Halfway between Hawai’i and Australia, sharks pursue small fish through turquoise water and vibrant coral reefs. Above the surface, turtles hatch while vast seabird colonies fill the air with calls as birds seek food and mates.

The scene at Bikar and Bokak suggests that these remote Pacific atolls may belong among the last ecosystems on Earth that remain effectively untouched by humans. Researchers describe them as unusually valuable places for examining ecological relationships with comparatively little direct local human pressure.

In 2023, marine biologists from the University of Hawai’i and the Pristine Seas project traveled to the two atolls. Their findings, reported this week, offer an extensive account of life across the land, surrounding reefs and deep ocean.

Bikar and Bokak are technically part of the Marshall Islands, but their isolation is extreme. They sit around 850 kilometers, or 528 miles, north of the nation’s capital, Majuro, and more than 116 kilometers, or 72 miles, from the nearest inhabited atoll, Utrik.

Researchers surveyed nearly everything the atolls had to offer, from resident rats, plants, birds and sea turtles to coral reefs and open water fish. They also examined reefs in the twilight zone, fish living at greater depths and communities on the deep ocean floor.

Across the broader ecosystem, biodiversity was remarkably rich. The study described the environment as “relatively intact,” a condition that gives scientists an uncommon view of a complex tropical ocean ecosystem facing limited direct local pressure.

Coral reefs were abundant around both atolls, although the strongest reefs were found at Bokak. Hard corals and encrusting algae covered between 60% and 80% of the surrounding shallow waters there.

Bird populations were thriving as well. Researchers recorded 17 seabird species across Bikar and Bokak, with 15 of those species confirmed or considered likely to be breeding on the atolls.

Sooty terns, frigatebirds, red footed boobies, black noddies and white terns were especially common. The survey also identified bristle thighed curlews, eastern reef egrets and wandering tattlers.

Sea turtles were particularly numerous on Bikar. The team documented 71 turtle crawls and 132 nest pits, with most of the nesting activity attributed to green sea turtles, known scientifically as Chelonia mydas.

The waters surrounding the atolls revealed diversity at several depths. Researchers documented 266 groups of reef fish, 138 groups of fish living in deeper waters and 79 groups of deep sea fishes.

Farther from the reefs, the team observed at least 15 groups of open water predators, including sharks, jacks and wahoo, a relative of the mackerel. Even that tally may represent only a portion of the species found in these isolated waters.

“There is also a bit of a lottery involved as to which species show up and persist at remote locations,” Dr Alan Friedlander said in a statement. Friedlander is a study author and marine ecologist at the University of Hawai’i who served as Pristine Seas’ chief scientist.

“These atolls have extensive seabird colonies, mature native vegetation, active turtle nesting, high reef-fish biomass and large predators, coral-dominated reefs at Bokak, structurally complex mesophotic habitats, and surprisingly diverse and poorly known deep-sea communities,” said Friedlander.

He described the atolls as valuable reference points for understanding damage in other ecosystems. “They are among the best ecological reference systems we have to better understand what we have lost elsewhere and to provide fundamental insights needed for effective conservation and restoration efforts.”

The research has already helped inform the creation of a protected area around Bikar and Bokak. Designated by the Marshall Islands Government, the area covers 48,000 square kilometers, or 18,532 square miles.

Keeping the atolls in their seemingly untouched condition will remain difficult amid climate change, pollution and overexploitation. However, the protected designation is expected to make conservation work somewhat easier.

“The Marshall Island government should be applauded for their foresight in protecting these global biodiversity hotspots, where human activity is distant and habitats teem with life,” Friedlander concluded.

“As human activities increasingly threaten biodiversity, areas devoid of intense human impacts serve as vital refugia. Large marine protected areas serve as globally important ecological reference systems for understanding the structure and future trajectories of tropical oceanic ecosystems in the Anthropocene.”

The study was published in the journal Frontiers in Marine Science.