How to Read a Fish Finder

Garmin Fishfinder GPS Combos

It is very important to know! These days any fishing boat (small boat, kayak) is equipped with a fish finder. This device clearly shows the fisherman the bottom topography, depth, concentration and size of fish and much more. It is difficult to imagine a more necessary device than an echo sounder, navigator, walkie-talkie (radio).

How to Read a Fish Finder – Understanding the Detail of a Fish Finder

In several articles, we talked in detail about how useful fish finders are for your fishing (especially trolling) and reviewed the list of the best fish finders. But do you know what we haven’t done yet? That’s right, we didn’t talk about how to use the fish finder and how to read it to know every little thing about the fish under your boat.

In this article, we will take a closer look at how the fish finder works, how to use it, and how to read fish with it.

What is a Fish Finder – Construction

Basic Designations and Functions Fish Finder

Always seeing and knowing where the largest number of fish swims is every fisherman’s dream. The fish finder is just the device that allows you to do this. It can perform the following functions:

  • Determination of depth, relief of the bottom surface.
  • Search for fish, its accumulations.
  • Special features depend on the manufacturer.

A fish finder must always come with at least two blocks. The first is a screen with a microcomputer that processes information, the second is a sensor that perceives information, which you drop in the water. The fish finder also includes an emitter and a transducer, which are built into the blocks.

How Does A Fish Finder Work

Now that we know what a fish finder is made of, let’s see how it works. The transmitter emits high-frequency pulses that are produced at a specific time interval. These signals are sent from the emitter to the transducer, where they are transformed into ultrasonic waves. Following this, such impulses are carried along the water surface at a speed of 1500 m/s.

Further, ultrasonic waves, falling on stones, fish, driftwood, the bottom, or any other solid object, are reflected from it. Then these signals are sent to the transducer. In it, they are again converted into high-frequency pulses and amplified by the sensor. Then they are sent to the screen, where they appear in the form of an alphanumeric or graphic image, which is convenient for perception and understanding. It sounds pretty complicated, so let’s figure out which of these you need to pay attention to.

How To Read A Fish Finder: A Simple Guide

This is the most basic and important part of our article. Below we will show you how to use the fish finder and distinguish on-screen data that is at first glance incomprehensible.

Signal Processing – What You Need To Know About Beams

Definition of Sonar Fish Finder

The size of the scanned area depends on the width of the scan beam angle which the emitter creates. The wide-angle is 40°–60° wide. The area it covers is quite large. The narrow beam is 10°–20° wide. When interpreting the data displayed by the fish finder, it is important to know which beam is being scanned (wide or narrow). Garmin fish finders have wide and narrow scanning beams (55° and 15°) and medium beams (40°).

Secondly, it is important to remember that the fish finder is continuously transmitting and receiving data, while the image on the screen moves all the time. The current scan data is displayed on the right. Old data is shifted to the left as new data is received.

Thus, when examining the data on the screen, it is important to remember two things: 1. Know which beam was scanning: wide or narrow. 2. Continuous movement of the image does not mean that the fish finder is moving.

Identifying Fish

Identifying Fish

Modern expensive high-tech fish finders can work in two modes – with fish icons and with fish arches. Cheap ones only work with fish arches. These icons, as you might have guessed from the name, show fish locations on your screen. The situation with arcs is a little more complicated. But each model has its own subtleties of work.

Fish Icons

Fish icons are a useful tool in the early stages of using a fish finder: the device interprets the received data about objects and tries to determine if it is fish or not. This takes into account factors such as the size of the object, speed, and the strength of the returned fish finder signal.

Garmin fish finders use a complex computer algorithm for data interpretation. But even the most complex devices cannot transmit data values with 100% accuracy.

Different fish swim at different speeds and in different directions relative to the fish finder beam. This means that their numbers may not always be read in the same way. Besides, some underwater objects or algae can sometimes give similar performance to fish.

Therefore, you need to learn how to independently interpret the data received by the fish finder to use it as efficiently as possible.

