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Understanding Sonar

Have you heard fishermen talk about great marks or claims of big fish on the screen? Have you caught good numbers of fish while you're not marking fish? Success on Lake Erie can often be directly linked to your ability to use your electronics.


For a good majority of the season lake Erie walleyes are found suspending. Unlike most inland lakes, where fish relate to some sort of cover such as: points, rock piles, river channels ect. On Lake Erie we find that walleyes tend to school up and follow bait fish and water quality (temp clarity dissolved oxygen). The single most important aspect of catching fish on Lake Erie is that you have to be on the fish.


Fish arches

Understanding why fish show up as arches can also help you understand a lot of what your sonar is telling you. To understand this you must understand how sonar works.

The transducer sends a sound signal down into the water and waits to hear an echo or return back. It measures the time it takes the signal to hit an object and then return back to the transducer. There is much more to it than that. We could spend days discussing all the technical details. The good news is you don't need an engineering degree to understand the important parts of sonar. What you do need to understand is that the signal is sent down to the bottom in a cone shape. I find a lot of anglers think of it as more of a triangle. This is not accurate it's a 3D cone. When I'm trying to explain this to someone, I like to have them imagine the beam from a flashlight. Take the light and set it down on the floor then slowly lift it up higher and higher. Take notice of how the area of coverage gets wider as you lift the light. the angle of the beam stays the same but as the distance increases the area of coverage gets wider. The beam or cone of the transducer works identically to the flashlight. The deeper the water, the wider the area of coverage will be at the bottom. The angle of the cone will usually be determined by the frequency settings, we'll touch more on cone angle later. We have a three-dimensional cone under the boat but only a two-dimensional screen to display the information on. The two dimensions the screen displays are time and distance. From top to bottom represents the distance. From right to left represents time. The right side of the screen is the present time and anything left of that point is in the past. The amount of time displayed is directly related to the scroll speed in your settings. The higher the scroll speed the less time or history is displayed, and the slower the speed the more time or history is displayed. Let's imagine the boat is sitting still. A fish enters the cone and swims directly through the middle of the cone and then exits the cone on the other side at a steady, but slow speed. The screen will start to show a return starting on the right side the moment the fish enters the cone. Thit'se return will continue to get longer much like its being printed out onto the screen. This will continue all the way up until the fish exits the cone on the other side. The length of the mark on the screen from right to left does not represent the size of the fish, it only represents the amount of time the fish spent in the cone. Once enough time has passed that mark will have made its way all the way to the left of the screen and eventually exit the screen. Now let's say that same fish was spooked by something and it came racing back through the cone at the exact same angle and the same depth, just in reverse entering where it left and exiting where it had previously entered the cone but at a very high rate of speed. Again, the return would begin to print out from the right the moment the fish entered the cone it will continue to print getting longer and longer until the fish exited the cone. The same fish will have spent much less time in the cone and will be displayed as a much shorter mark on the screen. Now that same fish has entered the cone at two different points in the cone and swam in opposite directions each time it passed through the cone. It could have entered from the bow and exited the stern side of the cone or it could have entered at the port side and exited the starboard side of the cone, or any angle in between. The sonar unit doesn't care it's only displaying a target in the cone for the amount of time it's in the cone no matter what side it enters or exits. If a fish hangs out in the cone for long enough it will print a mark that reaches all the way from one side of the screen to the other. It will only stop when the fish exits the cone. The other dimension is the distance from the transducer, this is displayed vertically on the screen from top to bottom. Let's say the fish we've been talking about (Joe) entered the cone 20ft down in the water column and we are in 30ft of water. Joe maintains the same depth as he enters the cone as when he leaves the cone. When Joe enters the very outermost edge of the cone the distance from the transducer to Joe is slightly longer then it will be once he makes his way directly under the transducer in the center of the cone. The sonar is only able to represent distance as depth as Joe makes his way to the center of the cone the distance shortens until he makes it to the halfway point. Then the distance starts to increase again as he moves away from the center, up until the point he exits the cone. The sonar will start to print out his return slightly deeper at the start and as the distance gets shorter. The mark will start to print out onto the screen higher until he reaches the center of the cone and then back down lower as he exits giving the mark on the screen an arch shape.

