Hailuoto (Finnish: [ˈhɑi̯ˌluo̯to]; Swedish: Karlö) is a Finnish island in the northern Baltic Sea and a municipality in Northern Ostrobothnia region. The population of Hailuoto is 924 (October 31, 2024), making it the smallest municipality in Northern Ostrobothnia and the former Oulu Province in terms of population. The municipality covers an area of 205.65 km2 (79.40 sq mi) (excluding sea areas) of which 1.70 km2 (0.66 sq mi) is inland water (January 1, 2018). The population density is 4.49/km2 (11.6/sq mi). Of all the Finnish sea islands, Hailuoto is the third largest after Fasta Åland and Kimitoön.
Hailuoto lies opposite the city of Oulu in the Gulf of Bothnia. The distance between Oulu and Hailuoto is 53 kilometres (33 mi), and the sea area between them is called Luodonselkä (literally "open water of the islet").
Land in the region is constantly rising due to post-glacial rebound. It is estimated that the first parts of Hailuoto appeared from the Baltic Sea about 1700 years ago. The current island of Hailuoto was formed from many smaller islands. Two large sections, Santonen and Hanhinen merged into the main island (Luoto) only about two centuries ago. The island is continuously expanding and eventually it will join with the mainland. Kirkkosalmi, a wetland region between Hanhinen and Luoto is renowned for being an important bird refuge where rare bird species are observed.
The theme of the coat of arms of Hailuoto describes the economy of the island municipality; the explanation of the coat of arms is "in a blue field with a silver rowlock." The coat of arms was designed by Teuvo-Pentti Pakkala and approved by the Hailuoto Municipal Council at its meeting on 28 April 1966. The Ministry of the Interior approved the use of the coat of arms on 19 December of the same year.
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