecedented ocean heatwave hit the reefs, causing roughly 60% of the islands surrounding corals to bleach and 30% to eventually die.

“We can’t be certain at this moment that we will have a huge bleaching event,” says Asner, “but all signs are bad and that we are headed that way.”

Currently, sea surface temperatures are warmer than normal for this time of year and have exceeded the temperatures preceding the catastrophic 2015 bleaching event. According to National Oceanic and Atmospheric Administration (NOAA) scientist Jamison Gove, “Ocean temperatures are extremely warm right now across Hawaii. They’re about 3°F (1.66°C) warmer than what we typically experience in mid-August.”

A map of ocean temperature anomaly in the Eastern Pacific region on August 21st 2019. A temperature anomaly is the difference from an average temperature for that period. Image courtesy of the National Oceanic and Atmospheric Administration (NOAA).

Most reef building corals rely on algae, tiny dinoflagellate organisms in the genus Symbiodinium, to survive. In this arrangement, the coral provides a structure for the algae to inhabit and the algae work for room and board, turning sunlight into energy to keep the coral nourished.

Stress, such as high temperatures or ocean acidification, can cause these tiny symbiotic partners to die. No longer useful, the dinoflagellates are expelled by the coral. This mass exodus of colorful symbionts from their white coral chambers causes the bleaching effect.

Bleached coral is not dead, but because the vast majority of the energy for the coral is coming from the algae’s activities, the vacated coral is severely weakened. And unless conditions stabilize quickly, coral in this state does not fare well over time. If temperatures stay high and/or the reefs are disturbed, the coral will die.

A Hawaiian coral which sustained bleaching in 2015. Image by Greg Asner.

However, scientists are aware of a range of things people can do to relieve secondary stress on corals once they go into a stressful hot water phase or heatwave.

Hawaii’s Division of Aquatic Resources (DAR), which has 100% jurisdiction over the island’s reefs, issued a press release today urging citizens to follow a code of conduct, informed by the most recent science, to protect the reefs from further damage.

In the press release, DAR Administrator Brian Neilson explained, “We know this bleaching event is coming and it’s probably going to be worse than the ones we experienced four and five years ago. West Hawai‘i experienced a 50% mortality rate and Maui experience 20-30% mortality rates on fixed monitoring sites we operate. We’re asking for everyone’s help in trying to be proactive and to minimize any additional stress we put on our corals.”

People can act to alleviate coral stress by not touching, standing or anchoring on the reef; keeping chemicals such as sunscreens with oxybenzone or octinoxate out of the water; and suspending fishing for herbivorous fish.

Though likely to be the most unpopular recommendation, laying off of fishing is critical. Herbivorous fish graze on algae and prevent it from overgrowing the bleached coral. Keeping the coral clean while waiting for temperatures to drop allows the coral a chance to attract new dinoflagellate partners, restoring life and color to the reef.

“Carbon dioxide emissions from human activities have committed coral reefs to periodic ocean heat waves,” says Asner. “While we need to work hard and fast to reduce our emissions, we can’t wait around in hopes that reefs will somehow survive. During each ocean heatwave that comes, our science clearly tells us that reducing secondary stress from physical and chemical reef damage, as well as from fishing, will reduce coral death and give reefs a far better chance of survival.”

Citizens and visitors to the Hawaiian reefs are also urged to participate in the real time monitoring of the reefs’ health using the newly launched Hawaiicoral.org website, which combines information from people and satellites to work towards better outcomes for corals. The site brings together two levels of coral monitoring: field-based (citizen) monitoring and satellite-based high-tech monitoring. The map looks simple, but it is quietly populated by some very high-tech engineering.

Map showing the status of Hawaii Coral Bleaching Tracker on August 23rd, 2019.

People can enter points where they’ve witnessed bleaching on the map, and the site also contains a map that come from more than 100 satellites tasked on a multi-time-per-day basis over all Hawaiian Islands. These satellites are from the company Planet, and are part of the Allen Coral Atlas project co-lead by Asner.

“We have turned all possible Planet satellite eyes on this bleaching event in hopes of monitoring the geographic pattern of change over the next 2+ months,” says Asner. “By doing so, we will be able to direct DAR and Hawaii’s communities as to where people can best reduce reef stress as outlined in the press release.”

The reason for this “eye in the sky” and “feet on the ground” (or fins in the water) approach is both technical and sociological. On the technical side, there is a need to calibrate the sensitivity of the satellite data to the field data to improve the accuracy and efficiency of the reef satellite monitoring program.

On the social side, says Asner, “this is a big socioecological experiment to engage people and to see if they will get off the reef. I’m crossing my fingers that the heat wave passes swiftly and that we can get through it and minimize the damage. The truth is we will never know what good we have done compared to what might have been without our help, but we are doing all we can.”

Banner image caption: A Hawaiian coral which sustained bleaching in 2015. Image by Greg Asner.

Editor’s note: Dr. Greg Asner has is a contributing author for Mongabay’s Reefscape series.