Take an important step and turn off fish icons – as a reward, you will receive more accurate indicators of the location of the fish. An additional bonus for decisiveness will be new useful knowledge and improved skills.

Fish Arches

If fish icons are disabled, how do I determine if there are fish on the fishfinder data screen? You need to look for fish arches. The fish will appear on the screen as arcs (since the sonar scans in a cone and the fish signal are reflected as an arc). It is important to remember that arcs can vary in size (length and width) and maybe incomplete – you should also pay attention to half arcs. They vary in length and width, some are incomplete, but they all represent fish.

Determining the Size of a Fish

Observing fish arches, you should understand how to determine the size of the fish from them. Let’s talk about their length and thickness, about full and half arcs.

Arch Length

When interpreting the indicators of a fish finder, fishermen are most often mistaken, taking long arcs for large fish. This is not true. On the fish finder data screen, the arch length indicates time. For example, imagine that the fish finder is stationary in the water (that is, you are not pulling or submerging the fish finder). If there is an equally motionless fish under it, how will it be displayed on the screen?

You will see one continuous line. This does not mean that you stumbled upon a huge blue whale in the lake. This means there is a stationary fish under your fishfinder, which can be very small in size. Or, for example, imagine that there are two fish swimming under your fishfinder – a large one and a small one. A large fish will swim much faster and leave a smaller arc than a small fish. A slow object always leaves a longer trail than a fast one.

Arch Width

So, we realized that it is difficult to determine the size of the fish from the length of the fish arches. The size is determined mainly by the width. The main rule when using a fish finder is to think vertically, not horizontally. Even if the arch is short but wide, it means that the signal has bounced off a large fish.

Full & Partial Arches

And the last point to remember when analyzing the data on the fish finder screen is that the arch does not have to be full. Half an arc or even a quarter also means fish. The fish can only swim through part of the fish finder beam and a half or quarter arch will appear on your screen. And an incomplete arch can also be from a large fish. Pay attention to incomplete arcs and remember that the most objective indicator is the thickness of the arch.


Let’s summarize what we said above and highlight three main points:

  • Fish icons do not always show exactly where the fish is. We recommend using them only in the early stages when you get used to the fish finder.
  • Always think vertically, not horizontally, and look first at the thickness of the arch. This will help you distinguish large fish from small ones.
  • Even if the arch is incomplete, this does not mean that there is no fish there. Pay attention to incomplete arches.

Determining Bottom Relief – Why It Is Sometimes More Important Than Finding Fish

Determining Bottom Relief

The fish finder can not only look for fish in the water column for you. It is also a great tool for analyzing bottom topography, structure, and depth. This can often help you find a lot more fish if you know how deep your species is. For example, predators often hide in pits and depressions waiting for their prey. Analysis of the bottom topography with the help of a fish finder will help you easily track possible habitats of predatory fish.

But first, remember that the fish finder scans continuously. This means the image will move and the bottom will be flat even if the fish finder is not moving. Lower and raise the fish finder slowly and smoothly to get an accurate bottom image. Also, pay attention to the depth scale on the right or left side of the screen. This is a very useful indicator that allows you to determine the depth of any object.

These are very useful functions when fishing for many types of fish. And you always see them when using the fish finder. As you lower or raise the fishfinder, you will see changes in the bottom topography. Do not forget to watch the depth indicator (in Garmin fish finders it is located on the right side of the screen) to track the dynamics of increasing or decreasing depth.

There is no need to analyze arches or icons here – the bottom is shown as a flat shape and you can easily identify it. We will just give you a few tips when scanning the bottom.

Switching from a wide beam to a narrow beam. When you find a hole or depression, switch from a wide scanning beam to a narrow one and scan that area again. This will give you a more accurate picture of the change in terrain and remove the blind spots that appeared when using a wide beam.

Depth alarm. If you are fishing at great depths and your finder has a depth alarm, be sure to turn it on. This will allow you to navigate shallow water without having to keep an eye on the fish finder.

Algae and vegetation. Often a bottom scan can help you find areas of algae. This can help you if you are going to fish for perch in the summer or, on the contrary, want to avoid algae so that your bait does not get caught on them.

