WHAT AM I SEEING ON MY SONAR SCREEN?
Think of your sonar screen as a composite picture of individual sonar returns. The transducer sends intermittent sound waves or “pings” down into the water and records how long it takes for each ping to return to the transducer as well as how loud that signal is when it comes back from the bottom or other object.
The sound waves are broadcast into the water in somewhat of a "cone" shape or circle that is narrow at the transducer and widens until it meets the bottom. Only objects that are in the cone or "view" of the transducer can be detected and displayed on your screen.
Think of each horizontal pixel on your screen as one ping or one still photograph of what was in the cone at that instant and how far that object was from the transducer (depth). The number of horizontal pixels depicts how long that object was in the cone and not necessarily how long the object is. As long as the object stays within the view of the transducer the object will continue to be displayed across the screen.
Think of each vertical pixel as one volume setting. The louder the ping comes back the more vertical pixels it will “paint” depicting higher volume. More vertical pixels means the sound wave bounced off a harder, thicker or denser object. The relative density of each object is also displayed as different colors (on a color sonar) and different levels of gray (on a grayscale graph).
The display on your screen is simply a short history of what your transducer passed over or what passed under your transducer. The most recent information comes into view on the right side of your screen and then moves across and off your screen to the left. Depending on your boat speed and Scroll Speed (speed at which information passes across your screen), what you’re seeing on your screen, especially as it approaches the left side of the screen, isn’t necessarily under your boat and can actually be quite a ways behind you if you’re moving very much at all.
In order to get “fish arches” that sonar manufacturers lead us to believe we should be seeing, either the boat or the fish must to be moving at a pretty good clip – a lot faster than most of us move when we’re crappie fishing and a lot faster than crappie move most of the time. Arches are created because as a fish passes through the “cone” of the transducer it is farther from the transducer when at the edge of the cone and closer to the transducer when in the center of the cone. The width of the arch depends on several factors including, cone angle (width), how deep the fish is, how fast the fish passes through the cone and also how fast your Scroll Speed is set.
The cone angle affects fish arches because at a given speed (boat or fish) and Scroll Speed it takes longer for a fish to pass through a wide cone than a narrow cone at any given depth. The varying depth of fish also affects the arches because the cone gets wider as it gets deeper so the deeper a fish is the longer it takes for it to pass through the cone and visa versa because of the difference in the distance across the cone at different depths.
The faster a fish passes through the cone and/or the slower your Scroll Speed is set the shorter the arch is going to be up to the point where it is just a blob instead of an arch. The slower a fish passes through the cone and/or the faster your Scroll Speed is set the longer the arch is going to be up to the point that it goes all the way across your screen.
If you get over an active school of fish that are moving up and down in the water column you’re going to see lines that go up and down your screen like a bunch of snakes. Crappie however are not usually that active and if you’re sitting still or moving very slowly over a brush pile you can expect them to show up as solid lines across your screen rather than arches. When a fish moves up or down in the water column the line will move up or down accordingly as it scrolls across your screen.
One of the most crucial adjustments on your sonar is Sensitivity. With Sensitivity properly adjusted you can distinguish between cover and the bottom because wood, bamboo, weeds, etc. have a different density than rock, silt or mud. Knowing what type of cover you are over, either because you put it there, you’ve looked at it with an underwater camera or you’ve seen it when the water level was down, is very beneficial in learning to identify what you’re seeing on your sonar.
You can distinguish between fish and cover because fish have a different density than wood, bamboo and weeds, etc. Bigger and shallower fish will show up as thicker lines because the ping comes back louder than it does on a smaller or deeper fish but each fish will show a consistent thickness or volume as the sonar return is displayed across the screen. If these solid lines going across your screen move up or down you can be sure it is fish you’re seeing and not branches. Tree limbs vary in thickness and therefore show up with an inconsistent thickness or volume and they don’t move up and down in the water column.
HOW DO I ADJUST SENSITIVITY FOR OPTIMUM PERFORMANCE?
First, make sure your transducer is pointing straight down and be sure there is no oil or wax residue on the face, which could cause small air bubbles to stick to the face and interfere with the sound waves. Also, make sure you have a good connection where your transducer cable connects to your graph - there should be no moisture or corrosion in this connection.
Turn your Fish ID off and suspend a 1/16th-ounce jig 10 to 15-feet deep and in the view of your transducer. Then move the jig up and down a foot or two at a time. It should show up on your sonar as a line that moves up and down on the screen just like you’re moving the jig.
If you don’t see your jig, slowly increase your sensitivity JUST until it starts to show up as a very faint line. If your jig shows up real dark then reduce your sensitivity until it only shows as a very faint line. Now your overall sensitivity should be about right but you can make minor adjustments from there if needed.
Most sonar units have an “Automatic Sensitivity” setting, which adjusts for different water conditions and depths. Even with the Automatic Sensitivity turned on you should be able to manually set the desired sensitivity range. The Automatic Sensitivity should then make adjustments for different depths and conditions so your display is more consistent. If you’re going to use your Automatic Sensitivity then be sure it is turned on before you make sensitivity adjustments. However, on some units you may have to switch to Manual Sensitivity to maintain the sensitivity level you want.
Many units (Lowrance in particular) filter out actual sonar returns that are “identified” as fish and replace them with fish symbols when “Fish ID” is turned on. In that case, you may not be able tell whether you’re seeing one fish moving up or down or several fish holding close together at different depths and it would be better to turn the Fish ID off.
If you are going to use your Fish ID turn it back on after you have adjusted your overall sensitivity. Now move your jig up and down as before. If you’re sonar shows fish symbols along with the line depicting your jig then reduce the Fish ID sensitivity (if available on your unit) just until your jig no longer shows as fish unless you want very small fish to show up as fish symbols. If your jig only shows as fish symbols with no line or "arch" then leave the Fish ID off.
With even an “entry level” sonar properly adjusted you should be able to move over a brush pile or other cover and quickly determine how deep the bottom is, how tall the cover is, whether or not it is holding fish, how deep and active those fish are and how closely they are holding to the cover.
With that information you should know about how deep to start fishing. If you’re not marking very many fish and don’t get a bite within a few minutes knowing that you are presenting your bait at the correct depth then it may be time to move on. On the other hand if you are marking a lot of fish all up and down the water column you may have to try several different depths to determine which fish if any are actively feeding.
Taking time to learn how to properly adjust your sonar and to identify what you’re seeing on the screen should save you a lot of time when searching for hungry crappie and help you put a lot more of them “in the box”.