Written by Tadashi Moody
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A fishfinder is a category of modern sonar device, using sound waves to gain a visual picture of what is below a vessel in the water. As the name implies, fishfinders are often used by anglers to locate fish or schools of fish, but they provide quite a bit more information than just that. They can give us information on everything from water depth to the best location to anchor.

How Do Fishfinders Work?

Sonar--an acronym for Sound Navigation and Ranging--is a time-tested technology wherein high frequency sound waves are transmitted through water, then received as they bounce off of objects of different density than water and return. Fishfinders, as do all sonar devices, use a transducer send lots of short bursts of high frequency sound (often about 24 per second) down below the vessel. The transducer also acts as a receiver, and since sound waves travel through water at known rates of speed (close to a mile per second), sonar devices can calculate the distance to these objects.

Fishfinders translate distances to these objects, whether it's the muddy bottom or a fish in a deep hole, into a visual representation of the what's below on an LCD screen. Since the flesh of a fish is mostly water, one might wonder how accurate sonar would be in locating them. Fish are, however, equipped with swim bladders which allow them to stay at a constant depth, and it is the different density of gas in the bladder which reflects the sound waves back so readily.

Important considerations for the recreational mariner shopping for a fishfinder include screen size and resolution, and features such as zoom, bottom-lock, and whiteline. It is also important to consider the average continual power output of a unit, and to make sure you get the right frequency transducer(s) for the type of water you will be in. Using manual features such as gain can help create a better picture for you, but will take some time to master.

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