COMMERCIAL FISHING

the taking of fish and other seafood from oceans, rivers, and lakes for the purpose of marketing them. Fishing is considered one of the primary forms of food production; it ranks with farming and probably predates it. Hunting-and-gathering peoples of prehistory took what they could from seas and lakes by hand. In many parts of the world, piles of mollusk shells dating back thousands of years attest to such early practices. Fishing techniques developed through the centuries, and, by the Middle Ages, bulk fisheries existed in Europe. So enterprising were the early fishing fleets that they were among the first Europeans to come to the New World, drawn to the excellent cod fishing in the Grand Banks off Newfoundland. The industry became mechanized in the 19th century, and modern commercial fishermen use a variety of power equipment, radar, and underwater sonar; computers are even used to operate large fishing vessels. The fishing industry employs more than 5,000,000 people worldwide. The major countries engaged in marine fishing are Japan, China, the United States, Chile, Peru, India, South Korea, Thailand, and the countries of northern Europe. The aquatic life that is harvested includes both marine and freshwater species of fish, shellfish, mammals, and seaweed. They are processed into a variety of products: food for human consumption, animal feeds, fertilizers, and as ingredients in other commercial commodities. Marine fish constitute about 80 percent of the world's total commercial catch. Those most commonly taken for human food are cod, the various flatfish (flounder, halibut, plaice, and sole), haddock, herring, salmon (Atlantic and Pacific), and tuna (albacore, bigeye, bluefin, bonito, skipjack, and yellowfin). Other important fish are anchovy, grouper, hake, mackerel, menhaden, pilchard, redfish (ocean perch), sardine, sea bass, shad, shark, snapper, sturgeon, and whiting. Freshwater fish are also taken commercially, although they comprise only about 10 percent of the annual global catch. Among those most commonly eaten are bass, carp, catfish, eel, perch, pike, trout, and whitefish. Shellfish, a term that includes all invertebrate marine organisms that have visible shells, are usually divided into two classifications: crustaceans and mollusks. Crustaceans taken for food include the varieties of crab, crayfish, lobster, and shrimp. The mollusks harvested include clams, mussels, oysters, scallops, snails (abalone being the best-known), and whelks. Cephalapods, comprising the species of octopus, squid, and cuttlefish, are also caught for food. The ocean mammals hunted for commercial uses include dolphins, porpoises, seals, walruses, and whales. (The taking of many species of aquatic mammals has been restricted or banned altogether in order to conserve their dwindling numbers or to save them from outright extinction.) Those oceanic mammals that are still taken are mainly used for products other than food, such as whale oil and sealskins. In the Arctic, however, these mammals are a major food source for Eskimos and other inhabitants. Freshwater dolphins are caught for food in such rivers as the Ganges and Amazon. Seaweed, a form of algae, is collected in its natural state and cultivated in various parts of the world, chiefly in the Far East. The different kinds of edible seaweed are usually processed into food and vitamin-rich animal feeds. The types of fish caught fall into two categories: demersal and pelagic. Demersal fish, such as cod, haddock, pollack, and the flatfishes, dwell in deep water, usually near the ocean floor. Pelagic fish, such as herring and tuna, are generally found near the surface. Fishing methods range from simply collecting them near shore without gear to trapping them in enormous mechanically hauled nets. Net fishing, the method that produces the highest yields, is done in several ways, depending on the type of fish sought. Surrounding nets are those that encircle fish before they are hauled in. The most basic of these is the seine net, which is a long net attached to a float line and weighted on the bottom. It is most commonly used on beaches and in freshwater lakes. In pelagic fisheries a frequently used surrounding net is the lampara net, which has a large central bunt (bagging portion) and short wings that trap the fish from the sides and underneath. The purse seine, in which the bottom of the net is drawn shut with a line running through rings, is the most productive net in sea fishery. A widely used net in demersal fisheries is the boat, or Danish, seine. The net is attached to two long towing ropes, one of which is anchored while the other is brought around by boat in a wide circle, trapping the fish. Trawling, in which a