Two ‘Untouched’ Pacific Atolls Sit Side by Side, but One Has Double the Sharks

Remoteness was supposed to make them twins. Instead, two uninhabited Marshall Islands atolls just a few hundred kilometers apart turned out to differ sharply: one holds nearly two and a half times the shark weight of the other and far more coral, while its neighbor tilts toward seaweed.

Bikar and Bokak are two tiny, uninhabited coral atolls sitting more than 850 kilometers north of Majuro, the capital of the Marshall Islands. Both rank among the least-disturbed reefs in the tropical Pacific, with no permanent residents and little direct human pressure. A team led by Alan Friedlander, working with National Geographic’s Pristine Seas project, surveyed both atolls in 2023 and published the results in the journal Frontiers in Marine Science, measuring everything from treetop forests to the deep-sea floor.

Despite their isolation and light human footprint, the two atolls proved strikingly different. Bokak, the larger atoll, had more fish overall, more coral-covered reefs, and far more sharks. Bikar had thicker native forest, far more sea turtle nesting, and more sightings of an invasive rat. Being remote and lightly disturbed, the researchers concluded, does not guarantee that two ecosystems will look or function the same way.

Surveying Two Pristine Reefs From the Treetops to the Deep Sea

Few reef studies attempt this much. Most stop at the reef itself, often in fairly shallow water. This team went further, studying the islands’ forests, counting seabirds and rats on foot, surveying turtle nesting beaches, diving the shallow reefs, dropping cameras into the deep sea, and sending a small submarine down to nearly 900 feet to reach reef habitat that sunlight barely touches.

Each part of the ecosystem called for a different tool. Divers swam fixed paths to count fish and coral. Field teams walked the islands, tallying birds and rats within about 100 meters. Waterproof cameras baited with chunks of fish sat on the seafloor for 14 hours straight, recording whatever predators and scavengers came to investigate the smell. A one-atmosphere submersible, a small submarine that keeps its crew at normal air pressure, carried observers past 800 feet into near-total darkness.

Across both atolls, researchers logged more than 260 reef fish species. Sharks, plant-eating fish, and other large predators made up a large share, a pattern usually seen on reefs with little fishing. Separate baited cameras lowered into the deep sea recorded 79 kinds of fish and other animals living thousands of feet down, several never officially recorded in the Marshall Islands before. A couple of fish filmed by the submarine could only be partly identified, matched to a broad group but not to any known species. That alone says a lot about how little science still knows about life in these deep, dark waters.

Two Pristine Reefs, Two Very Different Stories

Both atolls were in good ecological shape, exactly as their isolation predicted. What surprised the team was how differently that played out.

Bokak, the larger atoll, carried more fish by weight than Bikar in both shallow and deeper water. Coral dominated its reefs across depths, with hard coral and encrusting coralline algae covering 60 to 80 percent of the seafloor in places. Sharks followed the same pattern. Grey reef sharks, the most common shark on both reefs, weighed nearly twice as much at Bokak, and sharks overall were far heavier there too.

Bikar told a different story. Its reefs held noticeably more seaweed and less coral, and some deep, wave-battered stretches carried more seaweed than coral. On land, though, Bikar supported a mature, dense forest of a native tree called Pisonia grandis, with some trunks measuring more than 6 meters around, and large colonies of nesting seabirds. Bikar also had the most sea turtle nesting and more sightings of the introduced Polynesian rat.

Seabirds split along their own lines. Bikar hosted more black noddy nesting and more frigatebird sightings, while Bokak held more boobies, more common noddies, and more extensive frigatebird nesting. Neither atoll simply had more birds than the other; each carried a different mix of species in different numbers.

Researchers ran the numbers to rule out random chance. Statistical tests supported several differences between the atolls in coral and seaweed cover, reef fish numbers, and seabird communities, though the strength of the evidence varied by habitat and species. Why the two atolls diverged at all proved harder to answer. The team floated possibilities: differences in each atoll’s shape and size, the way water moves through their lagoons, and long-run differences in their ecological history, while stressing that these ideas remain untested.

Why Rats and Turtles Matter on These Atolls

One finding reminded us that even the most remote places on Earth are not entirely free of human fingerprints. Polynesian rats, which likely reached these islands sometime after people first settled the wider Pacific, turned up on both atolls, though sparsely and unevenly. Because the survey method was built to count birds rather than rodents, researchers treated the rat numbers as a rough sign of presence rather than an exact count. Their presence still matters, since rats are known elsewhere to disturb ground-nesting seabirds, meaning even a supposedly untouched atoll carries at least one trace of past human contact.

Sea turtles left a cleaner record. Nearly all the nesting activity, 71 crawls up the beach and more than 130 nest pits, was concentrated at Bikar, most of it from green sea turtles and at least one confirmed hawksbill. That concentration makes Bikar a nesting site worth watching over time, especially as sea turtles face mounting pressure across the Pacific from habitat loss and other threats.

What These Pristine Reefs Reveal About the Future

Researchers frame Bikar and Bokak as reference points, places scientists can return to and measure how climate change, ocean warming, and other global pressures reshape reefs that aren’t also battling overfishing or pollution. Both atolls now sit inside designated National Marine Sanctuaries covering tens of thousands of square kilometers of ocean, a rare chance to track long-term change without local human interference.

A single statistic is not the real takeaway here. The study shows why scientists cannot assume two remote, lightly disturbed reefs will look alike. Two atolls in the same ocean, under the same climate, with the same absence of permanent residents, still diverged sharply in coral and seaweed cover, fish numbers, forest growth, and the wildlife nesting on their shores. Protecting ocean ecosystems well cannot rely on a single picture of what a healthy reef looks like. Even in the same stretch of sea, nature rarely repeats itself.

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