A fishfinder is a type of fathometer, both being specialized types of echo sounding systems, a type of Active SONAR. ('Sounding' is the measurement of water depth, a historical nautical term of very long usage.) The fishfinder uses active sonar to detect fish and 'the bottom' and displays them on a graphical display device, generally a LCD or CRT screen. In contrast, the modern fathometer (from fathom plus meter, as in 'to measure') is designed specifically to show depth, so may use only a digital display (useless for fish finding) instead of a graphical display, and frequently will have some means of making a permanent recording of soundings (which are merely shown and subsequently electronically discarded in common sporting fishfinder technology) and are always principally instruments of navigation and safety.
