Surströmming is a Swedish dish that is made by fermenting herring. It has been made at least since the 16th century. The herring is caught in spring, and just enough salt will be added so it does not spoil. The herring will ferment for at least six months. As a result, the dish has a strong smell, and an acidic taste. Since the 1940s, it is forbidden to sell Surströmming of the same year before the third thursday in August. While the decree is no longer valid, the producers still keep that date as begin of the sales of this year's batch. Very often, Baltic herring is used instead of Atlantic herring. Baltic herring is much smaller.