Mackerel is preserved through fermentation. Surströmming, on the other hand, is a specific Swedish delicacy made from fermented Baltic Sea herring. Surströmming is initially salted just enough to prevent the fish from rotting. This allows the fermentation process to occur, and as a result, the fish develops its strong aroma and taste.The fish are fermented, first in barrels before being canned. The entire process, from the initial catch to when the cans are ready for sale, spans from spring to early September. However, many Swedes believe that Surströmming becomes best for consumption around a year after production, allowing the flavors to mature further.