boat pulls a bag-shaped net that scoops fish into its open end, follows purse seining in productivity. Heavy bottom trawls are used to catch demersal fish, while midwater trawls catch pelagic species below the surface of the sea. Other netting methods include gill and drift netting, in which long rows of net sections are anchored or allowed to drift, and lift netting, in which fish, attracted to an overhead light, are surrounded from underneath and lifted out of the water. Lining is the familiar method of fishing with hook and line. In pole-and-line fishing, manually operated bamboo poles or automated fibreglass rods are used to catch tuna species in the tropics. Off the coasts of Japan, Taiwan, and Korea, where tuna are also caught, a drifting longline is used. The line is composed of 400 to 450 sections, each with a number of hooked branch lines. The total line can measure up to 110 miles (180 km) in length with as many as 2,000 hooks. The bottom longline is used in northern waters to catch such demersal fish as cod and halibut. A main line is fitted with a great number of hooks and anchored at or near the bottom, with buoys above for markers. Lines are usually set to depths of about 300 to 900 feet (90 to 275 m). Hauling loaded lines or nets is hard labour, and power winches have become common equipment. Fishing is also done with traps. Most fishing traps are simple chambers that do not close mechanically but instead permit easy entry and then make exit difficult. Lobster pots are crates with such one-way apertures. Octopuses are captured in Italy and the Far East using traps that resemble natural hiding places, while the pound nets used for catching salmon resemble undersea coral. Fyke nets are long, tapering sacks with tight apertures fitted on hoops along their length. Miscellaneous fishing methods include the use of spears, especially the harpoons used in whaling; diving to collect various resources from the seafloor, such as oysters and abalone; the use of trained animals, such as the otters employed by Chinese fishermen; and the use of explosives and electric shocks to stun fish, which are then collected from the surface before they regain their senses. The idea of farming the sea is increasingly popular in the fishing industry. In some fishing boats, pumps are used to suck squid and small fish directly out of the water. Another such machine dredges up mud from the bottom and onto ships, where clams and mussels in the mud are flushed out with water jets. Kelp is harvested from the surface and loaded onto vessels with conveyer belts. Carp have been raised in ponds in China for thousands of years, and this practice has spread throughout the world. Natural bodies of water are stocked with artificially hatched trout. Virtual underwater oyster farms are operated, in which parallel vertical racks of growing oysters are hung. The collected fish and shellfish are processed and marketed fresh, frozen, or canned. Some are also preserved by salting or smoking. These products are high in protein and usually rich in vitamins and minerals but low in calories (compared to meats). Aquatic organisms have always been a staple in the diet of people living near bodies of water. Modern preserving techniques and transportation networks have made it possible for people throughout the world to consume these products on a regular basis. The principal by-products of fish are oils and meal, usually made from fish that are less desirable for human consumption. A large portion of the fish oils and meal that are processed comes from anchovy, herring, mackerel, menhaden, pilchard, and pollack, as well as from the wastes from fish industries. Fish oil is pressed from the flesh and then refined; it is used in products ranging from paints to margarine. Fish meal is made by drying and grinding the fish; it is used as a high-protein animal feed or as a feed additive. Other products include such varied items as salmon and whitefish roe (roe from sturgeon is called caviar), leather from shark skins, and fish protein concentrate from meal. By-products are also made into such items as glue, isinglass (fish gelatin), and pharmaceuticals. By-products from shellfish include natural and cultured pearls from oysters, animal feeds rendered from the ground shells of various species, and costume jewelry chips, buttons, and other ornamental items. The cellulose and polymer carbohydrates of seaweeds are made into such thickening agents as agar, algin, and carrageenan. the taking of fish and other seafood and resources from oceans, rivers, and lakes for the purpose of marketing them. Fishing is one of the oldest employments of humankind. Ancient heaps of discarded mollusk shells, some from prehistoric times, have