The lipid content of fillets from lean fish is low and stable whereas that from fatty species varies considerably. However, the variation in the percentage of fat is reflected in the percentage of water, since fat and water normally constitute around 80 percent of the fillet. As a rule of thumb, the amount of fat can be estimated from an analysis of the water content in the fillet.
The lipids present in teleost fish species may be divided into two major groups: the phospholipids and the triglycerides. The phospholipids make up the integral structure of the unit membranes in the cells; thus, they are often called structural lipids. The triglycerides are lipids used for energy storage in fat depots, usually within special fat cells surrounded by a phospholipid membrane and a rather weak collagen network. The triglycerides are often termed depot fat. A few fish have wax esters as part of their depot fats.
Fish and mammalian lipids differ mainly in that fish lipids include up to 40 percent of long-chain fatty acids (14-22 carbon atoms) which are highly unsaturated. Mammalian fat rarely contains more than two double bonds per fatty acid molecule while the depot fats of fish contain several fatty acids with five or six double bonds.
The percentage of polyunsaturated fatty acids with four, five or six double bonds is slightly lower in the polyunsaturated fatty acids of lipids from freshwater fish (approximately 70 percent) than in the corresponding lipids from marine fish (approximately 88 percent). However, the composition of the lipids is not completely fixed but can vary with the feed intake and season.