Surströmming

Surströmming is lightly salted, fermented Baltic Sea herring traditional to Swedish cuisine since at least the 16th century. It is distinct from fried or pickled herring. The Baltic herring, known as strömming in Swedish, is smaller than the Atlantic herring found in the North Sea. Traditionally, strömming is defined as herring caught in the brackish waters of the Baltic north of the Kalmar Strait. The herring used for surströmming are caught prior to spawning in April and May.

Source: Wikipedia