been found in coastal areas throughout the world, including those of China, Japan, Peru, Brazil, Portugal, and Denmark. These mounds, known as kitchen middens (from the Danish kkkenmdding), indicate that marine mollusks were among the early foods of humans. Archaeological evidence shows that humans next learned to catch fishes in traps and nets. These ventures were limited at first to the lakes and rivers, but as boats and fishing devices were improved, humans ventured into sheltered coastal areas and river mouths and eventually farther out onto the continental shelves, the relatively shallow ocean plains between the land and the deeper ocean areas. In some shelf areas where seaweed was abundant, this was also incorporated into the diet. Fishing technology continued to develop throughout history, employing improved and larger ships, more sophisticated fishing equipment, and various food preservation methods. Commercial fishing is now carried on in all types of waters, in all parts of the world, except where impeded by depth or dangerous currents or prohibited by law. Commercial fishing can be done in a simple manner with small vessels, little technical equipment, and little or no mechanization as in small local, traditional, or artisanal fisheries. It can also be done on a large scale with powerful deep-sea vessels and sophisticated mechanical equipment similar to that of modern industrial enterprises. Both plants and animals are taken from the sea. Two types of fish are caught: demersal, living at or near the bottom, although sometimes in mid-water; and pelagic, living in the open sea near the surface. Cod, haddock, hake, pollock, and all forms of flatfish are common demersal fish. Herring and related species and tuna and their relatives are examples of pelagic fish. Both demersal and pelagic fish can sometimes be found far from coastal regions. Other aquatic animals that may be the object of commercial fishery include, most notably, crustaceans (lobsters, spiny lobsters, crabs, prawns, shrimps, crayfish) and mollusks (oysters, scallops, mussels, snails, squid, octopuses). Certain mammals (whales, porpoises), reptiles (serpents, crocodiles), amphibians (frogs), many types of worms, coelenterates (coral, jellyfish), and sponges are also sought by commercial fishermen. Most of these animals are legally regarded as fish in many countries. The most important water plants commercially obtained in seawater and fresh water are algae. Seaweed is harvested in the water or collected on the seashore. Algae play an important ecological role in many countries, not only as human food but also as fodder for cattle, as fertilizer, and as a raw material for certain industries. Fisheries are classified in part by type of water: fresh waterlake, river, and pondand salt waterinshore, mid-water, and deep sea. Another classification is based on the objectas in whaling, salmon fishing, and sponge fishing. Sometimes fisheries are classified according to the method of fishing employed: harpooning, seining, trawling, and lining. This article discusses organized fishing for profit, with an emphasis on mechanized industrial methods, gear, and vessels. The history and methods of whaling, which is less fishing than the hunting of an aquatic mammal, are discussed separately in the article whaling. For angling, or recreational fishing, see the article fishing. For information on the use and value of fish and marine products as food, see the article nutrition, human. Andres R.F.T. von Brandt Georg A. Borgstrom Philip F. Purrington The Editors of the Encyclopdia Britannica Additional reading Commercial fishing The productivity of the seas and the exploitation of the living resources in the world's oceans are illustrated in Food and Agriculture Organization of the United Nations, Atlas of the Living Resources of the Seas, 4th ed. (1981). Organisation for Economic Co-operation and Development, Multilingual Dictionary of Fish and Fish Products, 2nd ed. (1978, reprinted with corrections, 1984), provides comprehensive information on fish, fish products, and other marine life used commercially. Scientific research supporting commercial fishing is discussed in Taivo Laevastu and Murray L. Hayes, Fisheries Oceanography and Ecology (1981); Tony J. Pitcher and Paul J.B. Hart, Fisheries Ecology (1982); and Maurice E. Stansby, Industrial Fishery Technology, 2nd ed. (1976). Developments in commercial fishing are detailed in studies of local areas, such as Peter Pownall, Fisheries of Australia (1979); and Peter R. Sinclair, State Intervention and the Newfoundland Fisheries (1987). C.R.P.C. Branson (ed.), Fishermen's Handbook, 2nd ed. (1987), explains and illustrates many situations occurring in commercial