Fish oil is now a valuable commodity of high nutritional value. However, this was not always so since fish oil was considered in the past a "second product" of the production of fishmeal, a product of great importance in animal nutrition. This second product, which was discarded initially began to be used in the manufacture of paints, varnishes, resins, etc. and also occasionally as a fuel. Later, it began to be used in the manufacture of shortenings and margarines, after partial hydrogenation, and in the preparation of edible oils mixed in different proportions with vegetable oils. However, the discovery of the beneficial properties of omega-3 marine oils, that fish oil contains in high proportion, and its use in the preparation of food for aquaculture, particularly salmon and trout, has transformed fish oil in a scarce, high commercial value and a growing demand product mainly for its nutritional properties.