Interpreting Your Echo Sounder

I found this on another forum, I think it is a good read for someone that has limited knowledge on how there echo sounder works, some of the info is a little old and doesn’t take into account some modern digital processers, however I still think it’s worth a read for an overview on the basics.


Far too often, I've had customers and fellow anglers tell me that their color sounders were not working properly. The majority of the time, once I step on board I find the only problem is operator error, or more to the point, operator ignorance. My own fishing experience with sonar started with a Gemtronics 1202S Curve-linear chart flasher/recorder, a very high tech unit when I purchased back in 1976, but a real dinosaur by today's standards.

At the start of each fishing day, I would get on the boat, turn it on and set the sensitivity level so it would produce a very precise bottom line on the chart paper at the dock. Upon getting to the offshore grounds, at that time offshore for me wasn't much more than six miles off the beach, I would look at the machine and not see a damn thing, no bottom reading whatsoever. Even when I was catching fish, I wasn't reading them on the recorder and it was frustrating. I cursed out the electronics dealer that sold it to the installer the manufacturer and me.

I'd send it out for repairs and the manufacturer would send it back to me with a note indicating they found nothing wrong with the unit and I would repeat the scenario again the next trip out. This went on for two months, back and forth, but all I really needed was a course in the unit's operation. It became obvious that I was the problem, not the machine.

The discount house I purchased the unit from was useless after the sale. The installer, although he did a fine job on both the transducer and unit installation, was not a fisherman and also offered little help with using it for fishing. It wasn't until I actually sat down and studied the manual, which appeared more like an ancient text in some dead language that I realize how little I knew about the workings of a fish finder.

I later found that the unit was capable of providing me with far more information than I ever imagined. This is the case with many fishermen when it comes to this most basic piece of marine electronics.

While my job here is not to teach you depthfinder theory, I do need to teach some of you the basics so you'll have a better idea of just how sonar operates. Once you understand the basics, you will see a new world develop right in front of your eyes from the same machine you thought was not performing properly or needed to be repaired or replaced.

Basic Depthfinder Operating Principles

A depthfinder generates an electrical impulse within the unit, which is converted, to mechanical energy in the form of sound waves in the transducer. The sound waves are transmitted downward through a heavy medium (water), where it bounces off objects and the signal is reflected back to the transducer. The return signal is then picked up by the transducer and displayed as an image on the unit's screen.

But how does the unit determine water depth and the position of objects in the water column between the transducer and the bottom? It's really very simple. The depthfinder is actually a very precise clock that measures the time it takes for the signal to travel from the transducer to the target and then back to the transducer.

By measuring the time from transmission to return, the unit calculates water depth and displays it in the form of lighted LCD dots on an LCD screen model, as a burn on chart paper in a paper recorder or as pixel illumination on a Cathode Ray Tube (video screen) in an amber or color sounder. A depthfinder is actually not much more than a sound emitting watch.

The power of the signal being transmitted through the transducer is adjusted by means of the sensitivity, gain or power control on the unit. Power output is measured in watts and frequency of the output is measure in kilohertz. When I turned on my old Gemtronics at the slip and it read the bottom perfectly in eight feet of water, what I didn't understand was that more power to gain was necessary for it to read accurately and clearly in 60 or 100 feet of water. Yeah, I felt pretty dumb after I figured that one out and I hope that this article might help you avoid feeling the same way

Now, realize that the more powerful the signal you pump through the transducer, the more information you will get back on the return signal, as long as you don't over-power the unit. You can easily increase the gain to the point that you totally fill the screen with clutter from interference. We will cover this in the section titled, "Peaking the Transducer."

Have you ever noticed how some rooms have an echo when you speak? Rooms that have an echo usually contain hardwood or tile floors, hard surface walls and ceilings. Conversely, in other rooms you nearly have to shout to be heard because sound doesn't carry well and there is little or no reverberation or echo at all. These rooms commonly have thick carpets on the floors, wallpaper, drapes and acoustical tiles on the ceilings. The difference is caused by the ability of the two rooms to reflect or absorb sound and the correlation to depthfinder use will come into play when we discuss bottom reading and how they differ between soft and hard bottom composition. That will be covered under, "Understanding the Return Signal."

Last, but not least, is understanding the different forms of structure and objects in the water that your machine is capable of reading and distinguishing on the screen. You'll be amazed by how much information you can obtain when you learn how to "Interpret the Screen."

