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French Frigate Shoals (23° 45' N - 166° 10' W)
French Frigate Shoals is a large 18-mile (29 km) wide, crescent-shaped atoll on a circular platform, approximately 718 miles (1,330 km) northwest of Honolulu and 702 miles (1,300 km) southeast of Kure Atoll. The Shoals’ lagoon contains two exposed volcanic rocks and 12 low, sandy islets. While the land area is only 67 acres (0.2 km2), the total coral reef area of the shoals is over 232,000 acres (938 km2), which makes it the largest atoll in the Northwestern Hawaiian Islands (NWHI).
Map of French Frigate Shoals (NOAA)
Satellite image of French Frigate Shoals (Photo: NOAA)
The reef forms a barrier against winds and currents around the north and east sides of the platform. The south and west sides of the platform are submerged, averaging about 98 feet (30 m) depth. Near the center of the platform is a small basalt pinnacle about 121 feet (37 m) high, La Perouse Pinnacle, which is the last remnant of the volcano still above sea level that formed the original foundation of the atoll. The pinnacle was named after Compte de La Pérouse, who visited the atoll in 1786. La Perouse supports a large number of seabirds. The bird guano provides high nitrogen levels in the surrounding waters, which promote the growth of algae in the pinnacle’s intertidal zone, including species of the green algae, Caulerpa and Cladophoropsis, and the red alga, Asparagopsis.
La Perouse Pinnacle, French Frigate Shoals (Photo: NOAA)
Some of the sandy islets have some sparse growth of grass and other low vegetation. East Island and Whale-Skate are the two largest naturally occurring islands. From 1944 to 1952, East Island was the site of a now-abandoned U.S. Coast Guard station. Tern Island, located 6 miles (11 km) to the northwest of East Island, is a reconstructed island that was enlarged by dredging and landfill. It was formed into a runway to serve as a refueling stop for planes en route to Midway during World War II. The original seawall, runway, and some of the buildings still remain. The U.S. Fish and Wildlife Service maintains a field station there, which is staffed year-round by two permanent employees and a few volunteers.
Birds, seals, and turtles
The islets' highest elevation is about three meters (9.8 ft). Most of the islets are lower. Their population consisted of thousands of nesting sea birds, most of them terns. Six different species of terns have been recorded from Tern Island. The islets attract the largest breeding colony of the endangered monk seal in the NWHI, while also serving as the breeding ground for 90% of threatened green sea turtles in the Hawaiian Islands. Satellite tagging of these turtles indicates that most of them migrate to the Main Hawaiian Islands (MHI) to feed and reach sexual maturity before returning to French Frigate Shoals to breed. Some of these turtles, however, migrate in a northwest direction to feed, while others migrate as far south as Johnston Atoll.
Tern Island, French Frigate Shoals (Photo: George H. Balazc/NOAA)
Sea turtles at French Frigate Shoals (Photo:Tom Ordway/ Ocean Futures Society)
French Frigate Shoals' substrate is primarily reef carbonate, which provides an abundance of diverse habitats, such as deep ocean reef slopes, terraces, spur and groove formations, pinnacles and mounds, caves and overhangs, and shallow perimeter reef flats. The Shoals' semi-enclosed lagoon gives protection from the destructive effects of storms and waves and provides many other reef habitats not found in exposed ocean reef environments. Due to the diversity and quantity of its habitats, coral species diversity and abundance is spectacular.The Shoals support the greatest variety of coral species in the NWHI, with 41 species of stony corals documented. These include table, finger, and lobe corals. Rare table corals of the genus Acropora, which are common throughout reefs of the central and south Pacific, are essentially absent in the Main Hawaiian Islands, but are common at French Frigate Shoals. Observed at almost all survey sites were Acropora cytherea, A. cerealis, A. gemmifera, A. nasuta, A. valida, and A. paniculata.
Giant tabletop coral
The best coral development occurs near the lagoon ends of the reef where exposure to waves and storms is reduced, and where the flow of clean ocean water promotes habitat diversity and good water quality. Poorer reef habitats were concentrated in the shallow eastern lagoon, which is dominated by shallow sediment deposits, strong currents, high turbidity, and poor water quality. However, reticulated reefs with protected habitats are concentrated in this area with finger and lobe corals (Porites) thriving. The western portion of the lagoon is open and live coral cover is higher in the transition zone between the two halves of the lagoon. Live coral cover is also high on patch reefs and on the deep reefs at the northern and southern ends of the lagoon. Table, finger, lobe, rose (Pocillopora), and other corals in the genera Cyphastrea and Leptastrea are abundant.
Many macro and turf algae find suitable habitats within the lagoon. The bottom of lagoon reef slopes provide habitat for large mats of the green alga, Halimeda. More than 150 species of algae are components of the reef community, including red, green and brown algae. Especially rich and diverse algal communities are found immediately adjacent to La Perouse Pinnacle. Pink and purple encrusting coralline algae cover much of the shallow pavement habitats and reef crests.
The outer reef waters support gray reef sharks, butterfly fish, and large schools of jacks and groupers. Endemic masked angelfish (Genicanthus personatus) are occasionally seen here at scuba diving depths. In the MHI, they are rarely seen shallower than 295 feet (90 m). The chevron butterflyfish (Chaetodon trifascialis), which feeds exclusively on polyps of Acropora corals, is also rare or absent in the MHI, but common at French Frigate Shoals. The Shoals’ many and diverse habitats also support more than 600 species of invertebrates, many of which are endemic.
The rare Chevron butterflyfish, Chaetodon fascialis, is almost absent from the Hawaiian Islands, except for French frigate shoals. (Photo: Keoki and Yuko Stender)
A male masked angelfish, Genicanthus personatus. This species is rare except in the NWHI, where it is found at scuba diving depths. In the Main Hawaiian Islands, they are found at 300 feet depths. (Photo: Keoki and Yuko Stender)
Derelict fishing gear and other types of marine debris have a major impact on the reefs and associated fauna of the French Frigate Shoals, especially the monk seals. Over the past several years, efforts have been undertaken to lessen the threat of this problem.
Terrestrial plants of Tern Island
Click here for a description of the terrestrial plants of French Frigate Shoals: S.S. Midway Expedition
Reference: Starr, F. and K. Martz. 1999. S.S. Midway Expedition. Trip report prepared for U. S. Fish and Wildlife Service, Honolulu, Hawai'i.
Link to metadata and data held by CoRIS
Click on the following URL to locate metadata and data on French Frigate Shoals in the CoRIS holdings. When the query screen comes up, enter "French Frigate Shoals" in the window and click on "Search".
Checklist of the near shore fishes of Tern Island, French Frigate Shoals, Northwestern Hawaiian Islands. Ian L. Jones and Daniel Mitchell.
Annotated list of the near shore fishes of Tern Island, French Frigate Shoals, Northwestern Hawaiian Islands. Ian L. Jones and Daniel Mitchell.