Just enough salt is used to prevent the raw fish from rotting . A fermentation process of at least six months gives the lightly-salted fish its characteristic strong smell and somewhat acidic taste.
When opened, the contents release a strong and sometimes overwhelming odour; the dish is ordinarily eaten outdoors. According to a Japanese study, a newly opened can of surströmming has one of the most putrid food smells in the world, even more so than similarly fermented fish dishes such as the Korean Hongeohoe or Japanese Kusaya. (source : wikipedia)