The same thing happens to the returns as the boat starts to move. The sonar doesn't care if Joe is moving or if the boat is moving over Joe. The same rules apply, sonar displays how long Joe was in the cone and what the distance from the transducer to Joe was at any given time as he passes through the cone or the cone passes over him. At higher boat speeds the marks on the screen are going to be shorter, and at slower boat speeds the marks are going to be longer. The distance will stay consistent regardless of how fast or slow we pass over Joe. As he enters and exits the cone the geometry of the cone and his position in the cone will change the distance to the transducer as he nears the center of the cone.


There is sort of a third dimension being displayed on your screen and that is the color. When joe passed under your transducer his return strength was depicted by the brightness of the color displayed on the screen. This will also increase and decrease as he enters and exits the cone. However, this can be deceiving as Joe might not always pass under the transducer in the center of the cone. He could easily be off the port or starboard side making him appear to be a weaker return that some anglers would simply call a smaller fish.


Too many unknowns

There are too many variables to take into consideration when trying to decide how big the fish you're marking really are. If you could keep the variables constant and then have a large fish enter the cone and then a smaller fish. The sonar will show the bigger fish as a larger mark and the smaller fish as a small mark. Here are some variables to consider. Fish in deep water can look small, and fish in really shallow water can look huge because they are much closer to the transducer and make a stronger return. I hear guys making claims on the radio and at the dock of some big fish marks they found, but ultimately it's too hard to say if the fish was simply swimming in the same direction at the boat, making it appear to be a longer mark. If the signal strength was much better showing some really strong return colors on the screen, could it be that there were a few fish that just happened to be positioned just perfectly center under the cone, while the rest of the fish were more off to the sides? Could 3 or 4 fish be in the same depth together in such a tight group that the sonar couldn't separate them, and showed one bigger stronger return as if it was one fish? I think it's quite possible that all of these situations are often claimed to be big fish when realistically it's your sonar playing tricks on you. Now I have spent countless hours watching sonar and I can usually tell the difference in small baitfish schools, large shad, white bass, or white perch as opposed to walleyes but trying to tell the difference in 20-inch walleye to 28-inch walleye just by the sonar returns, I think could prove to be next to impossible. There is one, 100% accurate way to tell if you're marking big fish and honestly, it's fairly simple. That method is to catch them!


Cone angle

The cone angle is an important thing to consider when using sonar. Generally lower frequencies tend to use a wider angle, and higher frequencies use narrower angles. Every manufacturer is slightly different but for the most part 83khz uses a 60-degree cone angle and 200khz is around 20 degrees. This is important to understand because at 40 feet of water, a 60-degree cone will have a cone diameter of 40ft (60-degree cone diameter is nearly equal to depth), and which is about 1250 sq ft of coverage area and 40ft with a 20-degree cone will have a diameter of about 1/3 the depth making it 13ft diameter or 130sq ft or coverage area. As you can start to imagine this will make a big difference in how many fish your sonar unit is going to mark, just based on coverage area. There really isn't an instance that I can think of on Lake Erie where I would want to run anything other than 200khz. It's capable of marking fish in depths of up to 200ft, I don't know about you but I have never caught a walleye that was 200 feet down.

Marking fish with a 20-degree cone although it's a fairly narrow cone, it can still be deceiving when you're fishing depths that we often see on Lake Erie (40-60 ft or even more). In 60 feet of water, the cone is covering around a 20 foot diameter or a surface area of over 300 sq ft. The cone area halfway up the water column or 30feett down would only be roughly half the area as 60 ft. If the fish were spread out evenly throughout all 60 feet of the water column, it would appear that there are nearly double the amount of fish marking on the screen at the bottom than halfway up and next to no fish near the surface. Just keep in mind that if your marking even just a few fish up high, it's very possible that there are way more fish up high than your sonar is showing because the cone is so narrow that high up, the sample area is much smaller. You might also find that your marking most fish around 30 ft down but still have quite a few down lower. However, it's more likely that you are being deceived by the sample size of the cone as it widens out down deeper and there are really way more fish around that 30 ft depth than there are down deeper.


