https://en.wikipedia.org/wiki/Surstr%C3%B6mming

Surströmming (pronounced [ˈsʉ̂ːˌʂʈrœmːɪŋ]; Swedish for ''sour herring'') is a lightly-salted fermented Baltic Sea herring traditional to Swedish cuisine since at least the 16th century.

The Baltic herring, known as strömming in Swedish, is smaller than the Atlantic herring, found in the North Sea. Traditionally, the definition of strömming is "herring fished in the brackish waters of the Baltic north of the Kalmar Strait".[1] The herring used for surströmming are caught just prior to spawning in April and May.

During the production of surströmming, just enough salt is used to prevent the raw herring from rotting while allowing it to ferment. A fermentation process of at least six months gives the fish its characteristic strong smell and somewhat acidic taste.[2] A newly opened can of surströmming has one of the most putrid food smells in the world, even stronger than similarly fermented fish dishes such as the Korean hongeohoe or Japanese kusaya.[3] (source: wikipedia)