Maro Reef is one of the largest and least familiar coral reef systems in the Hawaiian archipelago, sprawling across a vast area of shallow ocean northwest of the main Hawaiian Islands yet barely breaking the surface of the water. Situated roughly halfway along the Northwestern Hawaiian Island chain between Laysan Island and Gardner Pinnacles, it is part of Papahānaumokuākea Marine National Monument and ranks among the more ecologically significant yet understudied reef platforms in the Pacific. Its near-total submersion, extreme remoteness, and sheer scale make it a place that most people, including many marine scientists, have never visited or even seen clearly in photographs.
A Reef You Could Sail Over Without Noticing
Maro Reef sits at roughly 25°25′N, 170°35′W, about 1,300 kilometers northwest of Honolulu. What makes it unusual among Hawaiian reefs is its structure: it is an open atoll, sometimes called a table reef, meaning it is a broad, shallow platform of coral that never fully enclosed a central lagoon the way classic ring-shaped atolls do. The reef crest barely reaches the ocean surface in most places, and there is essentially no dry land. At high tide, virtually the entire formation is submerged. Waves occasionally break over the shallowest sections, but a passing ship in poor visibility could cross directly over it without ever seeing land.
Despite its low profile, the reef platform itself is enormous. Estimates of its area vary depending on how the boundaries are drawn, but Maro Reef covers well over a thousand square kilometers of shallow reef habitat, making it one of the largest reef structures in the entire Hawaiian chain. For comparison, that is larger than many well-known atoll systems in the central Pacific. The reef’s breadth supports an impressive diversity of habitats: forereef slopes dropping into deeper water, shallow reef flats with heavy wave energy, and scattered coral knolls and patch reefs spread across the platform.
What Lives on the Reef
Because Maro Reef has no permanent human population and sits inside one of the most strictly protected marine reserves on the planet, its ecosystems provide a useful reference point for what a relatively undisturbed subtropical reef looks like. Survey work on the benthic communities, the organisms covering the reef surface, has shown that the forereef environments at Maro and nearby non-atoll systems differ from the atolls farther along the chain. Crustose coralline red algae, the hard pink-and-purple algae that cement reef structures together, made up roughly 7 to 12 percent of cover on non-atoll forereefs including Maro, a higher proportion than on the atoll forereefs elsewhere in the chain. Meanwhile, green and brown fleshy macroalgae that were common at some atoll sites were not major components of the Maro forereef landscape.1PLoS ONE. Benthic Composition of a Healthy Subtropical Reef: Baseline Species-Level Cover, with an Emphasis on Algae, in the Northwestern Hawaiian Islands
That distinction matters because coralline algae are reef builders. They deposit calcium carbonate and literally glue the reef framework together, making it more resistant to wave damage and erosion. A reef with a healthy share of coralline algae is generally in decent structural shape. The relative scarcity of fleshy macroalgae on Maro’s forereef is also a positive sign, since unchecked fleshy algae growth on a reef often signals nutrient pollution or the loss of herbivorous fish that would normally keep algae in check. Maro, being far from agricultural runoff and largely free of fishing pressure, avoids both of those triggers.
The fish assemblages around Maro Reef are notable for being what ecologists call “top-heavy,” meaning that apex predators such as sharks and large jacks make up an unusually large fraction of the total fish biomass. In most reef systems near human populations, predators have been heavily fished and the food web is dominated by smaller herbivores and planktivores. In the Northwestern Hawaiian Islands, the predator-heavy structure is considered closer to a natural baseline. Maro’s remoteness and protected status help preserve that inverted biomass pyramid.
The Derelict Net Problem
Remoteness does not mean immunity from human impact. One of the most damaging and persistent threats to Maro Reef is marine debris, particularly derelict fishing nets. The North Pacific subtropical gyre, sometimes loosely called the Great Pacific Garbage Patch, funnels floating debris toward the Northwestern Hawaiian Islands. Discarded or lost commercial fishing nets drift with ocean currents and snag on the shallow reef structure at Maro and neighboring sites. Research estimates that roughly 52 metric tons of derelict nets accumulate in this region every year.2PubMed. Coral cover remains suppressed three years after derelict net removal in a remote shallow water coral reef ecosystem
Once tangled in the reef, these nets cause direct physical damage. They abrade and smother coral colonies, break branching corals, and shade out light. They also entangle marine animals, including endangered Hawaiian monk seals and green sea turtles that forage in the area. NOAA-led cleanup expeditions have removed thousands of tons of derelict nets from the Northwestern Hawaiian Islands over the past two decades, but the nets keep arriving.
