Ultimate Guide: How Does A Fish Finder Work?

Since time immemorial, fish have eluded fishers. And throughout history, fishers have dedicated themselves to studying the habitats, diets, and seasonal rhythms of the fish species they relied on for survival—not to mention the big ones they relied on for bragging rights! To imagine a magical tool that would reveal fish under the water’s surface was a wishful fantasy at best.

And yet today, that fantasy exists. And it’s called a fish finder. Used to its best advantage, a fish finder can augment more traditional fishing skills by revealing environmental information like water depth, bottom structure, and features. It can also show you the fish—provided you know how to use it.

Interpreting sonar readings from a fish finder isn’t hard, but like any skill worth learning, it does require some technique and practice. It’s also difficult to do if you don’t possess at least a basic understanding of how a fish finder works.

So, whether you’re a first-time Fish Finder Tech browser, have recently purchased a new fish finder, or just want to refine your fish-finding skills, the following information is for you. In this article, we’ll cover the basics of sonar as it applies to fish finder function and usage, explain how to interpret the information your fish finder provides, review the different types of fish finders, and discuss the features of some specialized fish finders, plus more. By the end of this article, you should be ready to head out on your next fishing excursion with the confidence to get the most out of your fish finder’s technology.

Let’s take the plunge!

A Brief History of Fish Finders

Before there were fish finders, there were fathometers—active sonar instruments that displayed water depth and made automatic permanent records of measurements. As far back as 1913, a German inventor named Alexander Behm was patenting the invention of an echo-sounding device for measuring the depths of the sea (or obstacles for ships) using reflected sound waves.

Though fish finders were initially derived from fathometers, today, the instruments and their technologies have merged.

Operating Theory

A fish finder consists of three main parts—a transducer, a processor, and a display screen (the processor and display screen are usually combined).

The transducer sends and receives sonar pulses underwater. It contains piezoelectric crystals and vibrates at a specific frequency.

The processor (basically, a small computer) interprets the information received by the transducer and transforms it into digital signals. It’s the brains of the operation, if you will.

The display screen—well—displays the information in a human-digestible format. Among other things, it can be programmed to show fish icons or raw sonar data (more on these options later).

How Exactly Does Sonar Work?

Sonar stands for Sound Navigation and Ranging. It’s a technology with wide-ranging underwater applications because sound waves travel farther in the water than light waves or radar. Marine animals such as whales and dolphins use built-in sonar for hunting and navigating. But how exactly does it work?

For sonar to work, a transducer emits pulses of sound waves into the water column. These pulses bounce off underwater objects such as the bottom features and—yes—fish, and are reflected back to the surface.

The sonar unit tracks the length of time a returning signal takes to travel through the water. The longer it takes, the further away an object is.

The sonar unit also measures the signal’s intensity. The stronger the return pulse, the harder the object is.

Then, the transducer sends out another pulse and repeats the process. Sound waves travel so rapidly underwater that a fish finder transducer sends out as many as dozens of pulses per second. Pulse frequency is one of the factors that impacts image accuracy and clarity. Newer CHIRP sonar takes advantage of multiple frequencies for this very reason (more on this later).

Based on the constant stream of sonar readings it receives, a fish finder generates live images of the underwater environment.

Sonars Scan in Cones Not Lines

The sonar technology used in traditional fish finders (known as 2D sonar) scans in the shape of a cone. This is important to note because fishers often assume that what they’re seeing on their fish finder display screen represents what’s occurring in real time directly under their boat. Often, this isn’t really the case. But why?

To explain, let’s think about the transducer shape. The shape of a standard 2D transducer is round. As a result, it emits a circular sonar signal that widens into a round cone as it travels downwards through the water.

The deeper the water, the wider the base of the sonar cone becomes. A wider scanning area correlates with lower accuracy and detail (and vice versa). What this means in practice is that adjusting your fish finder’s scanning beam frequency (which controls the range of its sound wave cone) can help you glean the best information possible based on your specific circumstances during any given fishing trip.

Wide beam scanning (usually forty to sixty degrees) will quickly provide you with an overall picture of the underwater landscape but will leave out the details. It’s best suited for shallower waters. Narrow beam scanning (usually ten to twenty degrees) covers a smaller area with greater precision. It’s better suited for deeper water since the sonar cone can’t spread as wide.

