This is the northernmost part of the Baltic Sea and also its largest gulf. About 270 km long and some 145 km wide, the gulf is 37,365 km3 in area and has a volume of 1462 km2. It lies between the west coast of Finland and the east coast of Sweden. The northern part of the gulf (known as the Bothnian Bay) is shallow, with a maximum depth of 141 m and an average of 42 m. In the south (also known as the Bothnian Sea) the greatest depth is 294 m. The two parts are separated by the 74-km wide Kvarken Strait. To the south the gulf is closed off by the archipelago of the Åland Islands. The seabed is morphologically diverse. The shallower Bothnian Bay is relatively flat, with a number of shoals. In the north-east, there are sands along the shores to a depth of about 50 m. Clay bottoms are overlain by boulders, pebbles and gravels. In the south-east, clays fill small depressions, while and glacial clays overlie elevations on the bottom. In the skerry area there are sedimentation basins and bottom depressions. Ferromanganese concretions are present. The eskers—post-glacial embankments of sandy-gravel sediments—are a characteristic feature. Arctic-continental influences are clearly evident in the climate, leading to very harsh winters. The warm period lasts from June to September. In July, the average temperature is 15 °C. In the warmest period in summer, the water temperature at the bottom is about 1.5 °C, while the surface water temperature is less than 14 °C. In winter the northern part of the gulf freezes over and is covered with ice, sometimes for up to 6–8 months a year. In the 1970s the ice cover persisted into mid-April, with ice floes remaining until May. This gulf is the least saline Baltic basin. In the south the salinity varies between 3 and 5‰, whereas in the north it is approximately 2‰. Herring Clupea harengus and cod Gadus morhua are caught in the gulf (Fig. 7.1).