Even after removal, recovery is slow. Studies tracking reef sites where nets were cleared found that coral cover remained significantly lower at impact sites compared to nearby controls, even three years after the debris was taken away. The bare substrate exposed when nets were removed tended to be colonized by algae rather than by the reef-building organisms, such as corals and coralline algae, that the reef needs to rebuild.3PubMed. Coral cover remains suppressed three years after derelict net removal in a remote shallow water coral reef ecosystem In other words, removing the nets is necessary but not sufficient. The reef does not simply bounce back once the debris is gone. Algae can establish themselves on the newly exposed rock before coral larvae get a chance to settle, creating a feedback loop that slows or prevents coral recovery.
Coral Disease at Maro
A separate and somewhat surprising threat is coral disease. The assumption that remote, unpolluted reefs should be free of disease does not hold up. Surveys across the entire Hawaiian archipelago have documented distinct disease signatures at different islands, and Maro Reef stands out for two conditions in particular.
One is Porites trematodiasis, a parasitic disease caused by a digenetic trematode that infects Porites corals, producing characteristic pink swellings on the colony surface. This disease was found to be particularly characteristic of both Laysan and Maro in archipelago-wide analyses. The other is Montipora white syndrome, a tissue-loss disease affecting Montipora corals, which was flagged as an important indicator disease that statistically distinguished Maro from other Northwestern Hawaiian Island sites.4PLoS ONE. Patterns of Coral Disease across the Hawaiian Archipelago: Relating Disease to Environment
Why Maro? The drivers are not fully understood. Coral disease in general is linked to a mix of factors including water temperature, microbial communities, host coral density, and stress from other sources. One possibility is that the dense coral assemblages on Maro’s shallow platform, combined with warm-water events that are becoming more frequent, create conditions favorable for pathogen transmission. The trematode responsible for Porites trematodiasis has a complex life cycle involving fish and snail hosts, and the abundant reef-fish populations at Maro could help sustain that cycle. Whatever the cause, the finding underscores that even well-protected reefs are not insulated from disease, and that monitoring disease at remote sites matters for understanding how reef health is shifting across the Pacific.
Why Maro Is Hard to Study
Maro Reef presents logistical challenges that most other research sites do not. There is no land to camp on, no harbor, and no shelter from open-ocean swells. Research vessels anchoring near the reef must contend with currents, weather windows, and the simple problem of being more than a day’s sail from the nearest port. Dive surveys are constrained by wave conditions, which over a shallow reef with no lagoon protection can deteriorate rapidly.
The reef’s sheer size adds another layer of difficulty. Surveying representative transects across a platform this large requires either many dive hours or the use of towed-camera systems and satellite imagery, each of which has its own limitations. Satellite-derived benthic maps work well for distinguishing sand from reef at broad scales but struggle to resolve the fine-scale composition, such as distinguishing live coral from coralline algae from turf algae, that matters most for tracking reef health. The result is that scientific coverage of Maro remains patchy. Certain areas of the forereef and shallow platform have been surveyed during multi-year NOAA expeditions, but large swaths of the reef have been visited only rarely or not at all.
This data gap creates a real problem for conservation managers. If baseline coral cover is not well documented across the whole reef, detecting declines caused by bleaching, disease, or debris becomes much harder. A monitoring program that revisits only a handful of fixed sites may miss damage unfolding in the unvisited majority of the platform. Scientists working in the area have called for expanded survey effort, but funding and ship time for expeditions to the Northwestern Hawaiian Islands are limited and compete with needs at dozens of other sites in the monument.
Papahānaumokuākea and What Protection Means in Practice
Maro Reef lies within Papahānaumokuākea Marine National Monument, which was established in 2006 and expanded in 2016 to cover roughly 1.5 million square kilometers of ocean. The monument is one of the largest marine protected areas on Earth and is also a UNESCO World Heritage Site. Within its boundaries, commercial fishing and resource extraction are prohibited, and access requires a permit. That regulatory framework gives Maro and its neighbors a level of legal protection that most reef systems worldwide lack.
Management of the monument is shared among four co-trustees: NOAA, the U.S. Fish and Wildlife Service, the State of Hawai’i, and the Office of Hawaiian Affairs. In recent years, there has been a significant push to incorporate Native Hawaiian knowledge and governance principles into how the monument is managed. The Northwestern Hawaiian Islands hold deep cultural significance for Native Hawaiians. In Hawaiian cosmology, the islands and reefs of the northwest chain are ancestral places connected to the origins of life and the journey of souls after death. Efforts led by Native Hawaiian practitioners and scholars resulted in the Mai Ka Pō Mai guidance document, which provides a framework rooted in Indigenous worldviews for managing the monument’s resources.5Ecology and Society. Mai Ka Pō Mai: applying Indigenous cosmology and worldview to empower and transform a management plan for Papahānaumokuākea Marine National Monument
This approach treats the reef systems not merely as ecological assets to be preserved but as living entities with cultural relationships to the Hawaiian people. It has practical implications for how management decisions are made, who participates in those decisions, and what kinds of activities are considered appropriate in the monument. For Maro Reef specifically, it means that conversations about debris cleanup, research access, and climate adaptation are increasingly informed by cultural perspectives alongside scientific ones.
