Invasive Species

As our world becomes more connected, species can travel across ocean basins in new ways—carried on ships, in ballast water, or on floating debris. When plants or animals arrive in places where they do not naturally occur, they can upset the ecological and cultural balance of Papahānaumokuākea.

Many non-native species do not become established, but others can become a nuisance. They can spread quickly without natural predators, outcompete native species, and even become invasive. Invasive species can disrupt fragile ecosystems and harm the cultural resources and living connections that make this place significant.

Rapidly Spreading Mat-Forming Alga, Chondria tumulosa

One example of a rapidly spreading invasive-like species in the sanctuary is the fast-growing species of red alga, Chondria tumulosa. Chondria can form large mats covering hundreds of square meters up to 9 inches thick—smothering native corals and virtually all other organisms under the mats. The alga is being referred to as a nuisance species for its mat-forming growth and its ability to easily detach and spread to new locations, much like a tumbleweed. Although C. tumulosa displays invasive characteristics, the origin of this alga is still a mystery, and very little is known about its ecology, physiology, or potential biological drivers. Once known only from Papahānaumokuākea, a similar alga was also detected and visually confirmed in the Marshall Islands in 2024, however the genetics of the Marshall Islands specimen are slightly different from that of the C. tumulosa at Papahānaumokuākea.

Timeline of Observations

Chondria tumulosa has invasive characteristics and is overgrowing some of the most pristine coral reefs in Hawai‘i. Below is a timeline of what researchers have observed about the spread of this mat-forming alga to date:

• 2016

  Chondria tumulosa is first observed in Papahānaumokuākea covering extensive areas of the forereef and northern backreef. The first voucher specimens of the species were collected.

• 2019

  The alga is seen growing over large swaths of reef habitat at Manawai (Pearl & Hermes Atoll).

• 2020

  C. tumulosa was identified as a species new to science and named by University of Hawai’i and Papahānaumokuākea researchers in a taxonomic publication.

• 2021

  C. tumulosa discovered at Kuaihelani (Midway Atoll) by Papahānaumokuākea Marine Debris Project partners.

• 2022

  C. tumulosa was found at about 30% of the roughly 150 sites surveyed around Kuaihelani (Midway Atoll), but was in high-abundance mat formation at only eight of the sites.

• 2023

  A study of the spatial and temporal expansion of C. tumulosa revealed through historical high-resolution satellite imagery that these accumulations first became visible in 2015.

• 2023

  C. tumulosa discovered at Hōlanikū (Kure Atoll). This invasive-like species was now confirmed to occur at the three northern-most atolls of the sanctuary—documented growing as thick mats at Manawai (Pearl & Hermes Atoll), and deemed present in lower abundances at Hōlanikū and Kuaihelani (Kure Atoll and Midway Atoll).

• 2024

  University of Hawai’i researchers reported environmental DNA detection and visual confirmation of C. tumulosa at Majuro Atoll, Republic of the Marshall Islands; however the place of origin remains unknown, and genetics reveal slight differences between Marshall Islands specimens and Papahānaumokuākea specimens.

• 2025

  C. tumulosa was not seen during visual surveys at Hōlanikū (Kure Atoll), researchers are awaiting eDNA data to determine if the species was still cryptically present at the atoll.

Building Our Knowledge of Nuisance Species

Since the discovery of large mats of C. tumulosa smothering Papahānaumokuākea reefs at Manawai (Pearl & Hermes Atoll), researchers have been investigating the distribution, population structure, physiology, reproductive ability, chemical composition, and overall ecology of this nuisance alga as well as studying the oceanographic conditions (e.g., temperature, nutrients, light levels) that might be fueling its rapid growth.

Researchers have quantified some of the impacts C. tumulosa can have on the reef ecosystem. This includes an ability to block almost all sunlight from reaching through its thick mats and changing the reef fish and invertebrate communities associated with affected reefs. Despite the presence of large schools of herbivorous reef fishes in Papahānaumokuākea, researchers have not found evidence that fish or other organisms are consuming C. tumulosa as part of their diet.

