Similarly operated are entangling nets, single or double walled, and three-walled trammel nets. These are used in sea fisheries for hake, shark, rays, salmon, sturgeons, halibut, plaice, shrimps, prawns, lobster, spiny lobster, king crabs, and turtles. Single-walled nets are used in the southern part of the Caspian Sea and in the Black Sea to catch sturgeons by entangling. Iranian fishermen set about 150 sturgeon nets in one row perpendicular to the shoreline. Setting requires much labour; between each two nets a line is tied, which is connected to a short wooden peg driven into the bottom. The Turkish Black Sea fishermen sometimes set sturgeon nets in another form. Two nets always form an angle open to the sea. The nets are held by sticks rammed into the bottom. Sturgeon nets are checked once or even twice each day, depending on weather. For this purpose an Iranian fisherman lies on the bow of his sailboat, towing the vessel along the float line of the net. The sturgeons are taken from the water by hand or with a gaff.

The most important sea fishery for crustaceans is the king crab fishery in the northern Pacific. For the Japanese, who use entangling nets, this is a very important distant fishery ranking with tuna and salmon fishing. Originally carried on close to shore, king crab fishing was extended in the northern Pacific after its beginnings in the 1870s. The old land stations for processing were replaced by floating factories that accompanied the fishing vessels. The entangling nets are set on the bottom, sometimes 200 nets with a total length of 10 kilometres in one row. Larger catching vessels set 1,200 to 1,300 nets a day, usually in parallel rows about 500 metres apart. Nets stay in the water from five to seven days and are hauled by small open vessels with motor-driven reels, which can take from 2,500 to 3,000 nets per day out of the water. When hauling, the floats and sinkers are untied and the entangled king crabs are taken from the netting. The catch and nets are then transported to the mother ship, where the catch is processed and the nets cleaned, an operation that may require 30 minutes per net. Large racks for drying and cleaning the entangling nets are characteristic of this type of vessel. A single fishing unit may own a permanent set of 15,000 to 30,000 nets.

Harvesting machines

A relatively new type of fishing gear is the harvesting machine combined with a pump, used in the northern part of the Caspian Sea for sardinelike fish and for squid off the California coast. In both cases the prey is attracted by light. Squid fishing can be done near the surface, but in the Caspian the fish are sucked on board with pumps from depths as great as 110 metres. In pumping, the suction nozzle is moved up and down with attracting lamps. Once on board the fish or squid are strained from the water. The difficulty in fish pumping is to avoid damage to the catch. Only small objects can be pumped without injury.

Another type of harvesting machine is the hydraulic dredge, with pumps and conveyors. These dredges wash out deeply buried mussels with jets of water under high pressure. The Americans operate such hydraulic dredges to harvest soft clams, and the British use similar machines for cockles. Harvesting machines also are used to cut kelp off California. Giant kelp is harvested by cutting to a maximum depth of 1.2 metres below the surface of the water and is transferred by conveyor belt into the open hold of the vessel.

Andres R.F.T. von Brandt John C. Sainsbury

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 species—feeding on bacteria or detritus, plants or plankton, or living as predators—are used for human consumption. Well-known species include trout and whitefish, carp and other cyprinids, catfish, murrals, and tilapias. The desirability of some anadromous fishes—those, such as salmon and sturgeon, that spawn in fresh water but live in the sea—and catadromous fishes—those, most notably the eel, that spawn in the sea but live in fresh water—has 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.

Methods

Many techniques are employed to catch fish in inland waters, some appropriate to lakes alone, some to rivers only, and some to both. Of the many methods employed worldwide, only a few are economical for large-scale operation. Commercial line fishing, which uses many hooked and baited branch lines tied to a single main line, is widely practiced. A simpler technique is handlining, in which single lines with baited hooks are tied to small sticks or trees along the shore or to special devices set along the side of a hole in ice. Handlining is used for deep fishing or for catching in rocky areas. Drifting lines with one or more hooks can also be used on lakes, though seldom in rivers. Lines may also be trolled (trailed) behind a moving boat. On some rivers in tropical and temperate areas, fish are caught by fouling with sharp-pointed hooks. The main difficulty in line fishing is to keep the lines clear and to obtain baits in needed quantity.

