or Kojima is an uninhabited island in the Sea of Japan

Sea of Japan

The Sea of Japan is a marginal sea of the western Pacific Ocean, between the Asian mainland, the Japanese archipelago and Sakhalin. It is bordered by Japan, North Korea, Russia and South Korea. Like the Mediterranean Sea, it has almost no tides due to its nearly complete enclosure from the Pacific...

, 23 kilometres (14.3 mi) southwest of the mainland portion of the town of Matsumae

Matsumae, Hokkaido

is a town located in Matsumae District, Oshima, Hokkaidō, Japan. The former home of the Matsumae Han, it has an Edo period castle, Matsumae Castle, the only one in Hokkaidō, and Ryūun-in.The total area of the town is .-Geography:...

 town and is the southernmost point in Hokkaidō. It is under the administration of the town of Matsumae in Matsumae District

Matsumae District, Hokkaido

is a district located in southwestern Oshima Subprefecture, Hokkaidō, Japan.As of 2004, the district has an estimated population of 16,068 and a density of 33.45 persons per km²...

, Oshima Subprefecture

Oshima Subprefecture

is a subprefecture of Hokkaidō, Japan. As of 2004 it had a population of 456,621 and an area of 3,715.38 km².Hakodate Airport is located in the City of Hakodate.- Geography :-Towns and villages by district:* Futami District** Yakumo* Kameda District...

 in Hokkaidō

Hokkaido

, formerly known as Ezo, Yezo, Yeso, or Yesso, is Japan's second largest island; it is also the largest and northernmost of Japan's 47 prefectural-level subdivisions. The Tsugaru Strait separates Hokkaido from Honshu, although the two islands are connected by the underwater railway Seikan Tunnel...

, Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

. To distinguish Ko Island from other islands with the same name, it is sometimes known as or .

Ko Island has an area of 1.54 square kilometre (0.594597324232504 sq mi). It is an andesitic stratovolcano

Stratovolcano

A stratovolcano, also known as a composite volcano, is a tall, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions...

. The peak consists of non-alkali mafic

Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...

 rocks, dating from the Early Pleistocene

Early Pleistocene

Calabrian is a subdivision of the Pleistocene Epoch of the Geologic time scale. ~1.8 Ma.—781,000 years ago ± 5,000 years, a period of ~.The end of the stage is defined by the last magnetic pole reversal and plunge in to an ice age and global drying possibly colder and drier than the late Miocene ...

 overtop of non-alkali felsic

Felsic

The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....

 rocks from the Late Miocene

Late Miocene

The Late Miocene is a sub-epoch of the Miocene Epoch made up of two stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch....

-Pliocene

Pliocene

The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

.

Several smaller islets surround Ko Island, including Daihiyakushima, Shohiyakushima, Tenjinshima, and Sazaeshima.

To provide refuge for fishing vessels, a small harbor has been put in place.

Flora and fauna

Ko Island has been designated a natural monument. It is a breeding ground for Common Guillemot

Common Guillemot

The Common Murre or Common Guillemot is a large auk. It is also known as the Thin-billed Murre in North America. It has a circumpolar distribution, occurring in low-Arctic and boreal waters in the North-Atlantic and North Pacific...

, Japanese Cormorant

Japanese Cormorant

The Japanese Cormorant , also known as Temminck's Cormorant, is a cormorant native to East Asia. It lives from Taiwan north through Korea and Japan to the Russian Far East....

, and Black-tailed Gull

Black-tailed Gull

The Black-tailed Gull is a medium-sized gull, with a wingspan of 126-128 cm. The bird is resident in East Asia, including China, Taiwan, Japan and Korea. It is a vagrant to Alaska and northeastern North America....

.

15 kilometres (9.3 mi) to the west of Ko Island is the Ko Island Bank, where migratory fish tend to gather. In the adjacent seas there is also tuna

Tuna

Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...

 and squid

Squid

Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...

. In previous times it was referred to as the . For this reason it is a major fishing area for fishermen out of Matsumae, especially because of the decline in the Arabesque greenling

Arabesque greenling

The Arabesque greenling is a species of mackerel used in Japanese cuisine.The primary population of the fish is found off of the Sea of Okhotsk. According to legend, it was discovered by Nichiji.-Uses:...

 over the last 24 to 25 years.

The predominant vegetation on the island is grasses, but there are stands of Acer pictum subsp. mono

Acer pictum subsp. mono

Acer pictum subsp. mono, commonly known as Painted Maple or Mono Maple in English, or Ezo Itaya in Japan, or 五角枫 in China, is a species of maple.-Description:...

at in the caldera.