Fish finder or Structure Finder?

how to use a fish finder

The most effective and productive way to use a fish finder is to search out all the right elements that hold fish. But first, before getting into all the techno-babble and how to decipher what you’re looking at on your screen, I believe it’s important to learn the basics. This will help you better understand what’s being displayed on your fish finder. Some of the returns (echoes) can be a bit quirky. Although many fishermen, especially beginners, believe when they press the on button a perfect picture of what’s under the boat will appear...not even close. But starting with the basics and understanding how sonar works will help separate real vs. perceived information.For example, did you know:

  • there may be more fish within the area that your fish finder sees but cannot separate them on your screen?
  • by the nature of sonar a large fish and small fish may appear just the opposite on your screen?
  • you can go right over a fish with your LCD and never see him because he is in the dead zone?
  • that the image (fish, hook, light) displayed is not created by the actual flesh and bone of the fish?
Also understanding the basic operation of your fish finder will help in troubleshooting, adjustment and ideal installation of your control head, routing cables and especially the optimum transducer location.


The word sonar stands for SOund Navigation And Ranging.


Webster’s definition of sonar is: so-nar n. an apparatus that transmits high-frequency sound waves through water and registers the vibrations reflected from an object, used in locating submarines, finding depths, etc.


Sonar dates back to the early 20th century when, in 1912, a patent was applied for at the British Patent Office just one month after the sinking of the Titanic. Since then there have been several patents. The U.S. had their first system in 1918 and in 1924 had four vessels equipped for training at the HMS Osprey anti-submarine school out of Portland. Since then the Navy has developed systems for submarine navigation, locating enemy subs and mine detection.

The first fish finder for freshwater fishermen was introduced in 1957 by Carl Lorance. It was the first high frequency transistorized sonar unit that sold for under $150. In November of 1959 the company introduced the “Little Green Box” portable unit. This was the beginning of the freshwater fish finder we now refer to as the flasher.

In 1972 Tom Mann introduced his high speed flasher. In 1975 the water proof Super Sixty was introduced under the Humminbird name.

Principle of Operation

Sonar is the principle your fish finder operates on. Sound travels through fresh water at a rate of approximately 4920 feet per second. Your fish finder measures the time it takes the sound, sent out through the transducer, to travel to the bottom and return. The time lapse, between the transmitted signal being sent out and the time it takes it to return to the transducer, is then calculated and converts this time lapse into a number that represents bottom depth or the distance of a target (fish). Before a signal or burst of sound can be sent out, the previous burst or echo must be received. The sound is then amplified to strengthen it enough to:

  • light a L.E.D. (light emitting diode) on a flashers dial
  • excite a Pixel on a L.C.D. (liquid crystal display)
  • light a color dot on a CRT (cathode ray tube or T.V. screen)
  • send an electronic pulse to a stylus needle which burns an image into an electrostatic paper.
No matter what type of depth finder you are using, the process that creates the image on your display is the same.

Note: If you put your ear to the transducer, you can hear a ticking noise which is the acoustic sound that is transmitted into the water. You can also rub your hand over the face, or eye, and feel its vibration. This is important to remember when troubleshooting to determine if the transducer may be the problem your fish finder is not displaying a bottom signal.

Each cycle takes a fraction of a second measured in milliseconds. In the case of a flasher this could be as much as 24 times per second and is synchronized with the RPM of the flasher’s disc. An LCR could be as little as once every 2 seconds depending on how fast it takes the display to update 1 row of pixels on the display. This is important to remember when you think about high speed readings. Most fully rigged bass boats still come with a factory installed in-dash flasher for this reason. As great as 2 and 3 dimensional displays are, they are not recommended for high speed performance.

Cone Angle

Diagram of sonar dead zone

In my seminars at the “American Fishing Institute” I would always talk about the cone angle. Most people want a fish finder with as large a cone angle as possible. My thought is just the opposite and I’ll explain why. The wider the angle, the more you hide. Let’s say you’re crossing a small ditch or culvert. Before the trailing edge of the cone leaves the back edge of the ditch, the front side of the cone is already picking up the other side. The ditch is completely hidden on your 2 dimensional display and the bottom will appear perfectly flat. A 2 dimensional display can only show the shallowest reading it is receiving back. Figure 2 is what your display would look like if the boat was setting still as illustrated in figure 1. You would get the same results if the boat was moving parallel along the ledge. These are referred to as dead zones. Using the ditch example, a flasher, which records everything coming back, the bottom return will widen, leaving you to believe you have just come across a harder bottom giving a stronger return. I always said I would like a fish finder with a 1 degree cone so I can pencil out an exact bottom contour. If you’re a bass fisherman, you should be looking for all the conditions that hold bass and not trying to find fish. Bass are ambush feeders and hide in and around structure or in those dead zones that LCD’s and other 2 dimensional type units cannot display.

