Diving Hachijō Island

If one was to summarize the appeal of Hachijō island’s underwater-world, you could say that it stands out by its biodiversity, endemic species, volcanic topography and crystal-clear vivid blue waters, dynamic conditions and marked seasonal changes.

Average Hachijō Temperatures

The Izu Island’s Biodiversity Hospot

Hachijō’s underwater environment is surprisingly diverse.
It is a remote island, directly affected by the Kuroshio, the powerful warm water current flowing northwards along the coasts of Japan, bringing warm, equatorial water up from the tropics and the Philippine’s section of the coral triangle.

This strong flow of warm water has a significant impact on marine ecosystems, bringing unexpected tropical species and also coral to what would otherwise be temperate waters. Warm-water, tropical species of fish and critters mingle with colder water ones, and adds to the area’s overall diversity.

While the area is quite exposed to oceanic currents the island’s topography of natural volcanic basalt rock or the Japanese coast’s inevitable concrete wave breakers, jetties and tetrapods all contribute to create protected areas in which simple hard coral reefs have flourished, such as Sokodo Bay.
Coral-wise, the island is said to host over 35 genera from 14 families of zooxanthellae coral, the most common genera being faviids (e.g., Favia, Favites and Goniastrea), along with Alveopora, Acanthastrea, Echinophyllia, Goniopora and small colonies of tabulate Acropora.

Hachijō’s seafloor is an interesting mix of basaltic volcanic rock and sand, algae and hard and soft coral, which also makes an interesting substrate for smaller marine life.

Furthermore, easy access to deeper water around the island (which features many drop-offs) exposure to currents and a strategic position on seasonal migration routes of various fish and cetaceans all combine to add to the richness of the area.

All in all, Hachijō’s waters, offer fishy diving with both local dewellers and seasonal migratory schools, resident green turtles, great macro, seasonal spawning events, and also the odd encounter with “bigger” marine life such as thresher and hammerhead sharks, dolphins and whales…

Endemism

Hachijō is a remote oceanic island, located more than 200 kilometres (120 miles) off the mainland’s Izu Peninsula, in the Pacific Ocean’s Philippine Sea.
Other than neighbouring Hachijō-Kojima closest islands are with Mikura-jima about 79 km (49 mi) to the north and Aogashima about 64 km (40 mi) to the south.

This position means that Hachijō is directly connected to other islands of the Izu Archipelago and to the Bonin/Ogasawara islands rather than the mainland. It forms a biological and geological corridor with the islands of the archipelago, where it is possible can observe many endemic species rarely found elsewhere, which developed in relative isolation.

Beyond isolation, Japan’s Kuroshio warm water current also plays a major role in the development of Japan’s endemic species.

The Kuroshio is the Pacific’s largest current, and one of the most powerful, its waters reaching over 3 knots in certain areas and seasons.
This north-flowing current creates a fluid barrier, blocking the southern migration of fishes, and effectively trapping them in Japanese waters, which contributes to the area’s endemism.

The Izu and Ogasawara Islands, all administered by the Tōkyō Metropolis, form an island chain called the Nanpō Island (literally the Southern islands), and have recently been rebranded the Tōkyō Eleven Islands for promotional purposes.
The remoter of these islands, such as Hachijō, Aogashima and the Ogasawara group a few hundred kilometres south, stand out by their numerous endemic species, some famous examples being the Wrought Island Butterflyfish (Chaetodon daedalma, Yūzen in Japanese), Hubbard’s Hawkfish (Cirrhitops hubbardi) or the Japanese pigmy seahorse (Hippocampus japapigu), just to name a few…

Volcanic Topography & Hachijō-blue Waters

Hachijō is a volcanic island, formed by a conjoined pair of ancient volcanoes created in different timeframes.
The oldest of these volcanoes is the Higashi-Yama (東山), also called Mihara-Yama (三原山), which has a height of 701 metres (2,300 ft) and was active from 100,000 BCE to around 1700 BCE.
Its eroded flanks are covered with a lush rainforest of ferns and other subtropical species.

The younger of the two volcanoes is the island’s Nishi-Yama (西山), better known as Hachijō-Fuji (八丈富士), which has a height of 854.3 metres (2,803 ft), making it both the highest point on the island and the tallest peak in the entire Izu island chain.
It is rated as a Class-C active volcano by the Japan Meteorological Agency, with recent eruptions recorded in 1487, 1518–1523, and 1605, with seismic activity as recently as 2002.

The volcanic origins of the island contribute to its amazing underwater topography, which consists primarily of black lava rock (basalt), shaped by the currents and waves into a dynamic scenery of arches, overhangs, caves and canyons.

The rocky sea floor is also covered in an interesting mix of algae, gorgonian sea fans and hard-coral formations in more protected areas, contrasting with black volcanic sand…
It is possible to do both shore-dives and boat dives all around the island and around its neighbouring uninhabited islet, Hachijō Kojima.
Some shore-diving sites are within the boundaries of disused artificial harbours, and feature interesting blend of natural and man-made topography (Kyū-Yaene, for instance).

The waters of Hachijō are also famous for their vivid blue colour, nicknamed Hachijō-Blue, which are a true photographer’s delight.
Visibility, though seasonal and directly linked to the flow of the Kuroshio, goes from good to stunning.
It is said that the vivid blue of the Hachijō sea is created by reflection of light on the island’s black volcanic sand and rocks, but also the water’s depth, composition, and the island’s many underwater water springs.

Dynamic Diving & Seasonal Changes

Again, the key word here is current, especially Hachijō particular exposure to the Kuroshio warm water current.
Hachijō is a subtropical island, with a humid subtropical climate (Köppen climate classification Cfa), which give it very warm summers and rather mild winters.

The island is subject to strong winds (including seasonal typhoons, the Pacific Ocean’s tropical cyclones) and exposed to strong currents, the major one being the Kuroshio, which directly affects the island, from different angles and with varying strength at different times of the year, along with masses of cold water.

This has an effect on land and water temperatures, which are warm (up to 29°C) in summer and rather mild in winter  – the island can be dived all-year round, though diving dry-suit is recommended in the cooler months.
The Kuroshio also brings along various seasonal occurrences, including coral and squid spawning events, and schools of migratory fish (which set the calendar of the island’s fishing activities).
More recently, whales and other cetaceans have been regularly observed in Hachijō waters in the winter months, which could be linked to climate change and modification of migration routes (whales are common in the Ogasawara/Bonin islands, but started to be seen regularly around Hachijō since roughly 2015…).

The whales, including humpbacks, can be heard singing underwater on winter dives, and are also sometimes seen by very lucky divers!

While most of the diving is done inside protected areas, Hachijō’s waters are also known for rapid tidal and current changes, especially on more exposed shore-diving sites such as Nazumado, or on boat diving sites.

The island offers a highly dynamic diving environment, which can change radically in a matter of days – if not hours – and also features marked differences between its diving seasons, both in terms of conditions and marine life.