Small grooves. Many fishermen know that carp make small depressions in the bottom. The fish finder can find such places and show them to you. And you can easily get a few carps without wasting time looking for a place where they are waiting for prey.

Determining Water Temperature Using Fish Finder

Fishfinders have additional sensors that provide information about the water temperature. This is an important indicator to consider when fishing. Fish are cold-blooded animals, their own thermoregulation is minimal, and the central nervous system is very simple. This means they follow the comfortable temperature.

What Types of Fish Finders Are There

Types of Fish Finders

There are different types of fish finders. It depends on the number of beams and their frequency. The higher the indicators, the more accurate the picture. Portable echo sounders and fish finders are very popular with amateur and professional fishermen.

Single-beam and most common devices. Are inexpensive. Suitable for small ponds. They have only one beam directed to the bottom. Calculates depth and shows all passing objects along the way.

Double-beam have two rays, one of which is directed to the bottom and, accordingly, it determines the depth and topography of the bottom, and the second, less powerful, is emitted into the very water column and is looking for fish.

Three-beam has three beams in a direct sequence, and due to this, they have a large coverage of the territory. This type also allows you to find out the location of the object.

Four-beam fish finders have the same design as in the previous version, only with the addition of another beam of smaller diameter in the central circle, which is not intended for studying the bottom, but for searching for fish in the water column.

Multibeam fish finders are the most expensive but also the best option. Containing about 11 rays, they can show a three-dimensional image, which greatly simplifies the understanding of space underwater.

It is worth choosing a fish finder not according to which one is better and more complex, but according to the purpose of the device.

For boat fishing, pay attention to the fish finder mount. Fastening methods can be of two types: transom and through. The transom is installed at the place where the motor is attached to the boat. Through fastening will require you to punch holes. Choose a model with an amount that suits your needs.

How to Install a Fish Finder Correctly

How to Install a Fish Finder

There are two main options for operating a fish finder. You can use it from a boat, or you can use it from the shore. Let’s consider the features of each method in more detail.

Boat Installation

Depending on the method of attachment, the fish finders are placed in different.

If you attach it to the bottom, place the transducer on the floor of the boat and attach it so that there is no air gap. You can glue it on, or put it in a puddle of water – as you please. Remember to set up the device during installation.

If you are going to sail slowly, the device is mounted on the front of the boat, if it is fast, it is better to install it at the back.

In winter, the method of attachment and the performance of the fish finder is different, which will be described below.

Shore Installation

This option is less common, so it all depends on the device itself. Basically, this device is attached to the boat. If you nevertheless decide to use it on the shore, you can, for example, throw a fish finder specialized for this business into the water, and then catch the signal using your smartphone.

Features of Using Ice Fishing Fish Finder

Going on ice fishing, you should consider the following features of using a fish finder during this period:

  • It is worth protecting the device, and especially the battery, from the cold. For the battery, low temperatures are a disaster, for the device itself, temperatures down to -10 are quite ok. In any case, it is better to build an insulated bag or just a box laid with foam plastic with your own hands.
  • There are two methods of using the fish finder: lower the transducer into the hole and freeze it into ice. Both are not perfect and can create additional difficulties. For example, make a holder for the transducer to be placed in water, or each time pick out the device from ice to change the place of fishing.
  • Using the automatic fish recognition mode is not effective in winter.
  • It will not be possible to find out the bottom topography, since the device is placed in one place, which does not allow studying a large surface.

Our recommendation for winter – Humminbird Ice Fishing Finder.

How to Read Garmin Fishfinder

The general rules for using the Garmin fish finder are no different from other fish finders. I will attach a helpful video below from the official brand to help you understand how the Garmin fish finders work.

Garmin Echo Series – Basic Settings

This video explores the basic operations of Garmin’s echo™ series fishfinders. For more information on Garmin marine products look on the official website of the brand.

FAQs – How To Read A Fish Finder

Despite the large amount of material that we have provided, many users still have questions regarding the operation of fish finders. In this section, we have answered all of them.

