Fermented fish is a traditional preservation of fish. Before refrigeration, canning and other modern preservation techniques became available, fermenting was an important preservation method. Fish rapidly spoils, or goes rotten, unless some method is applied to stop the bacteria that produce the spoilage. Fermentation is a method which attacks the ability of microbials to spoil fish. It does this by making the fish muscle more acidic; bacteria usually cease multiplying when the pH drops below 4.5.
A modern approach, biopreservation, adds lactic acid bacteria to the fish to be fermented. This produces active antimicrobials such as lactic and acetic acid, hydrogen peroxide, and peptide bacteriocins. It can also produce the antimicrobial nisin, a particularly effective preservative.[1][2]
Fermented fish preparations can be notable for their putrid smell. These days there are many other techniques of preserving fish, but fish is still fermented because some people enjoy the taste.