Traditionally, sea fishermen have known the time and place to find their catch, but the history of fishing has demonstrated more than once that even old and rich fishing places can become exhausted quite suddenly. This is especially true with pelagic fish like herring, pilchards, or sardines. The herring yields of the Schonen fishery and later on of the Bohuslaine fishery (1744–1809) in the Baltic Sea fell so severely that the very existence of the Hanseatic League was compromised. This sudden change did not result from overfishing but was caused instead by natural fluctuations in the development of stocks. In modern times, sardine fishing collapsed off the California coast in 1952, followed by the Peruvian anchovy fishery 20 years later. Similar disasters have occurred in other parts of the world not only because of overfishing but also for natural reasons. When this happens new fishing places must be found. It is difficult to explain how good fishing places in great depths were found in ancient times, but fish in shallow waters, fjords, or small bays can easily be seen. On the high seas, fish can be located when they surface temporarily, and fish searching by direct observation from a vessel is important even today. Airplanes and helicopters are commonly used in purse seine fishing. An experienced air-spotting pilot can detect fish under the surface and identify species by observing the shoal’s form or colour or behaviour and sometimes by the presence of accompanying birds. During the night, fishes can be located through the phenomenon of bioluminescence; i.e., when their passage through the water causes tiny marine organisms to luminesce. Accompanying birds have played an important part in fish searching for centuries, because a concentration of birds can be seen from a distance. Very often the birds are not attracted by the fish sought but by smaller fishes and squid, which may have taken refuge from large species by swimming to the surface. Other animals may also indicate fish concentrations by their presence. Porpoises, for example, are known companions of tuna, and tuna purse seiners often set their nets where porpoises have been seen. To find fish in deeper waters by other means was difficult if not impossible in the past. Herring fishermen used signal lines to find their prey in deep waters. These were long wires dropped from a boat; the fisherman holding the line in his hand could feel the vibration caused by the fish touching the line, which was named the herring’s telephone. Other fish were also found by signal lines, often tied with fishing gear. In modern industrial fisheries, experiments have been made with direct listening for fish, but this method has been found impractical. Sea fishermen have also learned to judge where fish can be expected by observing environmental conditions. The colour of the water and the presence of current or of a borderline between different water bodies are some common fish indicators. One of the most important physical properties for fish finding is the temperature of the water. The use of thermometers was one of the first practices fishermen learned from oceanographers, not only for fish finding but also for forecasting availability of the desired species. Aerial and satellite surveys of these properties are becoming of increasing importance.

The first experiments using electrically generated sound pulses and their echoes to locate fish were undertaken in Britain during the 1930s, and by the 1950s fish-finding echo sounders had become an essential aid to catching. As these units worked vertically, they only showed fish immediately below the vessel, so that a logical development was the application of sonar in order to search horizontally around the boat. For many years the machines provided only a “black on white” paper display of the resulting echoes, and interpretation of the displays was dependent upon the skill of the skipper. Gradually, improvements were made in the quality and quantity of information displayed, enabling monochrome signals to be displayed on a television-type screen. A big breakthrough came with the advent of microprocessor technology, which made it possible for fish-finding sonars and sounders to rapidly analyze the signals that their high performance transducers picked up from the sea. Information regarding size, abundance, and movement of the fish is now displayed in many colours, provides the skipper with a wide range of scales, and enables him to focus on and expand information at a particular depth or location.

Other instruments have become vital to fishing operations, especially radio- and satellite-transmitted position-fixing equipment such as Decca Navigator, Loran, and Satnav. These enable a skipper to return to the precise position where fish are spotted or to a particular location such as a coral reef or where gear has been set. Microprocessor technology allows information from various instruments such as sonar, radar, Satnav, and Loran to be fed into a single television screen that provides the skipper with information processed to suit his needs. The vessel’s movements, shown on the screen, can be integrated with navigation and fishing charts fed into the display from computer memory banks. By linking these instruments to the control of winches, engines, and rudder, fully computerized fishing operations are possible.