How to Read a Fish Finder – Understand Sonar and Find Fish

Everyone from experienced anglers to novices has to go through quite a steep learning curve to understand how to use their first fish finder correctly, and I’m here to have that learning curve a little easier.

In this article, we will run through fish finder basics so that you can learn how fish finders work and how to read a fish finder screen so you can find and catch fish using one.

How Does A Fish Finder Work?

How Does A Fish Finder Work

Understanding how a fish finder works is the first stepping in learning how to read a fish finder. When you receive your new fish finder, such as a Garmin fish finder or Humminbird fish finder GPS combo, it will come with a display unit that houses the display screen and a transducer plus some cables.

The fish finder display will be mounted in your boat and be connected to the transducer which will sit in the water, and both will be connected to some kind of power supply.

The transducer will then send out a sonar beam in the form of sonar waves into the water. When the sonar waves hit anything in the water column from underwater objects, the bottom, weeds, and underwater structure, to smaller fish and larger fish, they are reflected back to the transducer which collects them.

The transducer is constantly collecting data about the return times of the sonar waves and all this raw data is sent to the fish finder screen unit which turns the data into a sonar image for you to read.

The sonar image will include water depth, the bottom, underwater structures, fish of all sizes, drop-offs, and lots more details depending on the sonar waves frequencies sent out.

Understanding Sonar Wave Frequencies

It’s very important to know how does a fish finder work. Many fish finders come with the ability to send out different sonar wave frequencies ranging from 80 kHz to 1200 kHz. Low-frequency sonar waves don’t pick up as much detail as high-frequency sonar waves but they travel further and thus do not get absorbed by the water.

This means they can travel through the water column all the way to the bottom to give you a depth reading while picking up some detail of say bait fish underneath your boat and other underwater objects along the way.

Higher frequency waves get absorbed by the water and thus can not travel as deep but they pick up a huge amount of detail along the way.

A good fish finder will come with CHIRP sonar and the great thing about CHIRP is that it sends out both high, low, and medium frequency sonar waves to give you a detailed image of every part of the water column.

Now that we know how a fish finder works, let’s look at how to read a fish finder screen so you identify fish and hopefully start catching bigger fish than ever.

Understanding What You See On A Fish Finder Screen

Understanding What You See On A Fish Finder Screen

In this section we will run through everything you will see on your fish finder’s screen when using CHIRP or traditional sonar and at the end, we’ll go into each depth finder feature such as side or down imaging sonar, and how the image might change.

Temperature, Depth & Speed

Most fish finders these days come with a GPS and temperature sensor built-in. On the screen of your depth finder, you’ll see three different numbers – one is the depth of the water under your boat, the next will be the speed you’re traveling at, and the last will be the water temperature.

This is all incredibly useful information as fish behave differently depending on water temperature, how fast you’re going, and knowing the depth lets you find areas where fish congregate like ledges or drop-offs.

Identifying Fish On A Fish Finder Display

Chances are you will be using an arch fish finder as most fish finders today are these types but there are a few tricks you can do to make reading arches easier, depending on the fish finder you own. More that later.

Interpreting Fish Arches

If you see an arch on your screen close to the bottom or in the middle or near the top of your screen, it’s a fish arch. When you see a fish arch, it means there are fish under your boat – simple right. Larger fish arches usually mean big fish and smaller fish arches usually mean small fish.

Judging Fish Size Based On Arches

Judging Fish Size Based On Arches

Knowing whether you have just cruised over a big fish or a tiny fish is really good information to have, as if it’s a trophy fish, you can stop the boat and put in a lot of effort to try to catch it.

The way to determine if a fish is big or small from an arch is by looking at the fish arch width and length. If the arch you are looking at is deep along the vertical lines then it’s a solid mass that is reflecting the sonar waves, and thus a tank of a fish worth trying to catch.

What Are Half Arches?

Sometimes you will see a fish arch that looks like it’s been cut in half. You might think that a half arch is just a glitch but it’s not, it’s a fish. But, why does it come up with just half an arch?

If a fish swims through the entire sonar cone (a beam of sonar waves), it will be displayed as a full arch but if a fish only swims through a part of the sonar cone, it will be displayed as half an arch.

If you ever see half an arch, do not discount it as a fish or as a small fish either as it could well be the giant mirror you’ve been looking for all-day while carp fly fishing for example.

Using Fish ID Technology

Some fish finders will come with Fish ID technology, and the feature will be marked simply as Fish ID. Fish ID takes arches and turns them into fish icons on the screen for you so instead of seeing a screen full of arches you’ll see a screen full of cartoon fish.

Each fish will be marked with a depth so you know how deep it is and its relative size will be expressed in the icon too.

Using Fish ID is a great way to learn how to ready arches as with some fish finders you can ask it to show both the arch and the icon for a fish, helping you to connect the dots.

I would not recommend using Fish ID forever though as it’s not as accurate as reading arches yourself and will show things like a group of baitfish that are actually suspended weeds or weeds on the bottom.

Spotting Baitfish On Fish Finders

Spotting Baitfish On Fish Finders

Bait is not displayed in the form of arches on fish finders as they are too small to create an arch on the image. These will instead be displayed as small dots, dashes, or lines and usually suspended in mid-water.

