Vertebrate animals that live in water, fish have always been a staple of the human diet, especially among coastal populations. Although fish once constituted an abundant resource, the stocks of numerous species have declined alarmingly during the 20th century. This dramatic change is the result of many factors, including overfishing, pollution and the development of industrial fishing techniques.
Fish farming (or pisciculture), a rapidly expanding industry, has begun to compensate for declining fish stocks. Although fish farming has existed for over 4,000 years, it initially involved nothing more than keeping fish in captivity; fish were not bred in captivity until 1733, when a German succeeded in breeding trout. The numerous species now bred in captivity include salmon, carp, redfish and sturgeon.
There are over 20,000 different species of fish. The vast majority of these species live in the sea, but some are found in fresh water (rivers, lakes, streams). Certain so-called anadromous species, such as salmon, leave the sea to spawn in fresh water, while catadromous species, such as eels, leave fresh water to spawn in the sea.
The dorsal fins of some fish are spiny, providing a measure of protection, while the anal fins serve as rudders and the lateral fins as stabilizers; fish use their tails to propel themselves forward. These appendages vary greatly from one species to another, as do the shape, size, and color of the fish, and the texture and taste of the flesh.
There are several significant differences between fish and meat. The proportion of muscle is greater because the connective tissue that links the muscles together comprises only 3% of the flesh (as compared to 13% of meat), which is why fish can be cooked quickly and is always tender. The muscle fibers are shorter, which makes the flesh more tender. The flesh of fish contains very little fat; it is thus easy to digest, which explains why people often feel hungry after eating fish, especially leaner varieties. Since the flesh contains far fewer blood vessels than meat and little or no pigment, it is almost invariably white in color. The fat in fish consists mainly of omega-3 polyunsaturated fatty acids, which are found in few other foods (meat contains mainly saturated fatty acids). Numerous studies have demonstrated that omega-3 polyunsaturated fatty acids have a beneficial impact on health. For example, they slow the rate at which blood coagulates, lower blood pressure, and reduce the levels of triglycerides and lipoproteins in the blood, all of which help.