Check out this image from Lowrance



Marking at speed

This is a very important part of being able to use your sonar efficiently to eliminate water. Lake Erie is way too vast to have to be at an idle to mark fish. You need to be able to mark fish while the boat is on plane. It's really a fine art trying to get a sonar to work well at speed, and transducer placement is key. Some larger boats will use a through hull transducer with a fairing block, and even spacers to get the transducer under the turbulence caused by the strakes on the bottom of the boat. I would recommend doing plenty of hours of research on your particular boat and type of transducer prior to drilling any holes. Possibly even adding a piece of starboard to the transom to mount transducers to, so you can make adjustments without punching a ton of holes in the boat. Or even hire a professional to help you mount and adjust the transducer. Whatever you do, make sure you can mark fish at cruising speed.


Ghost fish

We have had this scenario play out so many times we made a name for it. Here is how it goes. You're running out and not really marking much, just a few fish up high but nothing crazy. You slow down to have a better look and hardly mark a single fish. Back on plane and you start to mark a few more up high but again not a ton of fish just a few. Finally, it hits you the few fish you're marking are the ones who didn't get out of the way of the boat in time because they are not very deep in the water. They spook away from the boat and the cone coming out of your transducer is much narrower up high, making it harder to mark shallow fish with such a small coverage area, then when you slow down and they have plenty of time to move off to the sides so you don't mark hardly any. You stop and set a few lines with short leads putting your baits less than 10 feet down and bam it's fish after fish, all the while you're not marking a single fish with the sonar but marking them with the rods. Even on days when you are marking good numbers of fish down lower, it's always a good idea to set a few lines up way higher than you would think they should be just to keep them honest.


Fish on the bottom

Your sonar unit is not going to show you fish that are sitting on the bottom. The fish that appear to be on the bottom are at least 8-12 inches off the bottom. Maybe more depending on how wide of a cone angle you have and the frequency settings. Higher frequencies give better target separation but they still have limits. Down imaging can be a much better way to find and target fish that are sitting near or on the bottom. If your unit doesn't do DI not all is lost. If you start to notice a lot of marks on the bottom you can assume there are more than what your sonar is showing you. If you have used a sonar unit while ice fishing you can see this happen in real time. Watch your bait as it drops down through cone. When it gets about a foot above the bottom the mark starts to blend into the bottom. When it's on the bottom lift it back up and watch how far you need to lift it before your sonar starts to mark it again.


When it matters

Finding fish on Lake Eire can be very easy depending on the time of year and what port you're fishing. If you fish out of Lorain in June you will have a hard time finding a bad area. Come back in mid-August and you will need to rely heavily on your electronics. When the screen is loaded with targets you can put just about anything out there and catch fish. When you're fishing an area where the migrating walleyes have already left or haven't arrived yet. A better understanding of what your sonar is telling you can help you dial in the program. Get your baits in front of fish faster and help you catch the few resident fish that are there. As far as trying to target big fish goes you can still use your sonar to help you narrow it down. It's not too often we find mixed schools of males and females usually it's one or the other. If you find an area that has a lot of marks fish that area a bit if you find yourself catching a bunch of small males it might be time to start burning some fuel. look for a different school of fish and repeat. Just keep in mind if you're catching males it's likely the large schools of females are many miles away farther to the east. Not all the females are going to migrate east as the summer progresses you can still find a few relating to cover, depth changes, and current breaks. This to me seems like a topic we could better cover with an article on utilizing Side imaging, down imaging, and GPS.


A very important aspect of becoming the best angler you can be is always being willing to learn. I acknowledge that I don't know it all. I am always eager to hear different anglers points of view on any fishing related subject. I would also point out that I am willing to give my point of view but I reserve the right to be wrong. If you feel there is something you would like to add we would be happy to hear it. leave a comment in the box below.


As always we would be more than happy to have you out fishing with us.

We are just a phone call away!

Captain Tom Ullum

Lake Erie Charter Serivce LLC

(330) 309-5734



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