The harvesting of fish and shellfish from the sea or from freshwater; for example, cod from the North Sea and carp from the lakes of China and India. Fish are an excellent source of protein for humans, and fish products such as oils and bones are used in industry to produce livestock feed, fertilizers, glues, and drugs. Most of the world's catch comes from the oceans. The world's total fish catch is about 100 million tonnes a year (1995).

The world fish catch increased by an average of 7% each year 1950–70. Refrigerated factory ships allowed filleting and processing to be done at sea, and Japan evolved new techniques for locating shoals (by sonar and radar) and catching them (for example, with electrical charges and chemical baits). By the 1970s, overfishing had led to serious depletion of stocks, and heated confrontations between countries using the same fishing grounds. A partial solution was the extension of fishing limits to 320 km/200 mi. The North Sea countries have experimented with the artificial breeding of fish eggs and release of small fry into the sea. In 1988, overfishing of the northeastern Atlantic led to hundreds of thousands of seals starving on the north coast of Norway. A United Nations (UN) resolution was passed in 1989 to end drift-net fishing (an indiscriminate method) by June 1992. Marine pollution is blamed for the increasing number (up to 30%) of diseased fish in the North Sea.

Stocks of several deep-water fish have been decreasing since the 1970s because of the global boom in deep-water fishing. As traditional fisheries in shallow water became depleted, there was a growth in commercial interest in deep-water fisheries, but deep-water fish are slow-growing and do not reproduce rapidly enough. For example, stocks of New Zealand's orange roughy, one of the first deep-water fish to be exploited, fell by 90% in the 1970s, and there are no signs of recovery. This may have a permanent effect on deep-water ecosystems, with the disappearance of slow-growing species in favour of faster-reproducing fish.