Fish oil is derived from the tissues of fishes, every type of fish contains oil. However, the quantity and composition of the oil varies with the type of fish, the season of the year, the geographical location and the diet of the fish. The oil is naturally rich in two important omega-3 fatty acids called eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Fish oils are produced in several countries for human consumption and when traded, they usually are described either by generic name of the fish or the name of the country in which the oil has been extracted. The major fish types which are caught commercially and processed for oil include, Anchovy, Capelin, Herring, Mackerel, Menhaden, Sardine and even some Shark. The uniqueness of fish oils has been recognized for some time, but its health significance began in the mid-1970s when the Eskimos tribe residing in northern Canada, Alaska, Greenland, and eastern Siberia consuming a very rich fish oil diet, rarely suffered death. The reason for this remained a mystery until, it was found that their marine-based diet was very rich in omega-3 fatty acids.

Fish oil is recommended for a healthy diet, and it is beneficial to eat fish once a week (or more) but care must be taken to avoid the fish species which contain mercury or other contaminants such as Chlordane. Fish oil are very similar to one another in their physical nature. A whole fish consists of protein, fat, ash and water irrespective of the species, however, its composition is greatly influenced by seasonal changes due to natural cycle, maturity stage, geographical location, feeding habit etc. Fish oil, in general are more complex than its land animals or vegetable oils due to the long chain unsaturated fatty acids. It is generally believed that fish oil odour is due to the unsaturated fatty acids, since hydrogenation causes the oil to lose its colour but fish caught in colder water have a higher degree of insaturation than that caught in warm water.