Fishing name applied both to the commercial enterprises that harvest fish and shellfish from waters, and to the waters themselves where such fishing takes place. Fisheries are mostly marine but also exist on large lakes and rivers. The most productive fisheries extend outward from coastlines as far as the continental shelf, an average of about 80 km (about 50 mi) from the shore. They are less than 200 m (less than 650 ft) deep, but due to favorable currents and temperatures and abundant plant life, they hold most of the oceans' fish. Fisheries are particularly productive in areas of upwelling, where cold, deep, and nutrient-rich waters are carried to the surface. The harvest of whales, mollusks, crustaceans, and kelp are also considered fishery industries.

Major Fisheries

Rich fisheries are found along the North Sea, the west coast of Great Britain, the continental shelf of Iceland, the Grand Banks off eastern Canada, the Georges Banks off New England, off the southwestern United States and Peru, in the Bering Sea, in the Gulf of Alaska, and off the coasts of Japan.

Types of Fish Harvested
Two kinds of finned fish are taken from saltwater fisheries: pelagic and demersal. Pelagic fishes frequent near-surface water, usually migrate seasonally, and travel in schools. They include tuna, salmon, anchovy, sardine, and herring. Demersal, or groundfish, frequent ocean bottoms and are less gregarious. They include cod, halibut, sole, haddock, and flounder. Invertebrates are abundant animal species in the ocean, but make up only a small percent by weight of the harvest. They are taken mostly in shallow water; oyster, clam, scallop, lobster, crab, shrimp, and squid are of greatest commercial importance.

Methods of Harvesting
Commercial fishers use nets that are either pulled close to the surface or trawled along the bottom. The most effective surface net is the purse seine, a long, curtainlike net that hangs into the water from floats. It is towed by a skiff in a circle around a school of fish and then drawn shut at the bottom, like a purse, with a rope. The gill net is made with mesh size just large enough for the head of the fish to pass through and the gills to hook into the mesh. Gill nets are most often left to drift on the surface for pelagic fish but are also anchored on the bottom for groundfish. The otter trawl dredges demersal fish such as cod and is the most important net for commercial fishing in deep water. It is towed by two long cables. Near the mouth of its sock-shaped net are two boards that are forced sideways and downward by their movement through the water to hold open the net. Longlines, which are used to catch both surface fish (such as tuna) and groundfish (such as halibut) are long, heavy ropes to which are attached auxiliary lines with baited hooks. They can extend for several kilometers and are attached to moored buoys or trolled from vessels. The laden lines are hauled in by winches. Deepwater shellfish, especially scallops and sea clams, are gathered by power dredges. Closer to shore, oysters are also collected with dredges or long-handled tongs. Intertida clams are dug at low tide with long-tined rakes. Crabs are dredged or caught in wire traps. Lobsters are caught in traps made of wood or wire.

Advanced Harvesting Technology
Modern fishing fleets, such as those of Russia and Japan, have large factory stern trawlers that can easily haul in netloads of up to 100 metric tons of fish, which are then cleaned and quick-frozen at sea. Such vessels can work distant fishing grounds for many months. Echo sounding has become an effective means of locating and determining the species and size of fish shoals. Airplanes and helicopters are commonly used to detect surface fish. Schools of some species such as squid are attracted with strong lights and sucked into the ship by powerful vacuum pumps.

Depletion of Ocean Stocks
Because of improved technologies, the total world catch of fish more than tripled during the two decades after World War II, then leveled off. Although natural causes for this decline exist, such as changes in ocean currents, human causes such as pollution and overfishing have definitely contributed. Halibut, herring, cod, salmon, anchovy, sardine, and some tuna are now being overfished. To manage and control exploitation of coastal fisheries, Chile declared a 200-nautical-mile (370-km) regulatory limit to foreign fishing in 1945. Several other South American countries did the same. While this was fought by the international community for a time, in 1976 the U.S. enacted the Fishery Conservation and Management Act, which allows foreign ships limited quotas of fish within 200 nautical miles of the U.S. shore. Most nations currently claim 200-mi “exclusionary zones” Because purse seines often trap marine mammals that travel with tuna, the Marine Mammal Protection Act of 1972 placed a quota (20,500) on the number of dolphins that could be killed by U.S. tuna boats yearly. Continued concern about the killing of dolphins resulted in a consumer boycott of tuna. In 1990, the three major canners of tuna sold in the United States stopped buying tuna caught in purse seines. Similar pressure has been exerted on use of drift nets because of the kill of nontarget species. Up to 64 km (40 mi) long, they are used by Japanese and other Asian fishing fleets primarily for squid; however, they also trap mammals, as well as other valuable fish. In 1992, the U.S. Congress banned the importation of fish caught by drift nets.

Future Management of Fisheries
Accurate information on the size of fish populations is hard to gather because ocean fish make long migrations or dwell too deep for proper counts to be made. Management for a sustainable yield is, therefore, very difficult. Investigators are trying to relieve the overfishing of traditional food fishes by exploring methods to process and market less popular species such as squid, hake, and pollack, or the fishes currently used for livestock food. Ocean ranching is common in many areas, and may use fish that gather for spawning. These fish are corralled with stationary weirs and can be used as food and as a source of spawn to propagate young for further ranching. These methods are commonly used for salmon in northwestern North America, where young are released in streams and return as adults to their “home” sites.
Ultimate control of fishery production is found in aquaculture. Eggs are hatched and the fry are put into pens and fed or stocked into seminatural ponds to grow on natural foods. This process, also called mericulture, has become popular because favored food fishes grown in this way fetch high prices. Pen culture of Atlantic salmon in Norway and rearing of marine shrimp in Ecuador are two examples. Culture of local fishes can also provide protein in many developing countries more efficiently and cheaply than wild fish capture.By the year 2000, 20 percent of fishery harvest may come from aquaculture systems.