Passive and stationary fishing gear is so important in many lakes and rivers that some fishermen specialize entirely in trapping. Since deep and rocky shores, however, do not favour the use of traps, these devices cannot be used in all areas. Fish seeking shelter may be caught in simple brushwood devices when the brushwood is lifted quickly. More important are traps, such as wooden baskets made of wickerwork or of split bamboo, with retarding devices such as funnels or valves at the entrance. Wooden baskets, generally used in rivers with strong currents, can be set according to the longline system in which the baskets are tied with branch lines on a main line lying across the bottom of the river. Such baskets are usually baited, with the bait sometimes held in small bags or boxes. Today, in river fisheries as in coastal sea fisheries, traps, especially those used for eels, are made of plastic.

A more modern type of trap is the bag-shaped fyke net, held open by hoops; linked together in long chains, these are used to catch eels in rivers. When equipped with wings and leaders, fyke nets are employed in lakes where there are sheltered places with abundant plant life. Hundreds of such nets can be combined into systems where it is not economical to build large traps.

Another fishing method important in freshwater fisheries employs small scoop nets or large net bags (stownets). Such gear is known on many European and Asian rivers. The net bag is fixed to the river bottom to catch migrating or drifting fish. Some human control may be necessary; sometimes a watchman lives on a vessel or raft next to the stownet or on a special platform. Though stownets are especially popular in European rivers for eel fishing, their importance is lessening owing to increased boat traffic and to pollution. Moreover, the gear can be too large to be moved easily. In Indonesia stownets up to 100 metres long and up to 40 metres across the mouth are used. Small scoop nets can be operated by hand and pushed or towed over the bottoms in shallow waters. Sometimes this is done by fishnetting parties, in which all the men of a village form a line across the river, with a scoop net in each hand. Sometimes the fisherman stands on a platform built on the side of the stream and simply scoops up fish as they pass; this is done by some African fishermen in Malaŵi and was done by American Indians on the Columbia River in Oregon.

One of the most common fishing methods in freshwater lakes and rivers is seining, which is done in temperate zones especially in autumn and winter when fish are concentrated in deeper parts of the lakes. Because part of the seine must be dragged over the lake bottom while it surrounds the fish, seining is practicable only where the lake bottom is smooth and where favourable areas (i.e., with fish concentrated near the bottom) are known. In some lakes such areas have been known for a long time and may be named and marked on fishing maps. Seine nets in lake fisheries can be very large, with wings of 1,000 metres each. Since traditional seining required considerable labour, mechanization became desirable. A modern mechanized seine-net fishery requires only a small labour force. In northern countries seine nets are used under ice. For this purpose a number of ice holes are needed for guiding the towing warps with the net on the underside of the ice sheet. Here also manpower is saved by motorized towing and coiling lines and by drilling the holes in the ice with power drills. Some success has been achieved in increasing the efficiency of seine nets by electric light. Fish trying to evade the net can be caught by stunning, or eels lying in the mud during the cold season (generally a time when eel fishing is poor) can be attracted out of the mud by an electrical current. The disadvantage of all seine nets is that they are not selective; many undersized fish that should be preserved cannot escape.

On larger lakes, sea-fishing methods such as trawling and purse seining are used. Two or more fishing boats are usually required to set a purse seine net, which can then be hauled in manually by people on the shore. Lift nets are often used in fresh water, not only to catch bait fish for line fishing but also to catch crayfish or other freshwater crabs. There are small hand-operated lift nets tightened by frames and larger ones lifted from a gallows or with one or two vessels. Unframed blanket nets are used in rivers in Italy, each corner held by a gallows placed on the banks. Cover pots and cast nets also have some importance in commercial freshwater fisheries. Cover pots are especially used in rice fields or shallow waters with rich vegetation. Cast nets are used more in clear waters in lakes and in rivers; considerable skill is required to cast these. In Russia shooting mechanisms are employed to cast larger nets. Much more important in freshwater fisheries, however, are gill nets. The mesh size of the net can be used to regulate the size of the fish caught; thus smaller, undesired species escape. Lake fishermen use mostly stationary gill nets, anchored near the bottom or floating. River fishermen use gill nets that drift with the current, with one side tied to the boat and the other to a drifting buoy. With entangling two-walled nets and trammel nets, yield can be increased by frightening the fish into the netting; this is accomplished by beating the water or throwing stones.

Andres R.F.T. von Brandt Dag Pike