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If you think reading a fish finder is like looking at a television monitor, you have a lot to learn. It would be great if it were that easy. But in reality, reading a graph can be a challenge even to the most advanced fishermen. As mentioned in the previous article, the sonar constantly sends signals in the shape of a cone and whatever gets picked up is displayed at the right side of the monitor. Unless you see a continuous line touching the right side of the monitor, what you see on the screen is no longer in the cone. The majority of what you see is what was under the boat - not what is under the boat.
To make matters more complicated, if a fish passes through the cone, you can't tell what direction it came from or which way it went. You also have to consider boat movement. If your boat is stationary and a fish passes through the cone, you know it was the fish that moved. But even if you are stationary, when a wave moves the boat up and down, everything in the cone will also go up and down. The bottom line is that you are always trying to interpret a moving target.


Obviously, a depth finder is designed to show the depth. Most units not only have a range on the side to show the depth, but they also have a digital reading of the current depth. As the boat moves, the distance between the transducer and the lake bottom at the center of the cone is displayed on the unit (in feet).

Bottom Composition

You can tell whether the bottom is hard or soft by how much gray is displayed at the bottom. Usually, the more gray, the harder the bottom.


You can get an idea of what kind of cover is below by how the objects look and the depth of the water. Trees will usually be spikes from the bottom and will often have gray inside them. Grass will be a thick, uneven band at the bottom with no gray inside it. Grass doesn't usually grow in deep water, so you will probably only see it in less than 15 ft of water.


During the summer months, most larger lakes will have a thermocline that you can actually see on a good graph. The thermocline is the area where the water temperature makes the most drastic change. This will show as a horizontal bar on your graph. It is very advantageous to find this thermocline in the summer because this is where a large majority of fish will be during the day.

Fish arcs

The reason a graph will display fish as arcs is because of the cone shape of the sonar waves. The further away from the center of the cone, the greater the distance to the signal and the weaker the signal. When a fish enters the center of the cone, the signal is stronger and the distance is shorter. So the signal starts out deep and faint, gets shallower and stronger, and then gets deep and faint again. The result is an arc.
An arc can be created by either the boat moving across a fish or a fish moving under the boat. An arc that starts low and goes up but doesn't go back down may well be a fish moving upward. Likewise, a downward arc indicates a fish going down. A unit must have a fast chart speed and high resolution in order to display an arc. If the fish ID is turned on, it will show a picture of a fish instead of an arc.


Baitfish usually group up close together by the hundreds or thousands. When you have this many signals constantly getting received all over the cone, the result is a blob or balls of baitfish.

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