Surface Clutter & Blind Zones

A fish finder blind zone occurs when sonar signals become too chaotic or confusing for the device to interpret. Often, this happens near the surface of the water and is known as surface clutter. Surface clutter is the result of sonar waves rapidly reflecting off aquatic elements like bubbles, currents, or floating debris. This gives inaccurate readings that make it impossible to identify fish.

Blind zones caused by surface clutter impact fish finder readings to a depth of just over 3 feet—sometimes less at higher sonar frequencies.

General Interpretation

Let’s talk about what actually appears on your fish finder display screen and how to interpret it. Statistics like water temperature, depth, and GPS coordinates (if your fish finder has this feature) are pretty straightforward.

But what about all the sonar data? Though it can be a little confusing at first, this is where you can really start to see the bigger picture.

As mentioned earlier, you can program your fish finder to fish icon mode, or you can just work with the raw data. The choice is yours, but understanding how to interpret the raw data will ultimately help you in either mode. Let’s dive in!

Many modern fish finders display sonar data scrolling from right to left on the screen, with the freshest data appearing on the right. This can take a little getting used to because the screen keeps moving even when your boat—or a fish—is stationary.

Many fishers got excited when their fish finder displayed four big fish, only to realize that it was the same stationary fish displayed four times from four different sets of sonar data. Don’t be fooled! But do pay attention to the changing positions of fish as your fish finder tracks them.

In general, the sonar display on your fish finder will use color intensity and brightness to indicate signal strength. In practical terms, this means that harder objects (such as a rock bottom) will appear thick, bold, and bright, while softer objects (like mud or weed beds) will appear in less contrast. With a little practice, you’ll be visualizing the aquatic world beneath you in no time. Combine your fish finder’s technological data with your own knowledge of the species you’re after to find the perfect location…and start casting or trolling!

Fish Arches

If you turn off fish icon mode, how will you identify fish on your display screen? The answer is that fish will often (but not always) appear as crescent-shaped arches. Why? Because if a fish swims through your cone, the impulses returned from the edges of the cone have to travel slightly further than those from the middle. Voila! An arch appears on your screen.

If a fish only swims through part of your sonar cone, you’ll see shapes more like dashes or half arches. Ultimately, keep an eye out for any shapes suspended in your sonar cone, and remember that long arches don’t necessarily correlate to big fish.

Because your sonar cone is wider at the bottom, deeper fish will appear longer on your display—regardless of their actual size. A fish that’s within your sonar cone for a long time will also create a longer arch.

Thick arches, on the other hand, do indicate bigger fish. Train your brain to think vertically and to watch for thick lines and you’ll be reeling in the big ones in no time!

Types of Fish Finders & How They Work

2D Sonar

We’ve already spent quite a bit of time in this article covering 2D sonar. 2D sonar’s cone shape means that you get a circular scan of the area under and around your transducer. In general, 2D sonar provides a good overall snapshot of features and fish, but objects are often fuzzy or indistinct, especially the deeper down you go.

CHIRP sonar has drastically improved 2D sonar imaging (more on this later), but many fishers still prefer to explore other sonar options, such as side imaging and down imaging.

Side Imaging

Side imaging sonar sends out signals directed outwards from both sides of your boat (right and left). It’s broader ranging than 2D sonar and is a great way to get a general sense of major features in the area surrounding your boat. However, side imaging sonar is not great at detecting small objects—such as fish. It’s usually used in combination with other sonar technologies.

Down Imaging

Down imaging sonar is by far the most precise and accurate of the three main types of fish finder sonar. By focusing directly downward in a narrow beam, it captures higher definition details such as individual baitfish or fish resting on the bottom. Down imaging’s main disadvantage is that it’s highly localized; it limits you to scanning a very narrow area.

2D Sonar Vs. Down Imaging

While 2D sonar provides a more comprehensive but less detailed scan of a larger area, down imaging provides a highly accurate scan of a very narrow area.

Down Imaging Vs. Side Imaging

Side imaging provides a panoramic view of your surroundings with little detail, while down imaging provides just the opposite—a precise picture limited within very specific parameters.

