Fish-Finding Equipment

devices and instruments for locating fish, marine mammals, and other aquatic organisms. The equipment is also used for studying the behavior of fish, for detecting any obstacles on the ocean floor that interfere with the efficiency of fishing gear, and for guiding vessels and fishing gear to aggregations of fish. Most widely used is hydroacoustic fish-finding equipment based on the principles of sonar detection (seeBIOHYDROACOUSTICS, HYDROACOUSTICS, and SONAR TECHNIQUES). In fish-finding equipment involving active detection, a signal given by the equipment is reflected by the aquatic organisms and recorded; passive-action fish-finding equipment detects acoustic signals emitted by the aquatic organisms.

Active fish-finding equipment includes echometers, which search for fish in a vertical plane (underneath the ship’s keel), and sonars, which are designed for locating commercial marine products in all directions relative to the vessel. Most of the equipment operates in a frequency range of 10–200 kHz, with a pulse duration of the signals ranging from 0.1 to several tenths of a millisecond. The detection range reaches a depth of 3–4 km for schools of fish and 1.5 km for an individual fish. Equipment operating on lower frequencies has been developed to increase this range, and equipment operating on higher frequencies has been produced to increase the resolution, which is essential, for example, in studying the structure of fish schools or in recording small objects. To evaluate schools of fish quantitatively, fish-finding equipment is combined with counters (with sparse schools) or integrators (with schools of high concentration). Active-type fish-finding equipment lies at the basis of systems for automating commercial fishing.

Passive fish-finding equipment includes hydrophones and noise indicators, which are used basically in fishery research for studying the sounds of marine organisms, for example, in devising fish barriers or traps. Such equipment is also used for detecting objects whose location by active-type equipment is difficult (for example, tuna and crustaceans). Television units are used for locating benthic commercial products (mollusks, crabs, seaweed) and for studying the behavior of fish in relation to fishing gear.

The use of aircraft and earth satellites is being developed to locate aquatic organisms in vast expanses of water (seeAERIAL METHODS OF EARTH STUDY). Also under development for this purpose are laser and infrared devices capable of detecting thermal contrasts of tenths or even hundredths of a degree.