Streaked Shearwater Breeding Grounds

The Streaked Shearwater Breeding Ground: Japan's Northernmost Seabird Sanctuary on a Volcanic Island

Roughly 50 kilometres off the southwest coast of Hokkaido, in the rough waters of the Sea of Japan, rises a wild and weather-beaten volcanic island called Oshima-Oshima (also written 渡島大島). The entire island has been designated a National Natural Monument as the northernmost breeding ground in the world for the Streaked Shearwater (Calonectris leucomelas), a graceful seabird that migrates each year between Japan and the seas of Australia. For travellers fascinated by Japan's natural heritage, untouched landscapes and the long tug-of-war between human history and wildlife, this remote uninhabited island offers a story unlike any other.

What This Cultural Property Protects

The designation "Streaked Shearwater Breeding Ground" (オオミズナギドリ繁殖地) refers to the entire territory of Oshima-Oshima, an uninhabited island administratively belonging to Matsumae Town in southern Hokkaido. The island is approximately 16 kilometres in circumference, covers about 9.73 square kilometres, and is the largest uninhabited island under Japanese administration. Three volcanic peaks rise dramatically from the sea, the tallest being Mount Era at roughly 737 metres. From the seabed the volcano stands nearly 2,000 metres high.

The Streaked Shearwater is a medium-sized seabird that nests in burrows dug into the soft volcanic soil. Each pair raises a single chick per season, and the birds rely on tall trees and cliff edges to take flight, since they cannot launch directly from flat ground. The combination of forested slopes, soft volcanic earth and steep coastal cliffs once made Oshima-Oshima a near-perfect habitat — a place the species is believed to have used for breeding for around 9,000 years.

Why It Was Designated a National Natural Monument

Oshima-Oshima was officially designated a National Natural Monument on 24 March 1928 (Showa 3) under Japan's Cultural Properties Protection Law. The designation recognises three intertwined values.

The first is biogeographical importance. Although Streaked Shearwaters breed at several sites in Japan — including the famous colony on Mikurajima in the Izu Islands — Oshima-Oshima is the northernmost confirmed breeding ground in the world. This northern outpost holds particular scientific value for understanding the species' range and migration ecology.

The second is the scarcity of suitable habitat. Streaked Shearwaters require remote, predator-free islands with the right combination of soil and vegetation. Such places are rare, and only six breeding grounds in Japan have received national designation, of which Oshima-Oshima is one.

The third is the dramatic conservation history of the site. The colony has survived volcanic eruptions, decades of human harvesting and the introduction of invasive rats and rabbits. Designating the entire island as a Natural Monument was a decisive step taken in 1928 to halt the rapid decline of the population.

Highlights and Points of Interest

A Volcanic Landscape Born of Catastrophe

Oshima-Oshima is a stratovolcano with a striking horseshoe-shaped caldera open to the north. In 1741 (Kanpo 1) the volcano erupted catastrophically, triggering a massive collapse of its northern flank. The resulting tsunami devastated the coast of southern Hokkaido and the Tohoku region, claiming roughly 1,500 to 2,000 lives in what is remembered today as the Kanpo Tsunami. Stone monuments to the victims still stand in the temples of Matsumae. The volcano has remained quiet since the late 18th century, but its slopes still bear scars of the eruption that reshaped a coastline.

The Annual Cycle of the Streaked Shearwater

Streaked Shearwaters return to Oshima-Oshima each year in March and April after wintering in the seas of Southeast Asia and northern Australia. Pairs reunite, refresh their burrows and lay a single egg in June. Chicks hatch in August and fledge in November before setting out on the long migration south. Their eerie, almost human-sounding calls — sometimes compared to the cry of a baby — have inspired folklore along the Japan Sea coast for centuries.

A Hard-Earned Conservation Story

The colony has lived through extraordinary trials. From the late Meiji era, around 1905 to 1909, fur traders from Hakodate are reported to have harvested over 220,000 birds for feathers used in winter coats and futons. Later, in the early 20th century, fishermen who came seasonally to gather wakame seaweed continued to take adult birds for food despite the 1928 designation. From 1938, European rabbits were released on the island for fur farming, and brown rats arrived as stowaways on boats. Both species devastated the vegetation and preyed on shearwater eggs and chicks. Today, only an estimated 60 to 100 breeding pairs remain. The island is now a protected reserve where landing is strictly controlled, and ongoing studies aim to restore the colony.

A Sister Island: Matsumae-Kojima

About 23 kilometres west of Matsumae lies Matsumae-Kojima, a smaller sister island that is also an important seabird habitat and is itself designated a National Natural Monument. Together the two islands form one of the most significant seabird ecosystems in northern Japan.

Visitor Information and Access

Oshima-Oshima is uninhabited and landing is strictly regulated. The entire island is designated as a National Natural Monument, a Hokkaido prefectural wildlife protection area and a Class 1 Special Zone of the Matsumae-Yagoshi Prefectural Natural Park. Stepping ashore changes the natural state of a protected site, so a notification or permit must be obtained from the Matsumae Town Board of Education in advance. There is no developed harbour on the island — only a small breakwater — and weather conditions in this stretch of the Sea of Japan are notoriously changeable, with frequent maritime accidents in the surrounding waters.

For the vast majority of visitors, then, Oshima-Oshima is best appreciated from the mainland or from the deck of a sightseeing boat. On clear days the silhouette of the island can be seen from the western coast of Matsumae Town, particularly from the headlands around the Era and Oshima districts. The Matsumae Town Folk Museum and the Matsumae Castle Museum both display materials related to the island's natural and human history, including records of the 1741 tsunami.

Surrounding Attractions

Matsumae Town, the only castle town in Hokkaido, makes an excellent base for exploring the region's culture and nature. Matsumae Castle, rebuilt as a museum, stands at the heart of a park famous for some 10,000 cherry trees of more than 250 varieties, with blossoms continuing well into May. The historic temple district preserves Edo-period architecture and Buddhist art, and tombs of successive Matsumae clan lords lie in quiet hillside cemeteries. Along the rugged Matsumae-Yagoshi Coast, dramatic sea cliffs, fishing villages and clear coves invite slow travel by car or bicycle. Seafood lovers can enjoy locally caught bluefin tuna and the famous "matsumae-zuke" — a preserved dish of seaweed, dried squid and herring roe — at restaurants throughout the town.