Wiktionary
n. (context fishing English) An electronic device that uses sonar to measure water depth and shows outlines of fish and bottom features on a LCD or CRT screen
Wikipedia
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.
Usage examples of "fishfinder".
He turned on his radar, fishfinder, and GPS and was comforted by the green dials lighting up and showing his position and speed.