Humminbird LCR
First look at figure 1. This represents how fish can be hidden on a flat 2 dimensional type screen as seen in figure 2. Now compare the same information as it is displayed on a dial or flasher type read out (Fig. 3). Not only do you see the 15 foot return at the top of the ledge but also the 28 foot return at the bottom of the ledge. All the returns in between the 15’ and 28’ bottom readings represent the fish you see in figure 1 that will not show on an LCD screen (Fig. 2). Also mixed in with the fish returns will be more bottom returns as the sound echoes off the bottom between the 15’ and 28’ returns. Flashers give you much more information than a fish finder with a 2 dimensional screen but trying to determine exactly what you’re looking at can be very difficult to interpret.

Flasher dial of ledge
Let me stop here and explain that I’m not trying to discredit what a LCD unit can or cannot do. The purpose of this is to get you to understand the limitations of sonar and how it effects what you see on your fish finder. If you search out a lake and wait for your fish finder to tell you where the fish are, you’ll be passing over some good potential fish holding spots. Given a choice, I’ll take the LCD type unit every time because of the detail it gives me when searching out structure. In Florida you primarily look for grass. A LCD unit can actually tell you what type of grass you’re looking at by the way it appears on the screen. A good example is hydrilla which looks like a mound rising up from the bottom while other grasses like eel grass or kissimmee grass are sparser with void areas. If I find an isolated patch of hydrilla, I’m going to stop and fish it because I know these are fish holding magnets. You don’t need to see the fish, you probably won’t see them anyway since they bury up in this stuff and use it to ambush their prey. Rarely will you ever see a fish, at least a bass, on top of the hydrilla. If you spend your day looking for fish on your display you’re wasting valuable fishing time. Use your fish finder to look for all the elements that go into fish holding areas i.e. structure, depth change, bait fish, current etc. and then fish those areas. If you see bass up off of the bottom, they’re suspending and probably not feeding. If you see them suspend under a school of baitfish, they are much more likely to eat than those out in the open away from any cover or baitfish.

How We See Fish

Fish are made up of soft tissue and water so most of the sound is absorbed and no echo will be returned. Sound travels through water faster than air. When a sonar signal hits air, like a fish’s air bladder, it’s like bouncing off a rock because of the sudden change in density and sends a strong signal back. So what you’re actually seeing on your fish finder is the echo off of the fish’s air bladder and not the body of fish.Do we need to know the approximate size of each type of fish’s air bladder in relation to his total body mass to identify what species were looking at? No, but we do need to know the habits of each specie so we can determine bass from catfish, crappie, brim etc.

The size of the target is in direct relation to the depth of water the fish is in. The cone angle of sound typically starts out narrow at the top and widens at the bottom. When a fish passes through the upper limits or the narrow part of the cone, the number of pulses that bounce off his air bladder are much less than if he were in the lower part of the cone, giving a smaller image on the display. The opposite is true when he passes through the lower portion of the cone resulting in a larger image due to the greater number of returns because of the increased time he’ll be in the cone. Scenario : Let’s say you pass over a large fish in 5’ foot of water and a smaller fish in 10’ of water. The fish in 10’ of water will be in the cone twice as long as the fish in 5 foot causing him to appear as the larger fish. So just keep in mind, those small images, close to the boat are actually larger than they appear.

How a graph displays fish
The arc (Fig. 5) is caused by the different distances the fish is from the center of the cone. When the first signals come back, they are from the fish entering the edge of the cone which is the furthest point from the center which starts the leading arm of the arc. As he nears the center of the cone he is actually closer to the transducer which will make him appear shallower and forms the peak of the arc. As he moves away from the center he appears further away giving the trailing arm of the arc. The example shows the perfect arc which you seldom see. Most of the time, on your fish finder, you will see an arc with only one leg or a full leg on one side and a partial leg on the other. Also a tightly bunched school of bait fish will appear as a large ball that will break up towards the edges. To get the image on your fish finder screen, as shown in figure 5, the fish will have to enter the leading edge of the cone directly inline with the boats path and stay there as the boat travels directly over it. Most of the time the fish is moving as you pass over him or he enters the cone at an oblique angle. You also will pass by fish that are only caught by the outer edge of the cone.

