Fishfinder

A fishfinder or sounder (Australia) is an instrument used to locate fish underwater by detecting reflected pulses of sound energy, as in sonar. A modern fishfinder displays measurements of reflected sound on a graphical display, allowing an operator to interpret information to locate schools of fish, underwater debris, and the bottom of body of water. Fishfinder instruments are used both by sport and commercial fishermen. Modern electronics allows a high degree of integration between the fishfinder system, marine radar, compass and GPS navigation systems.

Fish finders were derived from fathometers, active sonar instruments used for navigation and safety to determine the depth of water. The fathom is a unit of water depth, from which the instrument gets its name. The fathometer is an echo sounding system for measurement of water depth. A fathometer will display water depth and can make an automatic permanent record of measurements. Since both fathometers and fishfinders work the same way, and use similar frequencies and can detect both the bottom and fish, the instruments have merged.

In operation, an electrical impulse from a transmitter is converted into a sound wave by an underwater transducer, called a hydrophone, and sent into the water. When the wave strikes something such as a fish, it is reflected back and displays size, composition, and shape of the object. The exact extent of what can be discerned depends on the frequency and power of the pulse transmitted. Knowing the speed of the wave in the water, the distance to the object that reflected the wave can be determined. The speed of sound through the water column depends on the temperature, salinity and pressure (depth). This is approximately c = 1404.85 + 4.618T - 0.0523T² + 1.25S + 0.017D (where c = sound speed (m/s), T = temperature (degrees Celsius), S = salinity (per mille) and D = depth). Typical values used by commercial fish finders are 4921 ft/s (1500 m/s) in seawater and 4800 ft/s (1463 m/s) in freshwater.

The process can be repeated up to 40 times per second and eventually results in the bottom of the ocean being displayed versus time (the fathometer function that eventually spawned the sporting use of fishfinding.)

...
Wikipedia