What is happening under your boat is some of the most valuable information you can have when fishing. You can determine whether there is a thermocline, underwater upwelling or pod of bait down there with your depthfinder. You can visualize an underwater sea mount, drop-off, reef or a wreck that could be holding a school of tuna or that trophy blue marlin you seek.

Only your depthfinder can provide this information when you're on the water and it is important to practice using it properly. It is a critical tool for serious and novice anglers alike, one that demands your attention throughout a fishing trip. It has the potential to put more fish in the boat and the greater your understanding and ability to make it work to its fullest, the more beneficial it will be to your efforts.

If you are a serious bluewater hunter, you really should have a color video depthfinder, or color scope, to get the maximum amount of information on the offshore grounds. The deeper the waters you fish, the more important a color scope becomes. Yes, paper charter recorders can give you loads of valuable information and many are capable of deep-water operation, but they do take more getting used to and a greater degree of interpretation. They are also getting hard to find, as most sonar manufacturers no longer make paper graphs.

LCD units have come a long way and are taking over where the paper machines left off, but for deep water use on the offshore grounds, most lack sufficient power and resolution to be a true deep water hunter's first choice. For small boat, inshore tuna, shark and billfish, the LCD unit will do the job well. Keep in mind that much of the information we will discuss here can be applied to LCD and paper chart recorders, too, so do not despair when you hear me mentioning color sounders over and over. I have a personal preference for color sounders because I fish larger boats and spend a lot of time in the distant canyons of the Northeast, fishing waters that can run as deep as 1,000 fathoms or more.

Color sounders require operators with a Ph.D. in electronics and sound dynamics, or at least that's what I thought when I bought my first one back in 1983. It was as big as a small house, had more buttons, knobs, whistles and bells than I thought I could ever master and all I knew once I turned it on was the bottom was red or orange and the surface was a white line and everything in between meant that I was on to something below the boat. Exactly what, at that point, I didn't have the foggiest idea. But, as I began to learn how to use the machine, all those pretty colors began to take on meaning and the more meaning they acquired, the more information I was getting from the unit.

That's one of the beauties about fishing with a color sounder. Unless you're color blind, it's almost impossible to not get more information from them than other types of units. You'll see why.

Peaking the Transducer

The key to getting the most information from any depthfinder is in learning how to peak the transducer. Too much power will fill the screen with clutter and overpower the machine. Too little power will not show such important underwater features as thermoclines, schools of baitfish and even gamefish. The gain or sensitivity knob controls the signal power output of the unit.

The first step in peaking the transducer is [to] turn off any automatic gain controls the unit might have and go to manual control of the power output. Now, select a deeper depth range setting than the actual bottom depth. If you're in 300 feet of water set the depth range for 900 feet. If you're working shallower, say in 60 feet, set the range for 180 feet. Tripling the actual depth is necessary for this operation.

Start to turn up the power gain. First, you'll see the bottom, which only makes sense since the harder and larger the object, the stronger the return signal. The ocean floor provides a solid return. Continue to increase the gain until you get a second echo.

The second echo is nothing more than the return signal bouncing off the bottom once and then having enough energy to make the trip back to the bottom and up again to the transducer. The second echo shows up on the screen as a second bottom reading at double the depth of the real bottom. The screen will now be showing a bottom reading at say 250 feet and another bottom reading at 500 feet. Now you can understand why you need to have the depth range set at triple the actual depth to accomplish this procedure.

Once you have a strong second echo reading, you can return the unit back to the normal depth setting for actual fishing conditions. You have maximized the power being sent through the transducer so you will be able to acquire the most information from the depthfinder screen. You will begin to notice things on the screen that you were missing on lower power settings or on an automatic sensitivity mode. You'll mark fish, bait, and temperature changes in the water column and more. Do realize that once you peak the transducer for a specific depth range, you have only about a 20 percent window in actual depth before you have to repeat the procedure. In other words, if you peak the unit while fishing in 200 feet of water and you begin moving into deeper water, say into 300 feet, you will have to repeat the procedure for the deeper water, if reading everything is your aim.

Understanding the Returned Signal

The analogy of the two different rooms and how they reflect or absorb sound is important to understanding signal return. Remember that the room with that has hard floors, walls and ceiling bounces sound back strong enough to cause an echo. Another room with deep carpets, wallpaper and acoustical ceiling tiles has no echo, simply because the soft materials in the room absorb a good portion of the sound waves.

The same holds true for signal returns to your transducer in water. A soft bottom absorbs more signals and has a different appearance on the screen than a hard bottom that provides a stronger return signal. Peaking the transducer provides enough signal strength to compensate for any absorption.