Climate Change and the Future of a Submerged Reef
Maro Reef’s near-surface position makes it acutely vulnerable to ocean warming. Mass coral bleaching events in the Northwestern Hawaiian Islands have intensified since the early 2000s, driven by marine heat waves that push water temperatures above the threshold corals can tolerate. When bleaching occurs, corals expel the symbiotic algae living in their tissues, turning white. If the heat stress persists for weeks, the corals starve and die. Reefs at the scale of Maro, spread across a wide area at uniformly shallow depths, have little ability to escape warm surface waters. Deeper reefs elsewhere can act as refugia, but much of Maro’s platform sits in the warmest layer of the water column.
Sea-level rise introduces a different kind of uncertainty. For most coastal communities, rising seas are an unambiguous threat. For a submerged reef, the picture is more complicated. In theory, moderate sea-level rise could give corals at Maro more vertical room to grow, since many colonies near the surface are already bumping up against the low-tide line and cannot grow higher. But that potential benefit only materializes if corals are healthy enough to grow upward, and if the rate of sea-level rise does not outpace the rate at which the reef can accrete new carbonate. A reef weakened by bleaching, disease, and debris damage may not be able to keep up.
Ocean acidification adds yet another complication. As the ocean absorbs more atmospheric carbon dioxide, its pH drops and the concentration of carbonate ions, the building blocks corals and coralline algae use to construct their skeletons, decreases. Subtropical reefs in the central Pacific are projected to experience significant declines in carbonate saturation over the coming decades. For a reef like Maro that depends on coralline algae to cement its structure together, reduced calcification rates could undermine the reef’s physical integrity even if the organisms themselves survive.
How Maro Compares to Other Northwestern Hawaiian Island Reefs
The Northwestern Hawaiian Island chain includes a range of reef types: classic atolls like Midway and Kure, volcanic remnants with fringing reefs like Nihoa and Necker, and hybrid formations that fall somewhere in between. Maro Reef is unusual in being neither a classic atoll with a well-defined rim and lagoon nor a simple fringing reef around a volcanic island. Its open, table-like structure sets it apart morphologically, and its ecological character follows suit.
Compared to the atolls, Maro’s forereef environments tend to have more coralline algae and less fleshy macroalgae, as the benthic survey data indicate.6PLoS ONE. Benthic Composition of a Healthy Subtropical Reef: Baseline Species-Level Cover, with an Emphasis on Algae, in the Northwestern Hawaiian Islands Compared to the volcanic high-island reefs at the southeastern end of the chain, Maro lacks the terrestrial runoff and sediment input that can smother corals and fuel algal blooms. It is, in many respects, a best-case scenario for what a Hawaiian reef can look like when land-based stressors are absent. That baseline quality is precisely what makes it scientifically valuable and worth protecting, even as the threats it faces shift from local pollution, which it largely avoids, to global-scale warming and ocean chemistry changes, which it cannot escape.
Wildlife Beyond Coral
While the coral communities get most of the scientific attention, Maro Reef supports a broader web of life. Hawaiian monk seals, one of the most endangered marine mammals on Earth with a population numbering only in the low hundreds, use the Northwestern Hawaiian Islands as critical habitat. They forage on reef flats and slopes, feeding on fish, octopus, and crustaceans. Maro’s extensive shallow platform provides a large foraging ground, though monk seal research in the area has focused more heavily on nearby Laysan Island, where seals also haul out on land to rest and pup.
Green sea turtles are another prominent resident. The Northwestern Hawaiian Islands serve as the primary nesting ground for the Hawaiian green turtle population, and turtles forage across the reef systems in the chain. At Maro, the shallow reef flats with their algal turf and macroalgae offer grazing habitat. Both monk seals and green turtles are protected under the Endangered Species Act, and the monument’s no-take regulations help shield them from the direct threats of fishing bycatch and harvest that they face in the main Hawaiian Islands.
Seabirds also interact with Maro’s ecosystem, though indirectly. Laysan albatrosses, black-footed albatrosses, and various species of shearwaters and petrels nest on the islands and atolls nearest to Maro and forage in the surrounding waters. Their guano, deposited on nesting islands, washes into nearshore waters and provides nutrient input that can fertilize marine productivity. At Maro itself, with no nesting habitat above water, the link is more diffuse, but the surrounding ocean is part of the same nutrient cycle that sustains productivity across the region.
Spinner dolphins, reef sharks, and large pelagic fish round out the upper levels of the food web. The absence of fishing pressure at Maro means that populations of ulua (giant trevally), sharks, and other top predators remain at densities rarely seen on reefs closer to human populations. Researchers who have dived Maro consistently describe the experience as stepping back in time to what Hawaiian reefs may have looked like centuries ago, before overfishing reshaped the food web in the main islands. Whether Maro can hold onto that character through the coming decades of warming and acidification is one of the open questions in Pacific reef science.