In 2024, NOAA Office of National Marine Sanctuaries staff organized and hosted a C. tumulosa-focused conference in Honolulu to bring together researchers and managers conducting similar work globally. The 76 participants (representing 17 different organizations) learned the most up-to-date information on how C. tumulosa grows, reproduces, and impacts the reef habitat under it as well as the reef fish around it. Experts in the field discussed methods to avoid transporting the algae to new places, challenges linked to managing marine debris, and cutting-edge early detection methods. In addition to 20 different talks, the conference included hands-on training with previously frozen C. tumulosa—the first time many managers and scientists had seen C. tumulosa in person. The day ended with a discussion panel geared to link science objectives with manager needs, and to identify key knowledge gaps for advancing aquatic invasive species management strategies.

Preventing the Spread of Chondria tumulosa

Researchers and managers are concerned about the potential spread of Chondria tumulosa to other locations. Spread is possible due to the alga's ability to form dense mats, fragment easily, reproduce both sexually and asexually, and attach to marine debris such as abandoned fishing nets.

To minimize the risk of transporting non-indigenous species into Papahānaumokuākea National Marine Sanctuary, staff conduct pre-access vessel inspections for permitted ships traveling into the sanctuary.

To help prevent spread, co-trustee agencies developed best management practices for any vessels approved to travel near Manawai (Pearl & Hermes Atoll) or other atolls where this nuisance algae is known to occur.

Best practices to prevent the spread of C. tumulosa include:

• Soaking all dive gear used in the water of atolls where this nuisance algae occurs in a bleach solution
• Cleaning the vessel thoroughly with a bleach solution
• Examining gear and vessels for fragments of the alga prior to departure from those atolls

Exploring Other Options for Controlling the Spread of Chondria tumulosa

Scientists continue to conduct experiments to determine the most effective (and safe) ways to kill fragments of C. tumulosa. Researchers have explored a variety of chemical treatments, sun and shade exposures, freezing treatments, and heat treatments. The results of these studies will improve best management practices to prevent accidental spread via fragments that could be stowed away on boats, tucked into the crevices of dive gear, or entangled in marine debris.

Documentation Methods

To document spatial extent and detect spread, NOAA Office of National Marine Sanctuaries researchers and their partners use tools such as in-water surveys and video and satellite imagery. Through aerial imagery, a 20-foot-wide swath of unattached tumbleweed-like algae was detected within the lagoon of Manawai (Pearl & Hermes Atoll). Footage showed that the accumulation of tumbleweeds extended for several miles, earning it the nickname “Chondria Highway”. In-water surveys confirmed that these tumbleweeds are live and viable, and thus may represent a mode of dispersal for this alga.

Early detection and rapid response are critical to preventing the spread of nuisance species. Working with partners at the University of Hawaiʻi, a method was developed using water samples as a source of environmental DNA to quickly test for the presence of C. tumulosa on reefs. This provides a valuable management tool for detection of C. tumulosa when it is still at low levels and may go unnoticed by scuba divers.

Monitoring the Spread of Invasive Acanthophora spicifera

Acanthophora spicifera, a fast-growing red alga common across the Main Hawaiian Islands, was recently documented in Papahānaumokuākea for the first time during 2022 surveys at Kuaihelani (Midway Atoll). This species can spread quickly by fragmentation and attaching to a wide range of hard surfaces, including vessel hulls and hard substrates surrounding coral, which makes early detection especially important in remote ecosystems like the Northwestern Hawaiian Islands. Although it was only found in a few shallow sites and native algae were still present, its ability to form dense patches means continued monitoring is needed to understand its spread and potential ecological impacts. Sanctuary scientists and partners are improving field surveys and genetic identification tools to detect new introductions early and better protect Papahānaumokuākea's fragile reef communities.

The original discovery location is offshore of an area commonly referred to as “Rusty Bucket” because of the amount of steel remnants that reside there after allegedly being deposited off the end of the old runway upon the end of military activities. It is unknown if there is a correlation between algal presence and available iron in these nearshore waters.