fishing. Equipment, facilities, and methods John C. Sainsbury, Commercial Fishing Methods: An Introduction to Vessels and Gears, 2nd ed. (1986), describes the basics of the fishing industry. Andres von Brandt, Fish Catching Methods of the World, 3rd rev. ed. (1984), surveys fishing methods and equipment, from the most primitive to the totally computerized and automated. Dag Pike, Fishing Boats and Their Equipment (1979), examines the design factors of boats and their effect on fishing productivity. Jan-Olof Traung (ed.), Fishing Boats of the World, 3 vol. (195567), provides excellent basic information on boats for various fishing methods. Specific methods and technologies are discussed in Echo Sounding and Sonar for Fishing (1980), a Food and Agriculture Organization fishing manual; David B. Thomson, Seine Fishing: Bottom Fishing with Rope Warps and Wing Trawls, rev. ed. (1981), and Pair Trawling and Pair Seining: The Technology of Two-Boat Fishing (1978); J.H. Merritt, Refrigeration on Fishing Vessels, rev. ed. (1978); and J.J. Connell (ed.), Advances in Fish Science and Technology (1980). Fishing Ports and Markets (1970) is a collection of papers from a conference held under the auspices of the Food and Agriculture Organization. Fisheries The resources, ecology, and management of marine and freshwater fisheries are discussed in Robert J. Browning, Fisheries of the North Pacific: History, Species, Gear, & Processes, rev. ed. (1980); Robert T. Lackey and Larry A. Nielsen (eds.), Fisheries Management (1980); Frederick W. Bell, Food from the Sea: The Economics and Politics of Ocean Fisheries (1978); Stephen Cunningham, Michael R. Dunn, and David Whitmarsh, Fisheries Economics (1985); G.C. Eddie, Engineering, Economics, and Fisheries Management (1983); Toivo Laevastu and Herbert A. Larkins, Marine Fisheries Ecosystem: Its Quantitative Evaluation and Management (1981); Robin L. Welcomme, Fisheries Ecology of Floodplain Rivers (1979); Robin G. Templeton (ed.), Freshwater Fisheries Management (1984); and Shelby D. Gerking (ed.), Ecology of Freshwater Fish Production (1978). Ongoing research and developments in the field are reported in Fisheries of the United States (annual); and Ocean Yearbook. John C. Sainsbury Dag Pike Aquaculture Various aspects of commercial fish farming throughout the world are discussed in Elisabeth Mann Borgese, Seafarm: The Story of Aquaculture (1980); Robert Kirk, A History of Marine Fish Culture in Europe and North America (1987); Marcel Huet, Textbook of Fish Culture: Breeding and Cultivation of Fish, 2nd ed. (1986; originally published in French, 1970); E. Evan Brown, World Fish Farming: Cultivation and Economics, 2nd ed. (1983); Malcolm C.M. Beveridge, Cage Aquaculture (1987); James E. Lannan, R. Oneal Smitherman, and George Tchobanoglous (eds.), Principles and Practices of Pond Aquaculture (1986); Robert R. Stickney, Principles of Warmwater Aquaculture (1979); Fredrick W. Wheaton, Aquacultural Engineering (1977, reprinted 1985); John P. Stevenson, Trout Farming Manual, 2nd ed. (1987); Robert R. Stickney (ed.), Culture of Nonsalmonid Freshwater Fishes (1986); and Fredrick W. Wheaton and Thomas B. Lawson, Processing Aquatic Food Products (1985). A good overview article is William H. Queen, Aquaculture: A Renewal of Interest, Currents, 3(3):2025 (1988). Clyde H. Amundson Fishery equipment and facilities Gear An international classification of fishing methods includes 16 categories, depending upon the fishing gear and the manner in which the gear is used: (1) fishing without gear, (2) grappling and wounding gear, (3) stunning, (4) line fishing, (5) trapping, (6) trapping in the air, (7) fishing with bag nets, (8) dredging and trawling, (9) seining, (10) fishing with surrounding nets, (11) driving fish into nets, (12) fishing with lift nets, (13) fishing with falling gear, (14) gillnetting, (15) fishing with entangling nets, and (16) harvesting with machines. Hand tools The simplest and oldest form of fishing, collecting by hand, is still done today by both professionals and nonprofessionals along the shore during ebb tide in shallow water and in deeper water by divers with or without diving suits. Even when small tools such as knives or hoes are used, such collecting is classified as without gear. Diving to collect sponges, pearl oysters, or corals belongs under this classification, as does fishing with hunting animals. The Chinese still use trained otters, and the Japanese sometimes employ cormorants. To extend the reach of the human arm, long-handled tools were invented, such as spears, which can be thrust, thrown, or discharged, and clamps, tongs, and raking devices for shellfish harvesting. A special form is the harpoon, composed of a point and a stick joined together