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Understanding Your Fish Finder: Fish ID vs Arches

I began using sonar or fish finders back in the early 80s, with my first machine being a Lowrance Flasher, then on to my first paper graph X-15. The number one question that people always ask me is, “Should I use Fish ID?”

Back in the mid-80s, the first (very pixelated) liquid quartz sonar displays came on the market. Humminbird was the first manufacturer to introduce a special feature called Fish ID, with a red shadow dot behind the main black dot. Very soon, all the other brands followed suit, and it has been a feature option ever since. But one must remember it is strictly a sonar filter that receives the signal back from the transducer and interprets it into either a small, medium, or large fish symbol. Sounds pretty straightforward, but be wary, because anything that comes between surface and bottom will display as a fish symbol. That could be your downrigger ball or—if you are jigging for bottom fish and not trolling, your jig—showing up as a fish symbol.

Fish ID symbols just pop onto the screen! There is no way to tell when they enter & exit the cone angle.

Why Turn Off Fish ID on Your Fish Finder?

When it comes to truly understanding what your sonar is showing you, you must turn off the Fish ID feature. Then it will display a normal sonar image. If you are trolling, you will see arches, and the length of those arches only indicates how long objects spent in the cone of soundwaves. This can change if you are out in deeper water. A typical 200 kHz transducer emits a cone in a 20 ̊ angle, so in 30′ of water it is covering approximately 11′ diameter. In 60′ of water, that would be 22′ diameter. So, if you have two fish that are the same size but at two different depths, the one in deeper water will have a longer image than the one in shallower water. When you see a long arch displaying, it only means time spent in the cone angle; size is determined by the thickness of the arch or line. A larger fish would be thicker than a small fish.

You could be trolling along getting nice arches, but as you slow down to a slow drift, your arches will appear flatter, to the point that when you are no longer moving, you will get a completely flat line for each individual fish that is in the cone angle. Remember that however long the fish is in the cone angle is how long it is printing on the right edge of the screen. When it is no longer printing on the right edge of the screen, it has left the cone of sound waves. But its memory will still carry over to the left side of the screen. Also remember that what is displaying in the middle of the screen is not what is happening right now; it is only visible history. It is very important to remember that the right edge of the screen is what we call the “instantaneous edge of the screen.” It displays everything that is below the back transom of the boat.

Fish Finder showing Fish ID

So now you anchor in 30′ of water and the boat is completely stationary. Then you have three fish come by in the cone angle for 5 minutes. This means that you will be printing three individual lines on the right edge of the screen for 5 minutes. As soon as one of the fish leaves the cone angle, it no longer is printing on the right side, but its memory carries across while the other two are still printing. Now drop your jig down into the water inside that cone angle, and you will see the zigzag of your jigging action printing on the right side. If you had the Fish ID feature turned on, it would show the fish as multiple fish symbols— and your jig also would show up as fish symbols. You would never know what truly was going on down there.

Fish Finder showing Arches

Remember that size is determined only by thickness and arches only happen when you are moving—flat lines when you’re stationary. Also know that the likelihood of seeing fish show up on the screen in less than 20′ of water is very slim. This is because they are spooked by the passing of a boat over top of them, and cone diameter is even narrower in shallow water. Remember, with 200 kHz signal, the width is roughly 1/3 the depth. Once you get out deeper than 20′, it is easier to see fish on the screen. Use your sonar as a tool to find structure, first. Once you find the structure the fish will be there—they just might not be visible because of the shallow depth.

Fish Finder showing bait and predators

To recap, the size of fish is not determined by the length of the arch or line that is printing on the display; it is determined by the thickness of the line or arch, as well as the colour. Liquid graph displays for the past 8 to 10 years have been in colour, and the colour displayed relates to density or hardness. On Lowrance displays, a smaller fish like a baitfish will be a thin blue line if it is not in a school. As the fish get larger, their lines or arches will go from blue to red to orange, and finally to yellow for the largest, densest fish. They will also be much thicker. If you see a large cluster of baitfish, they can be packed so tightly that the density factor goes up and the colour scale will climb into the orange and yellow levels.

Fish Finder showing Chinook looking at bait (Courtesy of Murphy Sportfishing)

When it comes to bottom hardness, the same is true: A lower density (softer or muddy) bottom will display with more blue. As the bottom gets harder, with sand or gravel, it will have more red to orange. When you are over a rockier bottom, you will see a yellower lumpy bottom image. When you are trolling along and the bottom image goes from blue to yellow, that can be just as important as seeing a bottom dropoff edge. These transitions from a soft, mucky bottom to hard rock or gravel will attract fish big time!

Coho looking at big baitball of needlefish (Photo Courtesy of Murphy Sportfishing)

The next time you go out fishing, you will learn more about your unit if you turn off the Fish ID feature and watch the right edge of the screen more.

Good luck fishing!

This article appeared in Island Fisherman magazine in March 2022. Never miss another issue—subscribe today!


  1. This is really informative. Would you ever consider doing a post on the settings you would use for salmon fishing? Also what transducers are a minimum to have. I feel like I don’t see these results on what I have, I run a simrad go9xse and p66

  2. Yes Kyle, we wrote about that in a past issue as well. You will see it eventually online. You’ll get the freshest info if you subscribe to the magazine.


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