Most of the by-products from fish go into the production of fish meal and fish oil, the latter going directly to the human food chain, and therefore do not really come under the heading of industrial by-products and waste per se. Broadly speaking, fish meal made from fish offal is a by-product which otherwise would have been wasted. This paper discusses the quantities involved and the nutritional properties offish meal, and in addition, the contribution to fish meal and fish oil made from species which are not suitable for human consumption (e.g. sandeels) or where the quantities caught exceed the demand for human consumption (e.g. sprats).

Any method of utilizing fish by-products for animal feeding should minimize chemical changes in the product to avoid reduction in the nutrients which are present at the time of catching. In some respects chemical changes in fish by-products are brought about in a similar way to those in grass, cut for preservation. The fish material has a high water content, around 75%, and from the time of catching is subject to chemical changes by enzymes in the fish and also by bacterial action. Fish, however, differs from grass in that it contains oil and virtually no carbohydrates. The demersal, or lean fish, for example, cod, haddock, plaice, saithe, etc., contain high levels of oil in the liver which are removed for separate processing, but little in the flesh and in the offal produced. The ‘industrial’ fish caught are mainly pelagic species with high levels of oil in the flesh.