Advantages & Disadvantages

**Create a table with this data.

2D Sonar

  • Advantages: Comprehensive view of surroundings and nearby fish
  • Disadvantages: Limited clarity and object separation due to wide sonar cone

Side Imaging Sonar

  • Advantages: A wide snapshot of your boat’s surroundings
  • Disadvantages: Low detail quality makes it tough to spot fish

Down Imaging Sonar

  • Advantages: High precision and accuracy
  • Disadvantages: Very narrow focus limits your scope of information

Not surprisingly, many fishers opt to combine two or more types of sonar to make the most out of the data they’re receiving in any given situation.

How Do Ice-Fishing Fish Finders Work?

The technology for ice-fishing fish finders is essentially the same as for open-water fish finders, but there are a few key adaptations and specializations.

First of all, ice-fishing fish finders are specifically designed with transducers and screens that can withstand sub-zero temperatures (we don’t recommend exposing an open-water fish finder to these kinds of extremes). Ice transducers are also fitted with a flotation device to keep them pointing downwards in a vertical ice hole.

Speaking of vertical, ice-fishing fish finders are also optimized to eliminate lag time and return readings as close to real-time as possible. By switching to the RTS function, you can eliminate historical data and focus only on exactly what’s happening right under your ice hole at any given moment. Ice fishing, after all, is a vertical game, and there’s no way to relocate based on the big fish you saw heading a different direction a few minutes ago—so you may as well keep things real in the present moment.

Many ice fishers use flashers instead of fish finders, which only show real-time sonar data from directly under the ice hole. The choice is yours, but with the versatility of modern technology, you can get essentially the same function and features with a fish finder as you would with a flasher…plus more.

What Is CHIRP on a Fish Finder?

CHIRP sonar (Compressed High-Intensity Radar Pulse) is a newer sonar technology that utilizes multiple frequencies. By sending out both low and high-frequency signals in bursts (ascending from low to high throughout the duration of the pulse), CHIRP sonar provides higher-quality imaging with more resolution and clarity. It can be a game-changer compared to traditional 2D sonar, and you may wish to consider CHIRP technology if you’re on the hunt for a new fish finder. Read more about CHIRP technology here.

Fish Finder Batteries

Without quality batteries, your fish finder won’t do you any good at all. Be sure to research battery type, volts, special features, price, and reviews before purchasing. Our advice? Always have an extra on hand. Read this for a more comprehensive fish finder battery comparison.

Fish Finders With GPS

Fish finders with GPS capabilities can take your fishing game to new levels. Not only is it handy to have a built-in navigational aid for tracking your position on top of a marine map, but it’s a great way to chart your own favorite fishing spots. With chart plotting technology, you can create personalized maps as you explore and then return to the best locations again and again.

Some GPS fish finders also contain custom mapping software that works in conjunction with your transducer to create detailed records of underwater topography.

Types of Transducers & How They Work

If you’re serious about upping your fish tracking game, consider purchasing multiple types of transducers to connect to your fish finder. Select the correct frequency for your specific application, and be sure that the transducers you select are compatible with your fish finder’s technical specifications (e.g., CHIRP sonar).

Transducer Mounting Style

Before you purchase a fish finder, consider the boat you’ll be mounting it in and your personal preferences when it comes to mounting style.

In Hull

In-hull transducers don’t need to be in direct contact with water. Instead, they transmit sonar signals directly through the hull of your boat. Gluing a transducer directly to your boat’s hull can be a simple and permanent mounting solution.

Transom Mount

This type of mount involves an adjustable bracket attached at the stern of your boat. It’s the most common type of fish finder mount and requires that the transducer be inserted directly into the water.

Trolling Motor Mount

A transducer attached to your boat’s trolling motor ensures that it’s always in contact with the water when you need it to be. There are multiple motor mounting options.


In conclusion, fish finders are incredible tools that can dramatically improve your fishing technique and success rate. But, without understanding how they work, it’s difficult to use them with much finesse.

In this article, we’ve covered the basics of sonar, how to interpret sonar data on your fish finder, and special features such as GPS and tracking under ice. With this knowledge in hand, it’s time to get out to the nearest body of water and apply what you’ve learned here. 

Happy fishing!

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