Coral Reef Biodiversity of Midway Atoll

Midway Atoll is home to more than 3 million birds but the marine biodiversity of the surrounding coral reefs is equally amazing.


Beyond the blanket of several million birds atop three small islands lies an abundant and diverse marine world. Midway’s atoll is situated within a large, elliptical barrier reef measuring about 5 miles in diameter. A bright, turquoise-blue lagoon lies within the reef, dotted with numerous patch reefs. Outside of the lagoon, the Midway Atoll National Wildlife Refuge boundaries extend another 12 nautical miles, encompassing 580,740 acres of marine resources.

School of Convict Tang / Manini (Acanthurus triostegus). Credit: Dan Clark/USFWS

Located at the northern extent of one of the highest-latitude coral reef ecosystems in the world, the waters within the Refuge are relatively cold, making the area an important location for research on biological diversity as well as the effects of heat-induced coral bleaching. Midway is one of the few remaining predator-dominated coral reef ecosystems in North America. Midway’s waters brim with life, including a diverse range of algae, corals, worms, snails, and seashells. A total of 29 species of stony or “hard” coral have been recorded throughout the atoll, along with more than 100 species of algae, including a seaweed species new to science, Dudresnaya babbittiana, and over 250 fish species.

Spectacled Parrotfish / Uhu uliuli (Chlorurus perspicillatus). Credit: Angela Hansen

Midway is also home to the endangered Hawaiian Monk Seal / llio holo I ka uaua (Monachus schauinslandi), which now boasts a local population of greater than 60 seals. Hawaiian Monk Seals are among the rarest marine mammals in the world. Part of the “true seal” family (Phocidae), Hawaiian Monk Seals are one of only two remaining monk seal species; the other is the Mediterranean Monk Seal. Hunted to the brink of extinction in the late 19th century, Hawaiian Monk Seals have been declining since modern surveys began. In 1976, the Hawaiian Monk Seal was listed as Endangered under the Endangered Species Act.

Hawaiian Monk Seal / llio holo I ka uaua (Monachus schauinslandi) and a Laysan Albatross on Eastern Island, Midway Atoll National Wildlife Refuge. Credit: Dan Clark/USFWS

In addition to the Monk Seals, Hawaiian Green Sea Turtles / Honu (Chelonia mydas) bask on Midway’s beaches and a resident pod of about 200 Spinner Dolphins (Stenella longirostris) cavort and rest in the lagoon. The federally-recognized distinct population of Hawaiian Green Sea Turtles is listed as Threatened. Globally, Green Sea Turtle numbers are declining. In contrast, the Hawaiian population has been increasing and numbers are now approaching pre-exploitation levels.

The biodiversity on and surrounding Midway Atoll is precious and in need of conservation. Today, Midway’s seabirds are facing a threat—invasive, predatory mice. Island Conservation and our partners are going to remove them in July 2020 and restore the balance on the Atoll, but we need your help. Learn more at

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This blog post is part of an ongoing, collaborative series between Island Conservation, Northern Illinois University, and the U.S. Fish and Wildlife Service. Be sure to check back for more information about Midway and ongoing conservation and restoration efforts!

Featured photo: A Hawaiian monk seal / ilioholokauaua (Neomonachus schauinslandi) and green sea turtle / honu (Chelonia mydas) resting on the beach in Papahānaumokuākea Marine National Monument. Credit: Papahānaumokuākea Marine National Monument

About Wieteke Holthuijzen

Wieteke Holthuijzen is a National Science Foundation graduate fellow at Northern Illinois University, where she studies the ecological impacts of introduced house mice on Midway through a collaborative research effort with Island Conservation and the U.S. Fish and Wildlife Service. Previously, Wieteke served as the Invasive Plant Control Specialist on Midway, helping to restore the atoll to a bustling seabird colony. She is intrigued by the nexus of nature and human presence and seeks to study and contribute to the conservation of imperiled species. In her spare time, she enjoys playing the cello, ukulele, banjo, and electric bass.

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