The Gulf of Bothnia (Fin.; Pohjanlahti, Sw.; Botniska viken) is the northernmost arm of the Baltic Sea. It is situated between Finland's west coast and Sweden's east coast. In the south of the gulf lie the Åland Islands, between the Sea of Åland and the Archipelago Sea. The sea area is interesting because its salinity drops rapidly towards the North. In the South the water is the normal brackish water of the Baltic Sea, but in the North the salinity has dropped so low that one can no longer taste the salt in the water and many freshwater fish thrive in it.
Some historians consider Ottar refer to Gulf of Bothnia when he uses Cwen sea (9th c.). It is possible too that Claussön's usage Mare Gotticus (15th c.) refers to Gulf of Bothnia.