Why do fish appear as arches on the screen?
Fishfinders send sound pulses to search for objects. The physics of sound is designed so that sound travels in waves, not in straight lines, and these waves expand in a conical manner, becoming wider and wider. Most fish finders can control the cones of sound waves by changing the frequency of the scanning beam. This is important because different scanning beams are more or less effective in different situations.
What are fish finder blind spots? How do they arise?

In some cases, the fish finder does not detect objects that are immediately below the surface of the water. This is called the “blind spot” of the fish finder. This is caused by the reflection of waves from the surface that occurs when using any fish finders. Surface reflection occurs because water close to the surface reflects some of the waves emitted by the fish finder, and these reflections occur too quickly, preventing the fish finder from correctly processing the data.

Reflections can occur for several reasons. Most often these are waves on the surface of the water, bubbles, currents, and algae. They generate strong sonar noise near the surface. Because of this, a “blind spot” appear in which it is impossible to identify fish.

What are the arches that the fish finder shows me for?
In most cases, the fish will appear as an arch on your display. The reason is very simple. If the fish swims straight through the sonar cone, the signal will return from the edge of the cone, in the middle, and then from the other edge. Returns from two ends of the cone are slightly farther than returns from the middle. Thus, an arch will appear on your display.

A Couple Of Final Tips

Pay attention to such, at first glance, imperceptible details that at the decisive moment can ruin all the pleasure:

  • The battery of the fish finder in winter sits down much faster. If you plan to fish for more than a day, take a spare battery or, if you have a car, an appropriate charge.
  • The best and best quality fish finder is not always good and is often pointless. If you are going to use the device in a local lake three meters deep, the four-beam device will be superfluous. A cheaper single-beam, which will show all the information you need, will be enough for you.
  • Reading the information from the screen of the fish finder, be prepared for the fact that he is mistaken and this is not one big fish, but a flock of small ones or even a shoe. This can especially happen in automatic mode with icons. If you want more accurate results, learn to do the analysis yourself.

Use the fish finder according to all of the listed rules, and it will brighten up and facilitate fishing, will not spoil your nerves, and provide a rich catch.

Have a nice fishing!

Tags: #how to read a fish finder / #ice fishing fish finder / #how to read garmin fish finder / #how to use a fish finder / #how to install fish finder / #how to effectively use a fish finder / #how to read a fish finder screen

Sergio Smirnoff
Sergio Smirnoff
Professional fisherman with 40+ years of experience.
Fishing and the ocean is a passion of my life. I am passionate about all kinds of sea fishing: fishing from the shore (spinning, float, baitcasting), from a boat, and spearfishing day and night.

I live in Tenerife (Canary Islands) for the last 10+ years and share my daily fishing experiences on my website. Many years of personal experience as a fisherman and the vast experience of my friends allow me to write professionally on any fishing topics (from choosing a flashlight and equipment to deep-sea fishing).

All of my advice is based on practical real-world experience and will be useful to both novice anglers and professionals. Read more about the author.

Affiliate Disclosure: sometimes gets paid for listings, through sponsors or affiliate programs like Amazon, Ebay, Cabelas, Bass Pro Shop, Shimano, Daiwa, Rapala, Renn, Okuma, KastKing, etс. Clicking a link helps keep free, at no extra cost to you!

About the author: Each article is verified by the fishing expert Sergio Smirnoff. The articles are written by professional and amateur fishermen with 20+ years of fishing experience.

Note: The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of any agency. The articles are for informational purposes only, share your opinions in the comments and join the fishing discussions, let's share our fishing experiences together!

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7 months ago

I advise all fishermen to buy a fish finder with a big screen and a famous brand. The thing is expensive and you can not make a mistake and constantly change it. At night and when the waves the big screen is more comfortable. You get used to the menu, the design, and the keys. Water protection and the number of functions are important. I made my purchases on the recommendation of the sales clerk. You helped me to use it to the fullest. Thank you.

Product Name
navigator (fish finder) for sea fishing