The Gulf of Bothnia is the northernmost arm of the Baltic Sea. It is situated between Finland's west coast and Sweden's east coast. The gulf is 450 miles long, 50-150 miles wide and has an average depth of 200 ft. The surface area is 45,200 sq miles.

Into the gulf flow a number of rivers from both sides; consequently, a salinity gradient exists from north to south. In the south, the water is the normal brackish water of the Baltic Sea, but in the north, in the Bothnian Bay, the salinity is so low, from 0.4% near Kvarken to 0.2% in the northernmost part, that many freshwater fish such as the pike, whitefish and perch thrive in it.

Being nearly freshwater the Gulf of Bothnia is frozen over five months every year and plays a large part in Finland's shipyards producing 60% of the world's production of icebreakers.

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