How To Read a Fishfinder (2021 In-depth Guide)

Lance Wilkins
Lance Wilkins
Editor @ CallOutdoors. Outdoor gear-head and adventure addict. I fish, camp and enjoy to writing about my adventures.

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I caught my first fish with a cane pole and crickets.

No cutting-edge fishing tools —just a 7-year-old boy on a dock staring at a cork.

When I turned 15-years-old my whole fishing world changed.

That’s when my uncle took me out on his boat and taught me how to read a fish finder.

Having the power to use fish ID and see underwater structures made fishing so much fun. After a while I was able to read a fish finder like a pro and it turned into my lifelong sidekick.

To this day, I use sonar technology to find fish.

But not everyone had an awesome uncle to teach them about fish finders, sowo —

How Do Fishfinders Work?

Fishfinder Mounted on a Boat
Fishfinder Mounted on a Boat

What is Sonar?

Fish finders have something called a transducer that emits sonar waves into the water.

Sonar is short for “Sound Navigation and Ranging”.

The sonar technology waves bounce off underwater objects and report their size, shape and location back to the fish finder.

The data pops up on a display so you can see fish, structures and vegetation.

This is how anglers use traditional sonar fish finders to find wrecks, drop-offs and schools of fish.

Function

Once you learn to read the fish finder display, you can find the best fish hotspots.

Cool fact —Dolphins use “natural sonar” to navigate the ocean and find food —also called echolocation.

About Transducers

They Scout Out the Scene

The transducer is the hands and eyes of the sonar fish finder.

It emits sonar waves that feel out what’s in the water then relay the data back to the display.

You can buy a fish finder that comes with a transducer, or buy one separately.

Just make sure you understand how it’s installed and if it’s ideal for your boat.

Which Transducer Should I Get?

Fish finders come in a range of powers and frequencies, depending on your fishing needs.

Here are some things to consider —

How Deep Are You Fishing?

Underwater
Underwater

Ping Power (Measured in Watts)

The “ping” is the sonar waves/signal sent out by the transducer. The greater the watts the deeper it goes, so the power of the fish finder should be based on how deep you are fishing.

If you’re in freshwater a 300-watt ping will be plenty. In the shallows you can see the bottom using about 200 watts, so 300 is enough to see deeper areas.

If saltwater fishing is your thing, roll with a 500 – 1200 watt ping.

This range covers coastal fishing areas and the deeps, so it’s ideal for blue-water anglers.

Frequency Matters (Measured in Kilohertz)

Low Frequency

Low frequency emits a wider sonar cone, goes deeper and covers more area…

The downsides are that you need to troll at a slower speed to get accurate data, and it offers lesser detail of structures and fish.

Lower frequencies are good for maximum depths and used by deepwater anglers.

This frequency is ideal if you’re looking for bluewater lunkers or reds lurking around a wreck.

Mid to deepwater recommendations are 50kHz – 150kHz (low frequency)

High Frequency

High frequency gives you less coverage and depth, but provides better visual clarity in shallow to mid-depths

It comes in handy when targeting fish that are holding tight to structures.

Because it offers the best detail resolution at these depths, it helps you see smaller fish like schools of baitfish.

The greater clarity comes in handy when hitting lakes and coastal waters because you can go faster and still see sharp detail.

NOTE

This is also a good frequency for ice fishing because it’s medium-depth and offers a clear image. Shallow to mid-depth recommendations are 200kHz – 800kHz (high frequency)

What’s All the Noise About CHIRP Sonar?

CHIRP stands for Compressed High Intensity Radiated Pulse.

This fish finder technology is the best of both worlds and combines low and high frequencies for better depth and image detail.

Many anglers fall in love with these transducers and never go back to fish finders with fixed frequencies.

They are a little more expensive and everything but well worth it.

Okay, now that we’ve talked about sonar, here are some other things to consider before buying a fish finder —

The Size of Your Boat

Fisherman on a Float Tube
Fisherman on a Float Tube

Type of Boat Mount

You have options when it comes to how your transducer is mounted.

You could go with any of these main styles —

Transom mount

The transom is the part of the boat where the sides of the hull meet. It’s located in the rear, and is part of the stern.

These mounts are ideal for smaller boats because —

  • They’re lightweight mounts.
  • They are easy to repair compared to other mounts.
  • They take up less space and are less likely to come into contact with the propeller.

Through-hull mount —

These work best on larger sport fishing boats because they allow more control over the mount location.

The downside is that it requires a hole to be drilled through the hull.