by a rope. Such grappling and wounding gear also includes spears, blowpipes, bows and arrows, and rifles and guns, which are used in fish shooting. The method called stunning may involve poisoning with toxic plants and special chemicals or mechanical stunning by explosions under water. The most modern practice in this field is to stun the fish by means of an electrical shock. Fishery equipment and facilities Vessels Until the mid-20th century, fishing boats were largely of local design, with different types found even in adjacent ports. As fishermen started to roam farther afield for their catches, the vessels grew, and with this growth in size came an element of standardization in design. Today, fishing boat design and construction is an international industry, with the different vessel types dictated more by the fishing methods for which they are designed rather than by their port or country of origin. The establishment of 200-mile fishing limits (see above History of commercial fishing) has altered fishing patterns and, with them, the types of vessels used by many countries. In the United States and Canada, fishing vessels have grown with the introduction of processing or factory trawlers, while the huge fleets of this type of vessel operated by Soviet-bloc countries and Japan have shrunk. In western Europe, compact fishing vessels have been developed with high catching power. The advantage of these smaller vessels is their reduced capital and operating costs. Steel is the most common construction material, being used exclusively on larger vessels (above 25 metres). Traditional wood construction is less common because of cost and a lack of suitable timber in many areas. The use of fibreglass is increasing in smaller fishing vessels, and it is now used on vessels of up to 25 metres in length. Ferrocement has been used to a certain extent; it is mainly used in the artisanal fisheries of developing countries because, while its construction is labour intensive, its raw materials are cheap. The aim in all fishing-boat development is to improve efficiency by building vessels that have higher catching power, smaller crews, and reduced operating costs. This development must be matched against safety concerns, as commercial fishing is one of the highest risk industries in the world. Several countries have introduced regulations governing the construction and operation of fishing vessels. The International Maritime Organization, convened in 1959 under the auspices of the United Nations, is responsible for devising international regulations covering such aspects of fishing vessel design as construction, stability, safety equipment, and watertight integrity. These regulations are likely to lead to further standardization in design. The Food and Agriculture Organization of the United Nations has introduced a classification scheme of fishing vessels based primarily on the gear used. Trawlers Most trawlers are single-screw vessels with powerful engines and deck machinery for dragging the trawl nets. Types of fishery Salt water Fishing in salt water ranges from small, traditional operations involving one person and a rowboat to huge private or government enterprises with large fleets for deep-sea and distant fisheries. The Law of the Sea extended from 12 to 200 miles an exclusive economic zone (EEZ) within which a coastal country has control over fisheries and their exploitation. This effectively restricts most fishing operations on the continental shelves to national vessels or to craft licensed by that country. Within the EEZ, fresh water and coastal waters are often demarcated by law, with fishing within, for example, three miles of the coast allocated only to small-scale, non-trawling fishermen and larger industrial vessels required to remain farther offshore. Small-scale fishermen are usually not restricted to the three-mile zone, and they often may be found well offshore or along the coast from their home ports as they follow the fish. For example, West African canoe fishermen traditionally migrate hundreds of miles coastwise in open canoes, frequently fishing out of sight of land. Andres R.F.T. von Brandt John C. Sainsbury The oceans The oceans constitute the largest factories of living organic matter on Earth, in both magnitude and total productive biomass. Average organic production per acre is identical to that on land, although productivity varies greatly from one area to another, ranging from luxuriance to almost barren deserts. Production in any specific area varies with the seasons and is subject to large and sporadic fluctuations. The primary production area of the oceans is the photic zone, the relatively thin surface layer, 25 fathoms (50 metres) deep, that can be