Once you have your transducer peaked, select the range setting that will give you the most information on the screen. Leave some room at the bottom of the screen, which will be important for determining bottom hardness, but try to set the bottom range so that you are pretty much filling the screen with the water column from surface to bottom.

If you're fishing in 180 feet set the machine for 200 feet, etc. Hard bottom will reflect a sharp signal and will appear to be thicker on the screen because of the strong signal. Soft silt bottom will be represented as a thinner line on the screen because the soft bottom material is absorbing more of the signal and providing a weaker return signal.

Knowing the bottom make up can be important, especially after a storm, when soft silt bottoms do not hold baitfish as well as rock or gravel bottoms. The silt that is stirred up into the water during a violent storm or when particularly strong currents are present irritates the gills of baitfish, as well as other fish. You will most often find baitfish over hard bottom areas after such an occurrence.

Remember that you've peaked the transducer and you are getting more of this type of information on your machine than you ever could by simply leaving it in an automatic mode. But also understand that the higher power settings you're using will also produce more interference or clutter on the screen than you are used to seeing at low power settings.

The button or knob on your machine marked Clutter Control or Discrimination should be glued in the off position. These features are meant to remove clutter and by doing this, they can also filter out useful information, too.

Why bother setting up the machine to give you the greatest amount of information if you are only going to use filters to take some of it away again. Filters do just what the name implies, they filter out weak signals, which can often provide information that you want and need. Learn to read the important signals through the mild interference that is created by peaking the transducer and stay away from the filter controls.

A good example of a time for keeping the power up and the filters off is when you go night chunking for tuna. There's a lot more to chunking than just setting up on an edge or drop-off and tossing butterfish in the water. It helps to know if there is a thermocline in the area and at what depth.

It is also helpful to know if there is baitfish in the area, because you have a much greater chance of getting a bite started if the tuna are on bait in the area. By peaking the transducer and keeping the filters off, you will be able to interpret the return signals to show the thermocline and small pods of bait. Dealing with a little clutter or interference on the screen is worth the additional information you get from keeping the gain up.

Interference comes in two forms, returned signals from another machine or random electrical impulses that show up on the screen during normal operation. Interference from another sounder operating on the same frequency will show up on your screen as a series of dots or dashes that run in symmetrical lines, usually diagonally across the screen. The rhythm they exhibit is a constant and the marks on the screen will be, also. Do not bother trying to remove them with filters or by turning down the gain. If you know what it is that you're seeing, you can disregard that information and simply read through it.

Other forms of interference will be shown as small marks on the screen, usually just a blanket of lit up pixels in no particular form. It can resemble what we call snow on the TV screen at home. Squid, anchovies or other small baitfish will show on the screen as larger spots of color because of their size. When they are concentrated, they can show up as a large area of color or a blob on the screen.

Even an algae bloom will show up stronger than interference, if you know what your are looking at is algae. Again, my suggestion is to learn to read through interference and you will still be able to pick out schools of bait at mid-depths or on the bottom, once you get a little practice under your belt.

One depthfinder function that will help you better define difference return signals is screen speed, the speed at which images move across the screen. Slowing down the speed is like watching the Super Bowl on a Sony Watchman with a two-inch screen, as compared to watching it on a 27-inch television. The picture at higher screen speeds is cleaner, crisper, provide more detail, increased definition and higher resolution and is not distorted or squished as when the screen speed is slow. Leave the screen speed turned to its highest setting to expand the information that is passing across it. At faster speeds, you can also discern interference signals more easily.

Interpreting the Screen

Thermoclines are air conditioning for fish. Large tuna do not like the same warm surface water as blue marlin, but they do like the clean water and forage fish the warm water often holds. Large tuna will often hold close to the thermocline layer where they find cooler water that is more suited to their temperature preference. Hence, more tuna are caught chunking at night than during the day trolling during warm water conditions.

Radiant sun warming has subsided at night, surface temperatures cool a little and, quite often, the tuna will turn on. Sure, we can get a blast of surface feeding fish when a school goes on the feed during the day, but those blasts are usually early morning affairs and very short in duration. But, if you find a significant thermocline, you can increase trolling success by putting baits or lures on the thermocline with downriggers, but you have to be able to determine the depth of the thermocline, first.

Anglers chasing swordfish make it a point to place their baits just above or below the thermocline to get their best chance at catching one of these night feeding billfish. When they are cruising looking for bait or actively feeding the love the area around the thermocline because it provides cool, clean, highly oxygenated water usually full of life.

