This publication is a practical manual that will guide the reader through the main principles of producing fish silage by fermentation and explain each step of the fermentation process needed to successfully become a fish silage producer. Fish, including shellfish, are highly nutritious and in much demand all over the world. However, fish processing by-products, in particular viscera (guts), are highly perishable. If not preserved or processed within a relatively short time after harvest, they may deteriorate rapidly making them unfit for human consumption or other uses. In many cases, processing leads to the removal of significant parts of the fish, such as the viscera, head, belly flaps and backbone. Depending on the species, these parts may represent between 30 percent and 70 percent of the fish. Some parts, such as gonads, belly flaps and backbones, may be used directly for human consumption, but most of the by-products of fish processing have traditionally been wasted, leading to negative environmental impacts, or they have been used in fresh form as feed for livestock or as fertilizers.