Often, baitfish tend to hold very tightly together in a ball and you will see them doing so in your readings too, like a big ball of tiny dots on the fish finder screen.

As we all know, fishing a good ball of bait is the first step to finding the predators that want to eat them and we want to catch them.

Understanding Different Colors

Chances are the fish finder you are going to be using will be a color fish finder as they pretty much all are these days. Whether you’re using traditional sonar or down imaging sonar, the image will be displayed in color and how the colors are displayed shows you some useful information.

The stronger and darker the color you see the stronger the reflection of the sonar waves received by your fish finder. This means if you see an arch with a think dark color on it, you are looking at a big fish. The same goes with bottom types, which we will dive into next.

Judging Bottom Types & Hardness

Judging Bottom Types & Hardness

Now that we know that stronger returns equate to stronger, darker colors, let’s translate this into bottom types.

If you’re fishing over say a granite bottom, your fish finder will show the bottom as a bold line in a deep dark color like red. If you’re fishing over clay, for example, the return will be weaker and you’ll see a light line in a lighter, weaker color.

This allows you to map different bottom types in your fishing areas in order to find more fish and target different species.

You can also monitor your traditional sonar or down imaging readings for points and depressions for looking for little Vs in the bottom line which denotes a depression or points where the depth drops off a bit.

Weeds & Vegetation

Weeds and vegetation can be excellent places to find certain fish species such as bass or pike that like to lurk in the weeds for cover and then ambush their prey from them.

You can see weeds and vegetation on your fish finder and they will be displayed as vertical colored lines coming off the bottom. They will not be in the form of an arch, so you won’t mistake them for fish. The same goes for a sunken log or brush pile except these will be log-shaped or like a ball on the screen.

How Do Different Sonar Beams Affect What I’m Seeing?

How Do Different Sonar Beams Affect What I'm Seeing

Depending on the fish finder you’re using, you might have the option to scan with a wide or narrow sonar beam or both at the same time. The beam you choose makes a difference as to what you’re going to see on the screen.

A wide beam covers a large area but in less detail and thus might miss fish on the bottom whereas narrow beam scanning will penetrate to the same depth but won’t cast such a wide net. It thus picks up more detail of what is below your boat.

Some fish finders, such as a Simrad fish finder, allow you to scan with a wide and narrow beam at the same time, giving you both the detail and the coverage at the same time.

This is by far the best option as it combines traditional sonar with a down imaging overlay so you can pick up all the detail possible to help you find more fish.

Traditional, Down, and Side Imaging Sonar Explained

Depending on the fish finder you own, you might have these three sonar scanning features to help you find fish with. But what are they and how do they affect the image you’re reading on the screen?

Traditional & CHIRP

Traditional or CHIRP always scans under your boat using a wide beam and multiple sonar wave frequencies. This is the best feature to use when searching for fishy areas or moving at speed as it gives you a full picture of the water column, even at 10 knots.

Down Imaging

Down Imaging is a more intense version of traditional/CHIRP. It uses a narrower beam stuffed with higher frequency sonar waves to give you all the detail possible. It’s perfect for getting a better look at something fishy like a rock pile, brush pile, or drop-off as you’ll be able to see fish, their size, and where they are holding more clearly.

Side Imaging

Side Imaging

Fish Finders with side imaging scans on both sides of your boat and has a range of 150-300 ft on each side, depending on the fish finder you have. By scanning to the side, you will be able to see fish and structures that would have been missed using down or CHIRP.

The image you see on side imaging is a little different and it’s best to think of it as a birds-ey view. In the center will be a black line, this is your boat, and to the left and right is the image that will show fish, rocks, or structures as if you’re looking from above.

Side Imaging only works at slow speeds of 5 knots or less and it doesn’t penetrate very deep with a maximum depth of around 50 feet. Also, to get the cleanest image with side imaging, you should drive your boat as straight as possible.


Is dry fly fishing hard?

Dry fly fishing can be challenging, but it can also be quite rewarding. It requires a lot of patience and practice to master the techniques. It is important to understand the details of the habitat and the behavior of the fish, as well as the skills of casting and presenting the flies.

What time of day is best for dry fly fishing?

The best time of day for dry fly fishing is usually during the mid-morning and late afternoon when the light is lower and the water is calm. This is when the hatches of insects occur and the fish are actively feeding.

Are dry flies good for trout?

Yes, dry flies are a great choice for trout. They imitate the natural insects that trout feed on and can be fished in almost any water. They are especially useful for targeting trout in shallow water or in stillwater ponds.

What weight fly rod for dry flies?

The best weight fly rod for dry flies is usually a 3-weight or 4-weight. These are lightweight rods that make it easier to cast delicate dry flies and make accurate presentations. They are also easy to maneuver in tight spots and can be used in a variety of water types.

Photo of author

Jamie Melvin

Growing up fly fishing on trout streams in Kenya and the UK, Jamie has traveled the world in search of fly fishing nirvana. From his time managing bonefish lodges in the Bahamas and running fishing safaris in East Africa, all the way to guiding on the flats of Seychelles, there aren't many species or environments he hasn't experienced firsthand.

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