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Maro Reef (25° 22' N - 170° 35' W)

Geography and habitats

Maro Reef is the largest coral reef in the Northwestern Hawaiian Islands (NWHI), with approximately 746 square miles (478,000 acres or over 1,934 km2) of reef area. Maro Reef is an open atoll without any emergent land except for about an acre of large coral blocks on the reef crest at low tide. Much of the reef habitat is composed of an intricate network of reef crests and surrounding lagoons. Unlike the familiar circular atoll, Maro Reef has a complex geometry of linear reefs that radiate outward from a series of lagoons, like spokes of a wheel. Channels of deep water with highly irregular bottom topography lie between the shallow reef structures. The lagoon is the second largest in the archipelago, after French Frigate Shoals. Many large gaps in the reef's perimeter expose regions of the lagoon area to wave action and resuspension of fine bottom sediments, which makes the water silty and murky. The habitats of Maro Reef include shallow sandy lagoon bottoms, steep reef slopes, large coral heads, pinnacles, and patch reefs.

map of French Frigate Shoals
Maro Reef is a largely submerged atoll with only about one acre of land that is sometimes submerged. The surrounding reef habitat is about 475,000 acres in size (Illustration: Bishop Museum)
 Maro Reef satellite image
IKONOS satellite imagery of Maro Reef (Photo: NOAA)
Rice Coral
Rice Coral (Montipora Capitata) (Photo: UCSC-LLNL0Hyperspectral Imageing Project)


The eastern side of Maro Reef has little coral cover. Scientists observed that many pinnacles were covered with an algal “fuzz.” Boring bivalve mollusks also covered many of the pinnacles. Few sea urchins were found at Maro. The lack of these grazers may account for the amount of algal “fuzz.” In contrast, the western side of the reef has a high live coral cover. Maro Reef has a greater abundance and diversity of coral than most any other reef system in the NWHI. Rapid environmental assessment teams have recorded 37 species of stony corals, and live coral cover ranging from zero to 95 percent. The northwestern pinnacles and reticulated reefs are characterized by high coral species diversity. The rice coral, Montipora capitata, dominated the reef slopes and bases, and finger coral (Porites compressa), disc coral (Pavona duerdeni), and sheet-like growths of Porites lobata and Montipora species are common near the top of the reef slope. The large table coral, Acropora cytherea, and other smaller table corals are also quite common. A new species of Montipora was also discovered. Non reef-building tube corals were found under overhangs and caves.

One prominent coral reef scientist feels that Maro Reef is "surviving on the edge" because the reefs are narrow and not consolidated, making them vulnerable to storm surges and waves. Others suggest that Maro Reef is a healthy complicated reef system on a large seamount, living in balance with the environmental forces.


The bluefin trevally or omilu
The bluefin trevally or omilu, Caranx melampygus.

Maro Reef has a large amount of crustose coralline algae which lay down a cement that holds the coral together in rough water. The reef supports several species of butterflyfish and large numbers of surgeonfish. Large jacks, such as the giant trevally or ulua (Caranx ignobilis) and the bluefin trevally or omilu (Caranx melampygus) have been seen in the reef's open waters, along with white-tip and gray reef sharks. Large schools of Galapagos sharks are also a common in the shallow waters. Large manta rays and eagle rays have also been observed.

Dive team
On an Ocean futures Society expedition to NWHI, Jean-Michel Cousteau and a dive team member examine the divisity of " macro " life clinging to the underside of the ledge. (Photo:Tom, Ocean Future Society)
Maro Reef
Maro Reef (Photo: Phycology Dept., University of Hawaii at Manoa/L. Preskitt)

Seals, birds, and Marine debris

Monk seals are occasionally seen hunting in the area, but Maro Reef provides very few suitable mating sites and therefore is not considered a breeding area. Derelict fishing gear and other types of marine debris have a major impact on the shallow reefs and fauna. Birds are the only terrestrial animals that inhabit Maro Reef.

Link to metadata and data held by CoRIS

Click on the following URL to locate metadata and data on Maro Reef in the CoRIS holdings. When the query screen comes up, enter "Maro Reef" in the window and click on "Search".

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