1st detailed map of Hawaii reefs shows coral decline

The first detailed maps of nearly all the nearshore coral reefs of the main Hawaiian Islands, unveiled in a scientific paper released today, found vast areas of decline and degradation linked to shoreline development, overfishing and increasing water heat waves.

But the study also uncovered a number of spectacular, intact reefs that warrant protecting, as well as impaired reefs that are likely candidates for rehabilitation.

In addition, the study found that the reefs of Oahu scored poorly in comparison to those around most of the other islands.

The paper, published today in Proceedings of the National Academy of Sciences of the United States of America, identified the location of live corals in unprecedented detail up to 50 feet of water depth.

Researchers from Arizona State University took to the air at the beginning of the year to obtain a bird’s eye view of live coral using laser-guided imaging spectroscopy, the same technology the team has been using to map the state’s forests for years.

But instead of soaring over the interior of the islands, the team in its Global Airborne Observatory twin-engine turbo prop devoted about 30 days flying off Hawaii’s coasts taking pictures with the high-tech camera.

The maps were then combined with geospatial information on coastal and marine activities, and computer algorithms were used in an analysis that confirmed that nearshore development, with its runoff and pollution, has a major negative relationship with live corals.

Overfishing from the shoreline and coral bleaching are the other major impacts on the health of the reefs.

The most concerning of the impacts are the bleaching events caused by the warming ocean from climate change. Hawaii has experienced serious bleaching in 2015 and 2019, and the National Oceanic and Atmospheric Administration is predicting heat waves every year or two by 2030.

A copy of the new study and its maps was not available prior to today’s publication. But lead author Greg Asner said in an interview that he was surprised by how deeply degraded the reefs in Hawaii are.

“I had no sense of the vast amount of coral we have already lost and how degraded a lot of areas are,” he said. “It was eyeopening — like getting cold water splashed in your face.”

Asner, a veteran Hawaii ecologist and director of ASU’s Center for Global Discovery and Conservation Science based in Hilo, said that while the maps illustrate the steep decline, they also reveal “incredible hot spots of living coral,” called refugia.

These areas apparently are more resilient and generally located in leeward areas away from significant coastal development.

“These reefs are critical to save and maintain,” he said.

At the same time, the maps reveal a lot of reefs that fall into a “middle ground” category, with enough live coral to make them candidates for rehabilitative conservation.

The study was conducted in coordination with the state Department of Land and Natural Resources, and Brian Neilson, administrator of DLNR’s Division of Aquatic Resources, is listed among the paper’s co-authors.

“Hawaii (and most of the world) has never had coral maps that are this expansive and this detailed,” Neilson said in an email.

The maps are expected to establish better connections between science and management. Previously marine managers relied either on SCUBA diving surveys, which are detailed but limited in scope, or satellite surveys, which are vast but low on detail.

“This is incredible helpful information for the State of Hawaii to manage our coral reefs that are experiencing unprecedented loss related to climate change,” Neilson said.

Going forward managers can hone-in on these areas to determine what type of factors contribute to coral loss and coral resilience, and adjust their management strategies accordingly, he said.

The maps are also expected to play a role in meeting Gov. David Ige’s 30×30 marine initiative, which aims to expand the state’s network of marine managed areas to at least 30% of Hawaii’s nearshore region by 2030.

Currently, only 6% of Hawaii’s nearshore waters receive some level of management.

As for the study, Oahu’s reefs did not fare well compared to most of the rest of the state, Asner said.

Only Kaneohe Bay finished in the state’s top 20 reefs for amount of live coral, while Hanauma Bay, Oahu’s popular tourist snorkeling spot, ranked just outside the top 20.

Asner said Oahu really should have the most reefs of any island in the main archipelago. At 2.3 million years old, the state’s most populous island is just the right age to have lots of hard nearshore bottoms that reefs like to sit on.

“The problem is that it is almost all covered with algae,” he said, a consequence of the effects of onshore development and too much shoreline fishing.

Only 12% of Oahu’s ocean substrate is covered by live coral, the lowest percentage in the state.

Hawaii island has the largest percentage at 20%, a number that is deceptive because much of the hard bottom is still too young to support major reefs.

But the majority of intact reefs are found off the Big Island and other islands east of Oahu, according to the paper.

The Big Island scored seven of the top 10 coral reefs listed in the study (25 acres are larger). West Maui, Lanai and Molokai each had one reef in the top 10.

No. 11 was a reef off the southwest tip of Kahoolawe, whose size just shy of 25 acres kept it from being considered for the top 10. Asner described reef as “epic.”

Asner, who moved to Hawaii in 1987 and worked as a researcher for the Nature Conservancy and Carnegie Institution, invented the airborne mapping technology in Hilo in 2005.

Until recently it was used for mapping the condition of Hawaii’s forests. Among other things, it has helped to detect the presence of rapid ohia death, the fungal disease that has killed or injured a vast number of Hawaii’s most important native forest tree.

Asner said that when severe bleaching struck Hawaii’s reefs in 2015, he vowed to direct the effort toward the ocean.

“It woke me up,” he said.

After tweaking the technology, the laser-guided imaging spectroscopy now has the ability to penetrate sea water up to 70 feet and detect the chemical signature of both live and dead corals.

The first map of Hawaii’s reefs appears to be pretty accurate. The team, Asner said, compared the map to 1,132 existing transects of coral reef data, and it came out 94% accurate.

Now, Asner is now receiving interest in reef mapping from outside Hawaii, including a proposal for a project in the Red Sea.

The Hawaii maps are expected be posted soon at the project’s website, HawaiiCoral.org.

The study was supported by the Lenfest Ocean Program and the Battery Foundation.