Modern fish finders utilize technology that was invented during World War II to help destroyers locate German submarines. Although underwater locating devices have changed dramatically over the last 50 years, they still work on the same basic principle as the very first units.
“Sonar” is an acronym for sound, navigation and ranging and works like this: The unit sends out an electronic signal that bounces off objects and returns to your depth finder’s transducer. Because sound travels at a uniform speed, the depth finder can measure the distance to an object based on the amount of time it takes for the signal to return.
Although quality sonar units do indeed show fish, they can also show objects suspended in the water that many anglers mistake for fish. That’s why so many professional anglers don’t pay much attention to those fish-indicating arches that show up on their screens. Also, depth finders can’t determine the species of fish that might be under the boat, so even if you do see a fish holding on the lip of a ledge, it could be anything – a carp, catfish, striper or largemouth.