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Ogasawara NP, Tokyo
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At peak season, 50–100 dogtooth tuna (isomaguro) can whirl through Yomejima’s underwater arch. Some exceed 1 m and 100 kg—a remarkable crowd of heavyweight fish.
The arch looks like a stone doorway a boat could sail through, but the far side is too shallow for vessels to pass. Fish can use the underwater passage; boats cannot.
A receiver at Maguro Ana recorded a signal from a sand tiger shark tagged near Chichijima in 2023—evidence that at least one shark moved roughly 50 km to Yomejima.
Local guides report the tuna gathering mainly from around May to September, during their breeding season. They favor shade, so on cloudy days the shoal may move out of the arch.
Beyond the tuna arch, a dark overhang can shelter sand tiger sharks, known in Japanese as shirowani. A local guide reports seeing as many as 3 there, though their appearance is never guaranteed.
Divers line up along the arch’s sides so their bubbles are less likely to startle the tuna. Rush toward the school and it may scatter out of the opening—and sometimes not return.
The tuna are not always the only visitors: local guides report Indo-Pacific bottlenose dolphins approaching and circling divers at Maguro Ana. These are chance encounters, not a scheduled show.
