fishing in the pelagic zone of a lake, sea, or ocean—that is, in water that is substantially above the bottom, substantially below the surface, and far away from the shore. The implements used in mid-water fishing are determined by the behavior of various schools of fish.
Densely populated and highly mobile schools of fish at depths to 1 km are caught by trawls, which are towed by one vessel (otter trawls) or two vessels (twin trawls). Trawls are guided to schools of fish by means of special fish-finding equipment. Large schools of fish located close to the water’s surface (10–100 m) may be caught efficiently with pelagic seines, including purse seines, lamparas, alamans, and ring nets. Scattered schools of fish are caught with gill nets, which drift with the wind or the current. Drift nets are joined into formations 2–3 km or greater in length. As they drift, they catch fish over a vast area, thus compensating for the low fish concentration. Drift-net fishing is used at depths to 100 m. To catch large pelagic fish weighing 50–100 kg or greater, such as tuna, sailfish, and swordfish, baited drift lines reaching a length of 60–100 km are used. This method makes it possible to fish over an extensive area at depths of 100–150 m.
Through the use of light it is possible to attract large numbers of certain fish species to the water’s surface. Dip nets and fish pumps may then be used to catch the fish. An electric current that controls the behavior of fish and increases the fishing area is sometimes used to raise the efficiency of various types of mid-water fishing gear. Trawlers, seiners, drifters, and tuna-catching vessels are used for mid-water fishing.
A. L. FRIDMAN