Researchers observe coral reef damage and invasive alga in Papahānaumokuākea Marine National Monument

August 2019

NOAA and partner scientists recently completed a 22-day expedition aboard the NOAA Ship Rainier in Papahānaumokuākea Marine National Monument in the Northwestern Hawaiian Islands. They were surveying and monitoring coral reefs and associated reef fish communities, and searching for new species and habitat types on deep coral reefs. Researchers documented several new species of algae as well as an entirely new form of coral reef habitat. However, while conducting research dives, they observed reef destruction from Hurricane Walaka at French Frigate Shoals, and an invasive alga overgrowing native corals and other algae at Pearl and Hermes Atoll. Monument staff are now working to understand and respond to these impacts to the monument.

fish swimming around healthy coral
Fish school at Rapture Reef in French Frigate Shoals in Papahānaumokuākea Marine National Monument prior to Hurricane Walaka. Click the image to download a high-resolution version. Photo: Greg McFall/NOAA

French Frigate Shoals

French Frigate Shoals is home to abundant wildlife and one of the most significant coral reef systems in the monument. In October 2018, Hurricane Walaka passed through the Northwestern Hawaiian Islands at French Frigate Shoals as a Category 3 hurricane. The hurricane caused major damage to the islands of this atoll.

coral rubble on the seafloor
Divers observed devastating damage to coral reef sites at Rapture Reef in French Frigate Shoals. Click the image to download a high-resolution version. Photo: NOAA
Before and after satellite images show the extent of the impacts upon East Island from Hurricane Walaka in French Frigate Shoals. The first photo was taken in May 2018 and the second photo is from October 2018. Imagery courtesy of DigitalGlobe

During this research expedition, the first to assess coral reef impacts since Hurricane Walaka last year, divers observed devastating damage to coral reef sites at French Frigate Shoals. Before and after photos show rubble not recognizable as the former coral reef. Highly diverse fish communities associated with the reef are also gone.

While monument managers were aware of the terrestrial impacts from the storm, this expedition provides a more holistic understanding of the effects of the storm on the entire ecosystem, especially the devastation caused to several of the once healthy and abundant reefs in the atoll. It is unknown whether the corals will grow back, or if they do, how long it will take.

Pearl and Hermes Atoll

Pearl and Hermes Atoll is a true atoll: its expanse lies primarily underwater. It has several islets, seven of which are above sea level. While total land area is only 80 acres, the reef area is huge, over 194,000 acres (equivalent to a quarter of the area of Rhode Island.) The atoll is ever-changing, with islets emerging and subsiding. Hawaiian monk seals and sea turtles breed and feed at Pearl and Hermes, and it is a mating area for spinner dolphins.

Researchers observed large, thick mats of the invasive red alga on the west and northwest sides of Pearl and Hermes Atoll. Many of these mats were as large as multiple football fields. Underneath the mats were bare, dead native corals. In many areas where mats were not found, small, well-hidden growths of the alga were noted hidden among native alga and in cracks or crevices.

diver documenting algae that covers coral
University of Hawai‘i scientific diver Heather Spalding documents a mat of invasive algae at Pearl and Hermes Atoll. The alga has smothered all native algae and corals. Click the image to download a high-resolution version. Photo: Taylor Williams/NOAA

Planned activities at other locations in the monument were curtailed so that all research efforts could be focused on understanding and characterizing the effects of this invasive species on Pearl and Hermes Atoll. Researchers conducted bleach disinfection experiments aboard the research ship so that boats and dive gear could be cleaned with bleach concentrations that would be certain to kill any attached fragments of the alga.

The alga has not been identified, but it is almost certainly not a native species. Specimens were preserved for DNA analyses.

New discoveries from deep coral reefs

In spite of impacts to shallow reefs from hurricane Walaka and invasive algal outbreaks at Pearl and Hermes Atoll, the deep coral reefs of Papahānaumokuākea appear to be very healthy. They continue to support large numbers of apex predators such as sharks and giant trevally (Caranx ignobilis, or ulua in Hawaiian). These deep coral reefs are very poorly explored, and continue to yield discoveries that are completely new to science.

On this expedition, numerous new species of algae were discovered by divers using rebreather technology. Several of the algae are so distinct from any known forms that they likely represent not only new species, but entirely new genera of organisms. An entirely new form of coral reef habitat was discovered, namely lush beds of the alga Sargassum, at a depth of 300 feet off Pearl and Hermes Atoll. Typically found in tidepools and in very shallow reef habitats in Hawai‘i, this is the first time Sargassum has been found as a habitat-forming species at such great depths.

diver collecting algae specimens
NOAA research diver Dr. Randy Kosaki collects specimens at 310 feet at Lisianski Island. Click the image to download a high-resolution version. Photo: Rich Pyle/NOAA

Management response

For loss of reef at French Frigate Shoals, the Monument Management Board is working to understand the impacts of the hurricane and establish a baseline of those impacts to better assess management and response options. Hurricane damage from Walaka was very likely limited to French Frigate Shoals, since the storm did not approach any other reefs with the same intensity.

The alga found at Pearl and Hermes Atoll will be analyzed, including a detailed DNA analysis. Monument managers will determine next steps while observations and data collections are analyzed. Effective biosecurity protocols have been in place for all operations in the monument since 2010. One of the management measures being considered is to enhance some of these protocols to minimize the spread of this species to other areas of the monument.

a scientist looking at algae in a lab
University of Hawai‘i marine biology intern Taylor Williams examines a mat of invasive algae aboard the NOAA Ship Rainier at Pearl and Hermes Atoll. Click the image to download a high-resolution version. Photo: Heather Spalding/NOAA

The Monument Management Board is establishing a technical working group made up of expert researchers and biologists from each of the monument management agencies. Together this group will assess data gaps, additional research that is needed, and opportunities for response to each of the current management concerns identified on this research cruise.

These observations illustrate the need to continue monitoring, such as reef and research expeditions, by NOAA and partner scientists who work in the monument and have historical knowledge of observation and documentation.

Papahānaumokuākea Marine National Monument is cooperatively managed to ensure ecological integrity and achieve strong, long-term protection and perpetuation of Northwestern Hawaiian Island ecosystems, Native Hawaiian culture, and heritage resources for current and future generations. Four co-trustees – the Department of Commerce/NOAA, Department of the Interior/USFWS, State of Hawai‘i, and the Office of Hawaiian Affairs – protect this special place. Papahānaumokuākea Marine National Monument was inscribed as the first mixed (natural and cultural) UNESCO World Heritage Site in the United States in July 2010.

Sarah Marquis is the West Coast/Pacific Islands media coordinator for NOAA’s Office of National Marine Sanctuaries.