Here are some helpful tips for installing a through-hull mount —

  • For powerboats, mount near the centerline and inboard to ensure it’s in contact with the water at high speeds. This helps the propeller blades maintain a cleaner flow of water.
  • If you’re dealing with outboards and I/Os, be sure to mount it forward and to the side of the engine.
  • If you’re going inboard with the mount, make sure it’s ahead of the propeller and shaft.
  • For stepped hulls it’s important to mount ahead of the first step, so it doesn’t obstruct your ability to move around in the boat.

In-Hull Tank Mount

First off, these are only suitable for fiberglass hulls.

It’s mounted as a liquid-filled tank and bonded to the boat surface.

These are ideal for trailer boats because there is no external hardware exposed.

Here are some advantages of using and in-hull mount —

  • They reduce the chances of damaging your fishfinder.
  • Does not impact the speed of your boat
  • Has an accurate sonar readout at high speeds

Is It a Fish Finder GPS Combo?

Simrad Fishfinder
Simrad Fishfinder

Most fish finders today come with GPS, display monitor and transducer.

These are the best value, but you can also buy them separately if you like.

In case you don’t know, GPS stands for global positioning system.

It’s important to make sure you know what you’re getting in a fish finder, so go over the features and specs before snagging one.

You should be able to find all the necessary info from the manufacturer’s website.

How Much Bottom Does a Fish Finder Cover?

As mentioned above, this is determined by the power and frequency of the transducer

The more power, the stronger the sonar signal.

A transducer with higher power can reveal the bottom at greater depths.

Frequency is a little different. A transducer with lower frequency can go deeper and cover a wider area, but offers less detail.

Okay, now let’s dive deeper —

The Art of Reading a Fish Finder Screen

Identifying Fish

I bet you’re itching to know how fish are identified on a fish finder screen.

I won’t torture you, let’s begin —

The Easy Way

Fish IDs Technology

You’ll be glad to hear that most fish finders have Fish-ID technology.

It uses data to determine which objects are fish, and shows them on the fish finder screen.

Fish icons show up on the display to pinpoint their type, size and location.

Fish ID is pretty accurate, but occasionally it could mistake plants for a school of fish –it all depends on the fishing areas.

The Hard Way

Reading Fish Finder Arches and Raw Data

While some anglers keep it simple with fish IDs, others like the challenge of reading fish arches.

There is a bit of a learning curve, but it’s a more accurate way to find fish.

Fish arches show up as curved lines on a fish finder screen using traditional sonar.

You can determine their size by the width of the line and arch, whether they’re half or full.

Best Way to Spot a Trophy Fish

Garmin Striker 4 Display
Garmin Striker 4 Display

It’s a common misconception that a long fish arch indicates a big fish size. In reality, line width and full arch width are the true indicators.

This is because a big fish beams back a stronger signal, which means a thicker line and fuller fish arch width.

TIP

Keep in mind that smaller lines on a fishfinder can also indicate a big fish.

Trophy fish can show up as half a fish arch if they swim through a small portion of the sonar cone.

If you see a long fish arch, it’s likely a fish that slowly swam through the sonar cone.

What About The Depth of The Water?

Most fish finders have a depth finder located on the transducer.

Knowing the depth of the water is crucial because it can indicate the type of fish present.

Icons

For example, if you see some fish icons or fish arches sitting 25 feet deep in lake Michigan there’s a good chance you’re seeing summer smallmouth.

This is how a depth finder can help you pinpoint different types of fish.

The water bottom depth is usually displayed on the top left area of the screen, but different models may vary.

Keep in mind, some display depth in meters rather than feet.

Water Temperature

In most fish finder models, the water temp is located under the water depth reading.

This is pretty handy because it’s another way to target species of fish, based on their warm or cold preference.

Is There a Speed Sensor?

Most fish finders show the speed you’re moving on the display screen.

It’s basically a built-in speedometer which is helpful when you need to maintain a steady, slow speed while you read a fish finder.

What’s the Best Boat Speed?

Motorboat Water Wake
Motorboat Water Wake

Trolling speeds of 1 – 4 miles miles per hour give your fish finder enough time to see what’s under your boat.

They help your transducer relay accurate information to the display which makes arches, fish ID and structures easier to read.

What Are the Display Types?

Color vs Grayscale

You can get 2d sonar fish finders in grayscale, or color on the screen. Let’s look at each one…

Color Fish Finder Display

Garmin Striker Plus 7Cv
Garmin Color Display

This is the best way to see fish underwater.

