Fish silage is a liquid product made from whole fish or fish side streams that are hydrolyzed by endogenous enzymes in the presence of an added acid, usually formic acid. At acidic pH, the digestive enzymes break down the fish proteins into smaller peptides and free amino acids. The process is inexpensive and does not require high investment-costs and is regarded as especially useful when only small amounts of fishery side streams are available. The final liquid product can be concentrated to a fish protein concentrate. The process of producing silage entails several health and safety requirements and handling of acids should only be done while wearing personal protective equipment (goggles, rubber gloves etc.).

For preparation of fish silage, the raw material must be ground, and the mixture stirred to ensure good contact between the raw material and added acid. At pH <4, enzymes naturally present in fish viscera degrade and liquefy the fish tissue without risk of bacterial spoilage. Antioxidants should be added to prevent oxidation of fish oil. Production of fish silage is a relatively simple and low-cost technology but requires strict process control to avoid growth of spoilage bacteria. The rate of liquefaction depends on the type of raw material, its freshness, and the temperature of the process. Fatty fish liquefy more quickly than lean fish and the fish should be as fresh as possible. Moreover, the warmer the mixture, the faster the process. Fish silage of the correct acidity keeps at room temperature for at least two years without putrefaction. The final product is not suitable for human consumption, and silage technology should preferably be based on fish and residuals found unfit for food production. To readd more about silage, see more.