Fishfinder

A fishfinder or sounder (Australia) is a sonar instrument used on boats to identify aquatic animals, underwater topography and other objects by detecting reflected pulses of sound energy, usually during fishing activities.

Fishfinders were derived from fathometers, active sonar instruments used for navigation and safety to determine the depth of water.

[2] 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.

[3] When the wave strikes something such as a fish, it is reflected back and displays size, composition, and shape of the object.

Many modern fishfinders also have track-back capabilities to check changes in movement in order to switch position and location whilst fishing.

When the fish swims directly under the transducer, it is closer to the boat so the stronger signal shows a thicker line.

When no predators are nearby, a school of baitfish frequently appears as a thin horizontal line across the screen, at the depth where the temperature and oxygen levels are optimal.

Originally the subject of controversy due to its perceived unfair advantage, it faced the potential of being banned by some states but its use was eventually accepted.

[7][8] By the early 1970s, a common pattern of depth finder used an ultrasonic transducer immersed in water, and an electromechanical readout device.

A neon lamp mounted on the end of an arm was rotated around a circular scale at a fixed speed by a small electric motor.

Like today's low-end digital fathometers, they kept no record of the depth over time and provided no information about bottom structure.

Nowadays, many fishfinders available for hobby fishers have color LCD screens, built-in GPS, charting capabilities, and come bundled with transducers.

Fathometers of the constant recording type are still mandated for all large vessels (100+ tons displacement) in restricted waters (i.e. generally, within 15 miles (24 km) of land).