Depending on their lipid content, which varies greatly from 0.2 percent to 25 percent, fish are classified as lean, semi-fatty or fatty. Bottom-dwelling ground fish such as cod, saithe and hake are common lean species. Fatty species include pelagics such as herring, mackerel and sprat. Some species store lipids in limited parts of their body tissues only or in smaller quantities than typical fatty species, and are consequently termed semi-fatty (e.g. barracuda, mullet and shark).
Fish lipids contrast greatly from mammalian lipids in that they include up to 40 percent of long-chain fatty acids that are highly unsaturated and contain five or six double bonds. This difference entails both health (anti-thrombotic activity of polyunsaturated fatty acids) and technological (rapid development of rancidity) implications.
Proteins are the second-most important fish constituent. These comprise structural proteins (actin, myosin, tropormyosin and actomyosin), sarcoplasmic proteins (myoalbumin, globulin and enzymes) and connective tissue proteins (collagen). Fish proteins contain all the essential amino acids and, like milk, eggs and mammalian meat proteins, have a very high biological value. In addition, fish proteins are an excellent source of lysine, methionine and cysteine, and can significantly raise the value of cereal-based diets, which are poor in these essential amino acids.
Fish also has a non-protein nitrogen (NPN) fraction made of water-soluble, low molecular weight, nitrogen-containing compounds of a non-protein nature. This NPN-fraction constitutes from 9 to 18 percent of the total nitrogen in teleosts, including trimethylamine oxide (TMAO), free amino acids, creatine and carnosine. Despite their low levels, the constituents of the NPN fraction play a major role in fish quality.