penetrated by light, allowing the process of photosynthesis, the use of energy derived from sunlight in the manufacture of food, to take place. All marine life is directly or indirectly tied to the photic zone, on which both recycling and decomposition, also in other spheres of the ocean, depend. Those few microorganisms deriving their energy from sources other than light have relatively little significance in the overall productive balance of the oceans. In the photic zone, growth rate depends on light intensity and available nutrients. Nutrients are constantly depleted by the slow sinking toward the bottom of dead plankton, the floating and mainly miniature plant and animal life, which forms the primary link in the ocean food chain. Simultaneously, fertility is constantly restored as the nutrient-rich deeper waters are brought to the surface. The ocean is ploughed by the action of winds drifting surface waters away from coastal areas, by nutrient-rich waters welling up from the depths, and during the winter season of the temperate regions by cooled surface waters becoming heavier and sinking downward, forcing nutrient-rich waters to rise. As a rule tropical surface waters do not interchange with the mineral-supplying waters below as much as those of colder regions and are therefore less productive. However, under certain conditions in some regions of the tropics and subtropics, currents and winds induce a sustained upwelling of mineral nutrition from lower strata, producing spectacular results. Such regions include the waters around the west coasts of southern Africa and South America. Consideration of such conditions demonstrates that the production of fish-supporting plankton is not related to latitude but depends upon the presence of new water high in nutrient salts. The marine food chains, ranging from minute floating phytoplankton, sometimes called the grass of the sea, to the large predatory species, have many more links than terrestrial equivalents. Each transfer of food value from a lower to a higher level involves a considerable loss in the amount of recoverable organic matter, and consequently of food, so that the amount of organic matter is much greater at the plankton level than it is in fishes. The daily production of dry organic matter in kilograms per square metre beneath the surface of the English Channel is as follows: phytoplankton (plant life) 45; zooplankton (animal life) 1.5; pelagic fish (living near the surface) 0.0016; bottom fish 0.0010. The plankton eaters, although they tend generally to be small in size, include the basking and whale sharks, the largest of all fishes. Typical consumers of marine plankton include such species as herring, menhaden, sardines, and pilchards. Because of this plentiful food source, these fish exist in tremendous numbers, forming the basis of important fisheries. Demersal fishes, including such species as haddock and halibut, live primarily near the ocean floor, where they feed on various invertebrate marine animals. Most of the large fish, such as tuna, swordfish, and salmon, feed on smaller fishes. Types of fishery Aquaculture Aquaculture is the propagation and husbandry of aquatic plants and animals for commercial, recreational, and scientific purposes. This includes production for supplying other aquaculture operations, for food and industrial products, for stocking sport fisheries, for producing aquatic bait animals, for fee fishing, for ornamental purposes, and for use by the pharmaceutical and chemical industries. These activities can occur both in natural waters and in artificial aquatic impoundments. Aquaculture has been in existence since at least 500 BC. However, only in recent times has it assumed commercial importance, with world production more than doubling between 1970 and 1975. The rapid expansion of aquaculture has been to a large extent in the production of relatively high-priced species frequently consumed as a fresh product. Examples are shrimp, crayfish, prawns, trout, salmon, and oysters. However, also increasing is the production of catfish, carp, and tilapias, which are reared in extensive, low-energy systems. For example, catfish farming in the United States has more than quintupled its production since it began to grow in the 1960s. The growth of world aquaculture has been stimulated by a number of factors, including population increases, dietary shifts, and advances in aquaculture technology. Limited ocean resources have also helped to create a growing role for aquaculture in helping to meet increasing demands for fish and shellfish. Farming and rearing in hatcheries Fish farming as originally practiced involved capturing immature specimens and then raising them under optimal conditions in