Hello everyone, I'm Ishino, a photographer. "Japan Underwater Odyssey" is a series that follows the rhythm of the four seasons to explore the underwater world across Japan. For the fifth installment, I traveled to Aguni Island — a small island situated to the west of Okinawa.
In early summer, the ocean warms gradually, nourished by seaweed, and marine life bursts with energy. Aguni Island is no exception. Though home to only a few hundred residents and known for its quiet, unhurried way of life, every year from April to June the island draws divers from far and wide — all chasing the breathtaking "Bigeye Trevally Tornado."
Tens of thousands of Bigeye Trevally rise from the seafloor, forming a living wall that shifts and transforms continuously in the water. The massive school morphs from one shape to the next over time, conjuring what can only be described as a work of art crafted by nature itself.
Encountering Bigeye Trevally at Aguni Island
Bigeye Trevally are a migratory species that inhabit warm seas, and the "筆ん崎 Fudenzaki" reef off Aguni Island is a particularly special location where they gather simultaneously to feed and spawn. The density of Bigeye Trevally during this season is staggering — the enormous school rotates slowly through the water, at times stretching out like a flowing river.
At the center of the school, the fish are packed so tightly that light is almost entirely blocked out, while individuals on the outer edge catch the sunlight and flash brilliantly, their silver bodies glinting with every movement. When divers approach calmly, the school responds with quiet elegance, shifting its shape as though a colossal creature is slowly turning.
Bigeye Trevally are a rather sensitive species — sudden approaches or aggressive movements can easily cause the school to scatter. On Aguni Island, however, they appear to have grown accustomed to the presence of divers, and with a respectful distance maintained, extended observation becomes entirely possible. It is precisely this high-likelihood encounter with the "Bigeye Trevally Tornado" that has earned Aguni Island its excellent reputation in the diving community.
Two Ways to Dive Aguni Island: Day Trip or Island Stay
There are broadly two approaches to diving Aguni Island.
1. Day Trip from Okinawa's Main Island
This is the most common and convenient option. Departing from Naha, a large dive boat takes approximately two hours to reach Aguni Island. With no overnight accommodation to arrange, it is a practical and straightforward choice.
- No overnight stay required
- Dive using Naha as your base
- Relatively comfortable passage aboard a large vessel
Since accommodation options on Aguni Island are extremely limited — especially during peak season, when rooms are nearly impossible to find — a day trip is a smart alternative. Weather can occasionally affect departures, but planning a multi-day itinerary significantly increases your chances of making it out.
2. Staying on Aguni Island
Choosing to stay on the island unlocks a completely different and deeply rewarding experience.
Aguni Island is small and tranquil. At sunset, the sky and sea are painted in deep shades of crimson; after dark, thousands of stars scatter across the night sky. Spending time in the island's slow, unhurried rhythm feels like a true "remote island escape." This approach is especially popular among divers who want to savor a relaxed atmosphere both above and below the surface.
- Dive at a leisurely island pace
- Flexible scheduling based on morning sea conditions
- Enjoy the island's scenery and laid-back lifestyle after each dive
That said, the number of guesthouses on the island is very small, and some accommodations have fixed booking windows during peak season — so advance research is essential. The combination of ocean diving and the island's unhurried way of life creates a sense of relaxation that is truly unmatched.
Why Aguni Island Is the Perfect Place to Observe Bigeye Trevally
The Bigeye Trevally at Aguni Island are exceptional in terms of sheer numbers, density, and scale.
- High probability of witnessing tornado-formation schooling behavior
- Observed in excellent visibility within clear, deep-blue water
- Constantly shifting shapes create endlessly dynamic spectacles
- The school's tolerance of divers makes extended photography sessions possible
For underwater photographers in particular, the sight of sunlight piercing through the heart of the school and setting each Bigeye Trevally ablaze with silver light is genuinely one of a kind. A wide-angle lens allows you to capture both the depth of the school and the sheer magnitude of its spiraling motion in a single frame.
Because observing such a massive school is inherently seasonal, this is a rare phenomenon visible only at a specific time of year. May marks the peak, making it the busiest period on Aguni Island. By June, schools of Dogtooth Tuna numbering in the hundreds may also appear, adding yet another exciting dimension to each dive.
The Mystery of the School
The peak diving season for Bigeye Trevally on Aguni Island falls in early summer — April through June.
During this period, they form these enormous, swirling tornado formations. But why do they spiral like this? One widely cited explanation is predator defense. By forming a dense, continuously rotating aggregation, the fish make it extremely difficult for predators to single out an individual target — a classic collective defense strategy.
However, there may be an even more significant driving force at work: reproduction. During the spawning season, mature males and females need to find each other efficiently, and high-density aggregations dramatically increase the chances of successful pairing.
While the act of spawning itself has not yet been directly observed, careful observation of the school reveals that some individuals will suddenly turn a deep, dark black. It is known that male Bigeye Trevally darken their body color to attract females during courtship. This dramatic color change appears only briefly during the breeding season and vanishes once it ends — making it a compelling indicator that reproductive behavior is actively taking place.
Witnessing a "Moment of Life"
Encountering such a vast aggregation of Bigeye Trevally in the early summer waters of Aguni Island is far more than simply "seeing a lot of fish." It is a rare opportunity to witness, with your own eyes, a behavior these creatures have chosen in order to carry life forward — and to feel firsthand the immense energy contained within a living marine ecosystem.
Come next May, I encourage everyone to make their way to Aguni Island and witness for themselves the very moment this "vortex of life" is born.
Whether you come for the day or linger on the island, an underwater experience powerful enough to move you to your core is waiting.
Further Reading
