Demersel fish in general do not have a great deal of lipid in their flesh. Pelagic fish on the other hand, tend to store their lipid in head and muscle tissue. Some types, such as halibut, store some lipid in the liver, and some in muscle. In general, fish do not feed when they are spawning, in spite of the higher nutritional demands of spawning. Their nutrient supply is thus provided by body stores, and as a result, the level of lipid falls steadily as spawning progresses.
Herring show a more seasonal variation in lipid level.
During winter, lower sea temperatures and a scarcity of food causes the level to continue falling. When lipid is drawn from muscle reserves, it is replaced by water, so that the total mass of the fish remains much the same. The muscle water content obviously rises, and this makes the flesh weak and accounts for the poor eating quality of spawning fish. The livers are removed at the time of evisceration of the fish, and processed to separate the oil. This can be by simple steam cooking to obtain the highest quality medicinal oil, with various other techniques used to extract the residual oil which, being of poorer quality, is used for veterinary or industrial purposes.