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FishingHow Do Fish Finders Work?

How Do Fish Finders Work?

The fish finder devices you see advertised in stores that cater to fishing and outdoor activities are based on technology that is several decades old. Understanding how it works will help you get the most out of your fish finder so you can make the big catch.

A fish finder is basically a SONAR device, something that if you have watched old movies about World War II with submarines you probably understand the basics of how it works. An acronym for Sound Navigation Ranging, SONAR sends out waves of sound that travel through the water. When the sound waves strike a fish, vegetation, debris, or the bottom of the body of water, they are reflected back to the device.

SONAR measures how long the sound waves take to return and then translates the pulse into a visual image of what it strikes. The concept is basically the same technique as what bats and dolphins use to find prey. A typical SONAR device will send out up to 15 waves per second. When they return, the waves are converted to electronic signals and translated to visual images that show everything that the sound waves bounced off of on its way to the bottom.

How SONAR Works for Finding Fish

To understand what you are seeing, you have to first know that the sound waves spread out from the device before they return. This means that what you see is not directly under the boat, but rather in a wider area much like a cone that starts at the top and spreads out at the bottom.


SONAR displays its information based on the latest scans which show up on the right side of the screen. Previous scans are still present to the left. This means for the latest results you will need to focus your attention on the right side of the screen.

Surface Clutter

Next, the images you see will need to be interpreted as there is more than fish between the device and the bottom of the body of water. Just as the radar used to track the weather may have reflections of ground clutter, so too will SONAR have reflections of surface clutter. This may be bubbles, currents, or even algae that is below the surface.

Some fish finding devices can be adjusted to compensate for the clutter, but it is possible that it can hide fish that otherwise be in plain view.

Thick Lines

If you see thicker lines, that means the bottom of the body of water is harder. The thickness of the lines helps you determine which objects are softer, such as fish or vegetation. And which objects are harder, such as rocks, debris, and the bottom of the body of water.

Look for the Arches

If you see arches between the bottom and the surface, that is the fish swimming about. This is because the SONAR shows the image of the sound waves striking the fish which from the top would appear to be an arch. This fingernail shape can be tracked to help you find the fish.

And that is how a fish finder works. The more experience you have with the fish finder, the better you can locate the fish.


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