Thermoclines normally appear as a constant faint blue line that holds even in relationship to the bottom contour. They can be found as close as 50 feet from the surface when there is a strong upwelling present, but normally will be found somewhere in the top third of the water strata. Some fishermen misread them as interference, but they do not resemble true interference when you know what you're looking at.

Just as using downriggers during the daytime increases your chances of trolling a fish on the thermocline, you can use them to get chunk or whole dead baits deep on the thermocline at night. You can also use jigging techniques for tuna near the thermocline both during the day and at night, but the key is knowing where the thermocline is positioned below the boat. We often catch tuna on 24-ounce jigs tipped with a whole squid when the fish are holding on the thermocline.

It's important to know that thermoclines are not always found at the same depth throughout an area. They can vary when upwellings in one area are stronger than in others. When trolling and marking thermoclines, it's a good practice to mark the loran TDs or Lat/Long of significant thermoclines so you can return to them for night chunking or for another pass later in the day.

Upwellings occur when strong tidal influences converge at a drop-off, along the edge of the shelf or near a seamount. When the tidal flow crashes into an area of dramatic bottom depth change, it forces nutrient rich, cold water nearer the surface. These upwellings very often never make it all the way to the surface, so they can only be detected through the use of a depthfinder set up properly to read them. When upwellings make it all the way to the surface, they are easy to see as rips and tidal edges.

Most offshore fishermen have seen tuna stacked up on the edge of a sea mount or drop-off, holding patiently in position like ranked soldiers, and no matter how many times you trolled over them, you did not get them to come up and hit a lure. These fish are sitting on that faint blue line you now know how to detect. When you find fish like this, you've found the perfect place to start chunking, put out a bait on the downrigger or to stop at and drop a jig.

Color machines can tell you the type and amount of baitfish and gamefish you have under the boat, but it requires you to use the SWAG (Scientific Wild-Ass Guess) method in the beginning. When you are catching gamefish and there are schools of bait concentrated in the area and being marked on your depthfinder, when you put the first fish on the deck, check to see what it is spitting up. If it doesn't oblige, check the stomach contents when you collar the fish.

Is it squid or anchovies? Let's say the fish was helpful and spit up squid. You know what the fish were eating when you passed over the school of bait and hooked up, so you now know what a school of squid looks like on your color sounder screen. That provides a reference for future fishing and you can determine whether you're marking squid or other bait, the next time out.

A couple of references that I have found from my experience have shown that squid school evenly and their image is returned to the screen in a relatively even coloration solid right to the edge of the school. Anchovies and other small baitfish are represented with thinner edges around the school because they try to ball up tightly toward the center to stay away from predators that feed around the edges. The school usually shows up with light blue edges, fading into green or yellow and then even red, depending upon the size and mass of the school.

Knowing and understanding the way your machine represents different types of bait and gamefish on the screen comes from observing what you are catching and what the fish caught were feeding on. You must catalog these readings in your mind's eye for future reference, so the next time you come across similar readings, you know what they are long before you put the first fish in the boat. Combine all the information you get from your depthfinder and you will have a winning combination that will put more fish in your boat and make your fishing efforts more interesting, too.

Remember the four basic procedures for getting the most from your depthfinder offshore: 1) Always peak your transducer; 2) Leave clutter and discrimination mode controls turned off; 3) Learn to read through minor interference; 4) Always work with the screen speed turned to its highest setting. By following these simply procedures, you'll be seeing a much wider range of information on your depthfinder's screen, and putting more fish on ice.


Brock O's picture

Posts: 2777

Date Joined: 11/01/08

Cool read

Sun, 2012-04-08 23:32

 Thanks for sharing, I always take the time to read any info regarding sounders.

Picked up a few more things to look at next time I'm out!!

Posts: 348

Date Joined: 24/11/10


Sun, 2012-04-08 23:40

 Let's see if it will work tomorrow !


Lifes a game but fishing is serious !!!

Gricko's picture

Posts: 109

Date Joined: 09/07/10

Good post, makes for an

Mon, 2012-04-09 08:31

Good post, makes for an interesting read.


All this working keeps getting in the way of fishing!

Posts: 170

Date Joined: 27/10/11


Mon, 2012-04-09 09:01

Excellant info, as a beginner I have been trying to interpret the sounder and am slowly working up some knowledge. Thanks for the tips Simo as there is very little information out there.

Cant wait to try some of your tips and see how I get on.