Why fish appear to be in same depth
Figure 4 shows how fish, at different depths, will appear as one on your fish finder. This is due to the fact that the fish at the outer limits of the transducers cone angle will appear farther away. Why? Because they are, at least that’s the way the transducer's eye measures them. Because of the angle of the cone, the fish actually are farther away from the transducer and thus are displayed accordingly even though their true depth is 12 feet below the surface. The fish in the center of the cone is the only one that will be recoded at its true depth of 13’. Likewise they will all appear on your fish finder as one fish at 13 feet. All of this is occurring with the boat setting static and the fish holding their position. A common mistake made is that all the fish displayed on the screen are directly under the boat. Because of the expanse of the cone, especially in deep water, fish could be several feet away from the boat in any direction. If your fish finder uses a wide angle cone, like a 32 degree, in 40’ of water you would have a 25’ circle of bottom coverage. Any fish within that circle would appear to be under the boat but could actually be 10’ away on either side of the boat.

Looking down in cone as camera would
The illustration (Fig. 6) represents three fish within the circle of the cone as they would appear through the lens of a camera. A camera lens has field depth and objects will appear to be at different distances. A fish finders range is determined by how long it takes the sound to travel to the bottom or targets (exp. fish) and back to the transducer. In this case, even though the fish are in different depths, the fish on the out edge appear to be the same distance as the one in the center since they are farther away from the eye of the transducer. However sonar sees them as targets all at an equal distance from the center of the transducers eye. The fish on the outer edge of the circle are actually shallower than the fish in the center, we’ll say 12’ actual depth, however the transducer measures how far away from the center they are. It sees them as the same distance away as the one in the center. As the boat moves and the fish remain fixed in there positions the ones on the outer perimeter, as they move closer to the center of the eye, will become closer and then the transducer will measure them at their true depth and then you will see them separate on your fish finder display.

How fish are hidden when in equal depth
In figure 7 we see a lot of fish both inside and outside of the cone. First you see a mark at the “0” line on your flasher which is a reference point and always displayed when the unit is on. Next you see a mark at the 18’ range on the flasher’s dial indicating the depth the boat is in. Between these lines are 7 marks indicating targets (fish). Of course the fish outside of the cone are not seen on the display until the boat passes over them. Notice the number of fish within the transducers sound range (14). The ones appearing on the same plane will be seen as one. There are 5 fish along the 16’ parallel (equal distance from the transducer’s eye) but the flasher only can display one. This is another example of how fish, within the circle of the cone, can be hidden from your view and not displayed. Actually they are displayed but cannot be separated because they fall on the same parallel with other fish yet there is only one place the depth finder can place them. A LCD would just show a long line across the screen.

Why is this important to know? If you’re trying to make a vertical presentation, you must know the depth of the fish so you can stop your bait just above the fish. Fish are more likely to move up to attack a bait than follow it down. This is a technique used by a lot of anglers. I first heard of it from Dion Hibdon when he asked me for a paper chart to mount on the bow of his boat. You can actually watch your bait and know exactly where it is in relation to the fish. When the fish comes up for the bait, it appears as a diagonal line. Dion explained to me that you don’t always feel the bite since the fish is moving up and towards you when he takes the bait. While watching on a paper graph, when the fish reaches the bait…SET! The first generation of LCD type units weren’t capable of displaying enough detail to duplicate the diagonal image. Now, with the more recent model fish finders and there greater pixel counts, units can show much more detail that allows for the same representation that you would see with a paper graph. If this sounds like your style of fishing it is well worth the money to spend on the best unit you can afford. This is just one reason to consider the best depth finder your budget (or spouse) will allow. You also need this detail for any type of vertical fishing over brush piles, ledges, rock piles and to more accurately identify bottom structure.

Sonar vs. Video

All of this is designed to help separate sonar from video. Sometimes it’s difficult to understand why images appear as they do. You heard the expression, “blind as a bat”. Well bats use sonar instead of vision like you and me. That’s why they’re always chirping. When he receives an echo back he must process the information to determine if its food (and attack) or a solid object in his path…splat! As complicated as LCD displays, micro processors, amplifiers and transducers are, sonar is really simple. Send out a sound and wait for its echo to come back. All that high tech stuff then determines how far away it is and shows the best image it can on your display. It’s up to you to interpret what’s on the screen. Even the little fishes, some of the units draw on the screen, may not be fish at all. The processor only knows that it received a target back that is separated from the bottom so it must be a fish. Tree limbs, anchor ropes and boat dock cables can create a target that resembles a fish. Things like turtles, alligators and even dying vegetation can disperse air bubbles that sound will bounce off of just like a fish’s swim bladder. Never assume anything but use your experience and knowledge of the species your fishing for to accept or reject the information given.

Remember what I said earlier about looking for all the right elements like structure, current, deep water near by and bait fish in the area. You would do far better if you concentrated on learning what the different structures look like on your screen and then determine which ones are the best fish magnets.

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