May in Aguni Island: The Astonishing Bigeye Trevally Tornado – Japan Underwater Odyssey

Hello everyone, I'm Ishino, a photographer. "Japan Underwater Odyssey" is a series that follows the rhythm of the seasons to explore the ocean at various locations across Japan. For the fifth installment, I head to Aguni Island — a small island off the western coast of Okinawa.

In early summer, the sea warms gradually, nourished by seaweed, and marine life thrives in abundance. Aguni Island is no exception. Though the island is home to only a few hundred residents and moves at a quiet, unhurried pace, every year between April and June it draws divers from near and far — all chasing the breathtaking "Bigeye Trevally Tornado."

Tens of thousands — sometimes hundreds of thousands — of Bigeye Trevally rise from the seafloor like a living wall, endlessly shifting shape in the water. As time passes, the enormous school continuously transforms, creating what can only be described as a work of art crafted by nature itself.

Encountering Bigeye Trevally at Aguni Island

Bigeye Trevally are migratory fish that inhabit warm waters, and the "Fudenzaki" reef off the coast of Aguni Island is a remarkably special site where they gather simultaneously to feed and breed. During this season, the density of Bigeye Trevally is extraordinary — the massive school slowly spirals through the water, and at other times stretches out like a flowing river.

At the center of the school, light is nearly blocked out by the tightly packed fish, while individuals on the outer edge catch the sunlight, their silver bodies glittering with every flick of a fin. When divers approach calmly, the school responds gracefully, slowly shifting shape as if a great creature is turning to face them.

Bigeye Trevally are quite sensitive by nature; if a diver approaches too abruptly or makes aggressive movements, the school can easily scatter. In Aguni, however, they seem accustomed to the presence of divers — maintain a respectful distance, and you may be rewarded with a prolonged encounter. It is precisely this high-probability encounter with the "Bigeye Trevally Tornado" that has earned Aguni Island its outstanding reputation in the diving world.

Two Ways to Dive Aguni Island: Day Trip or Overnight Stay

There are broadly two approaches to diving at 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 reaches Aguni Island in approximately two hours. With no need to arrange accommodation on the island, it is a highly practical and hassle-free choice.

• No overnight stay required
• Naha serves as your base
• Travel on a large vessel for a relatively comfortable crossing

Accommodation on Aguni Island is extremely limited, and during peak season rooms are nearly impossible to find, making a day trip a sensible alternative. While weather can sometimes affect departures, planning a multi-day itinerary greatly increases the chances of a successful outing.

2. Staying on Aguni Island

Choosing to stay on the island offers an entirely different and deeply appealing experience.

Aguni Island is small and serene. At sunset, the sky and sea are painted in deep shades of crimson; at night, countless stars are scattered across the heavens. Spending time within the island's slow, unhurried rhythm of life feels like a true "remote island holiday." This style of trip is especially popular among divers who want to savour a relaxed atmosphere both in the ocean and on land.

• Dive within the laid-back flow of island life
• Flexible departure decisions based on morning sea conditions
• Enjoy the island's scenery and lifestyle to the fullest after diving

That said, on-island accommodation is extremely scarce, and during peak season some guesthouses have fixed booking-open dates — confirming details well in advance is essential. The unparalleled sense of ease that comes from enjoying both the ocean and the island lifestyle together is simply incomparable.

Why Aguni Island Is the Perfect Place to Observe Bigeye Trevally

The Bigeye Trevally at Aguni Island stand out in terms of numbers, density, and sheer scale.

• High probability of witnessing the tornado formation
• Observable in excellent visibility, crystal-clear blue water
• The school constantly changes shape, making it a highly dynamic spectacle
• The fish are relatively tolerant of divers, ideal for extended photography sessions

For underwater photographers in particular, the sight of sunlight piercing through the core of the school and setting the Bigeye Trevally ablaze with light is truly one of a kind. A wide-angle lens allows you to capture both the depth of the school and the sweeping dynamism of its rotation simultaneously.

Because observing such a large school is strictly seasonal, this is a rare phenomenon that can only be witnessed at a specific time of year. May marks the absolute peak, making it the liveliest period on Aguni Island; by June, schools of hundreds of Dogtooth Tuna may also appear, adding yet another layer of excitement to every dive.

The Mystery of the School

The peak diving season for Bigeye Trevally at Aguni Island falls in early summer — April through June.

During this period, they form these enormous tornado-like schools. But why do they spiral in this way? One widely cited explanation is predator defense. By forming a high-density, continuously rotating aggregation, they make it extremely difficult for predators to lock onto any single individual — a classic example of collective defense strategy.

However, there is another reason that may be even more significant: reproduction. During spawning season, mature males and females need to encounter each other efficiently, and high-density aggregation greatly increases the chances of successful pairing.

Although actual spawning behavior 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 during courtship to attract females. This dramatic color change appears only briefly during the breeding season and disappears once it ends — making it a compelling clue that reproductive behavior is actively taking place.

Witnessing a "Moment of Life"

Encountering such a vast aggregation of Bigeye Trevally at Aguni Island in early summer is far more than simply "seeing a lot of fish." It is a rare opportunity to witness firsthand the behaviors these creatures have chosen in order to continue their existence — and to feel, with your whole body, the immense energy contained within the ocean ecosystem.

Next May, I encourage you to make the journey to Aguni Island and witness for yourself the moment this "vortex of life" is born.

Whether you choose a day trip or an overnight stay, a deeply moving underwater experience is waiting for you.

Further Reading

• Shota Ishino on Instagram: shota_gorillahouse

• Diving Japan in Spring: A Kelp Forest Adventure in Kagoshima – Japan Underwater Odyssey

• Where the Kuroshio Passes: Yonaguni Island, Japan's Westernmost Point – Japan Underwater Odyssey

• The Drift Ice Story Anchored in the Sea of Japan's World Natural Heritage – Japan Underwater Odyssey