Fishfinder

Cabin display of a commercial or oceanographic fathometer sonar
Fathometer sonar

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 a body of water. Fishfinder instruments are used both by sport and commercial fishermen. Modern electronics allow a high degree of integration between the fishfinder system, marine radar, compass and GPS navigation systems.