Many of Hawaii’s once-thriving coral reefs are now struggling to recover from recent extreme coral bleaching caused by rising water temperatures. These periodic increased temperatures combined with coastal runoff, fishing pressure and other impacts are all suspected of contributing to slow reef recovery.

As a way of understanding which factors had the biggest impacts on Hawaii’s corals, a group of researchers from the collaborative Ocean Tipping Points project, co-led by Larry Crowder, the Edward Ricketts Provostial Professor of Marine Ecology and Conservation at Stanford’s Hopkins Marine Station and senior fellow at the Stanford Woods Institute for the Environment, completed the first-ever comprehensive map of how both humans and natural events influence overall reef health. This new study was published March 1 in PLOS One.

“When we jumped into the water in west Hawaii, over half of the coral reef was dead,” said Lisa Wedding, research associate at Stanford’s Center for Ocean Solutions and a lead author on the paper. “These are some of Hawaii’s most vibrant coral reefs, so we were heartbroken – and determined to better understand how reef ecosystems could be more resilient in the future.”

Big step for Hawaii

Reefs across the Hawaiian Islands have both cultural and economic value. Although people have known that natural and human-caused phenomena affect the health and resilience of coral reef ecosystems, little is known about which factors are more important in each region.

To find out what factors play the largest role in reef resilience, the group synthesized 10 years of datasets from university and government sources examining factors they knew had an impact on coral reefs, such as sedimentation, development and fishing.

This analysis revealed variations in what was inhibiting reef recovery across the islands. On the densely populated island of Oahu, dominant stressors were human activities, such as fishing and loss of natural habitat to coastal development. Sedimentation and nutrient runoff were dominant forces on less populated islands.

“This area of research has been a long-term need for coral reef conservation and management. These findings will allow us to take a big step forward in understanding how corals are impacted by both human activities and by environmental stressors, in a place with incredible value,” said Joey Lecky, co-author on the paper and a geographic information system analyst for NOAA Pacific Islands Fisheries Science Center.

Bigger steps beyond

The research team’s findings highlight the importance of tailoring strategies based on location to effectively address local impacts. This approach, synthesizing data from a large geographic area and over a long period of time to get a big-picture perspective on reef health and regional impacts, provides a foundation for further research and informs policies to protect coral reefs.

Data created by this mapping study are available for free at the Pacific Islands Ocean Observing System, where scientists, managers and members of the public can explore and further analyze what drives variation on coral reefs. Users can download data layers in various formats and explore all layers in an interactive map viewer.

“We live in a changing world, and changing oceans are a big part of that. Studies like this one provide crucial insights into how we can act locally to improve the resilience of reefs to global changes,” said Ocean Tipping Points lead investigator Carrie Kappel of the National Center for Ecological Analysis and Synthesis. “This is an approach that can be replicated for reefs elsewhere.”

Co-authors of the publication include scientists from the University of Hawai‘i at Mānoa; NOAA (National Oceanic and Atmospheric Administration); University of California, Santa Barbara; Bangor University; Stockholm University; National Geographic Society; Conservation International; Arizona State University; Royal Swedish Academy of Sciences; Curtin University; and California Polytechnic State University.

This research was supported by the Gordon and Betty Moore Foundation, NOAA Coral Reef Conservation Program, U.S. Department of Agriculture National Institute of Food and Agriculture, and NOAA Hawaiian Islands Humpback Whale National Marine Sanctuary.