which they were well fed and protected from predators and competitors for light and space. It was not until 1733, however, that a German farmer successfully raised fish from eggs that he had artificially obtained and fertilized. Male and female trout were collected when ready for spawning. Eggs and sperm were pressed from their bodies and mixed together under favourable conditions. After the eggs hatched, the fish fry were taken to tanks or ponds for further cultivation. Methods have also been developed for artificial breeding of saltwater fish, and it now appears possible not only to rear sea animals but also to have the complete life cycle under hatchery control. Types of fishery Fresh water Freshwater fishing is carried out in lakes and rivers or streams and to a growing extent in natural and artificial ponds. In some tropical areas, swamps with shallow water, sometimes overgrown with vegetation, are important inland fisheries. Before efficient transportation and distribution of ocean fish was organized, fresh waters were the only resource available for fish and other aquatic products for the inland population. Their importance decreased with the growing bulk fisheries of the seas. Freshwater fish now compose only about 5 percent of the total catch of water products of the world. General characteristics Widely different freshwater speciesfeeding on bacteria or detritus, plants or plankton, or living as predatorsare used for human consumption. Well-known species include trout and whitefish, carp and other cyprinids, catfish, murrals, and tilapias. The desirability of some anadromous fishesthose, such as salmon and sturgeon, that spawn in fresh water but live in the seaand catadromous fishesthose, most notably the eel, that spawn in the sea but live in fresh waterhas led to specialized fisheries in inland waters. The kind and quantity of fish found in lakes and rivers vary greatly with the physical and chemical condition of the water. Limnologists, scientists who study conditions in fresh water, classify fresh waters by the quantity of oxygen and essential nutrient salts (nitrates, phosphates, and potash) they contain. Fishermen classify waters by the principal fish to be caught therein. Rivers, for example, are divided into different zones beginning with the source, which is often good trout water, and ending in the estuary, where many coastal varieties of ocean fish can be caught. In like manner, fishermen classify lakes by expected catch (e.g., eels, tilapias, or crayfish). The great variations in the productivity of inland waters are explained by differences in their physical and chemical properties. Though some rivers may produce as much as 200 kilograms per hectare (180 pounds per acre) each year and some lakes may yield 160 kilograms per hectare, the world average is about eight kilograms per hectare. Pollution produced by chemical preparations applied for agricultural purposes has created serious problems for the world's freshwater fisheries; fish cultivation is increasingly restricted to man-made waters. Traditional freshwater fisheries still supply basic protein to China, Southeast Asia, and tropical Africa but have been seriously affected in the United Kingdom, continental Europe, Japan, Central Asia, and the United States. Because of pollution, freshwater fishing in natural waters has declined in industrial countries, but pollution is not totally to blame. The rapid rise in angling as a leisure pastime has created competition for the available waters and the fish in them. Because angling interests can afford higher prices for the rights to available waters, angling is now virtually the only fishing for wild fish that takes place in natural waters in industrialized countries. Some fish species that are considered delicacies and attract high prices are exempt from this trend. Fishing for salmon, eels, and crayfish is still very active on a commercial basis. With these fisheries there are many traditional rights to fishing certain waters. In nonindustrialized countries freshwater fishing has increased considerably, mainly under the influence of aid programs. Some of these programs have tried to introduce new and more efficient fishing methods, but the main improvement has been in mechanization of the fishing boats used and in improved methods of preserving and distributing the catch. On some of the larger inland lakes, freshwater fishing is still the primary occupation in the villages along the shore. Fish farming for freshwater species is being introduced in developing countries to produce a valuable source of protein. Where natural waters are fished in developing countries, fish management techniques are being used to improve the catch and to prevent overfishing.