Color gives more screen contrast, so you can tell fish and structures apart. It’s better for identifying different types of fish and hard bottom.

It’s easier for most anglers to read a fish finder screen in color, and I highly recommend going that route.

How to Read a Fish Finder Structure in Color

Brighter colors indicate a softer material like clay, weeds or plants.

That’s because less dense objects produce a weaker signal return.

The opposite is true for denser objects like rocks or reefs.

Denser materials emit a stronger return so they show up as darker and richer colors.

Grayscale Display

Humminbird 780
Humminbird 780

This is also called a black and white screen fish finder.

Reading one is a little harder because it’s more difficult to tell fish apart from structures. But on the flipside, it’s great for judging bottom hardness.

They work like color displays — denser objects show up darker while less dense objects appear lighter. Here are some tips for using grayscale —

  • A thick line on the bottom means hard bottom material, whereas a thin line means it’s soft bottom material like clay.
  • Fish can appear as white specks suspended in water columns on the screen
  • A larger fish may appear as a cluster of white specks
  • Bottom growth like weeds appear more as a solid mass on your screen

What Is Echo Return Strength?

Another perk to using a color fish finder screen is that they easily show how dense a structure is.

The denser the object, the higher the return strength —

  • Denser objects appear darker in color
  • Less dense objects appear lighter in color

How to Find Baitfish

The key to finding baitfish on a fish finder is to look very carefully.

They display on your screen as dots, dashes and tiny lines.

Spotting a school of fish is easier because they tend to look like big round balls and are always suspended in water.

Typically they will have a yellow hue compared to the green vegetation.

How to Read a Fish Finder Data Display Sequence

Traditional 2D Sonar Screen

When you read a 2D display sequence, remember that it’s not like reading a book. Data is viewed from right to left – the opposite of how you read books from left to right.

What Do Bass Look Like on a Fish Finder Screen?

If you’re looking to find largemouth bass with a fish finder it’s important to know how to recognize them.

Here is a trick you can use —

Largies look like white or light yellow beans on a traditional fish finder. Look for them to be spaced apart from other fish, not heavily clustered.

Smaller bass tend to cluster and look more like blips or blobs, but largies like their space.

TIP

You can also tell them apart from structure and hard bottom because they are lighter in color and circular in shape. Look for recesses and holes, then look for a bean-shaped lunker!

Down Imaging

Humminbird Down Imaging Feature
Humminbird Down Imaging Feature

It’s the same way with down-imaging —the data flows from right to left across the screen. This commonality comes in handy when you’re using Fishing Reveal technology, but I’ll talk more about that in a bit.

Remember, new data from the transducer flows in from the right side of the screen, so make sure you don’t read it backward. You must read a fish finder correctly to know what’s underwater.

Side Imaging

Humminbird Side Imaging
Humminbird Side Imaging

Side-imaging data is very different. It flows from the top of the screen to the bottom. You’ll also notice it reveals more details of structures.

This is because higher concentrations of silt and mud are present in the water below the boat.

Down-Imaging vs Side Imaging

Lots of fish finders come with traditional imaging, down-imaging and side-imaging ability.

The coolest ones combine them to give you an all-around view on your fishfinder.

Here are the strengths of down and side imaging —

Down-Imaging Pros

  • Gives photo-realistic image of what’s under the boat
  • Shows fish in better detail, easier to identify
  • Structures appear more pronounced and recognizable (easier to find weed lines)
  • Ideal for deepwater jigging
  • Using a fish finder this way provides a clear image of the bottom

Side Viewing Pros

  • Ideal for finding schools of fish while trolling
  • Saves time on the water —find hotspots on your fishfinder quicker
  • Provides 180 degree screen view of what’s around you in the water
  • Lets you gauge how recessed fish are into structures
  • Fish show up as white streaks on the fishfinder screen that are recognizable

How to Find Different Kinds of Fish Hiding in Structures

Make Multiple Passes Over an Area

One pass doesn’t always give the fish finder enough data to pinpoint honey-holes. I’d recommend making two or three passes over the area so you can get the most detail and accuracy.

Use Dual Frequency Fish Finders

One way to do this is using traditional sonar and down-imaging together. These frequencies work together to give a more robust view.

When you combine traditional sonar, down-imaging and side-imaging you get a complete visual.

You can start out with a low frequency fishfinder, then switch to a higher frequency fish finder to hone in on hotspots.

Fish Reveal Technology

Using FishReveal on the Water

Some fish finders like the Lowrance brand let you view a split screen of traditional sonar and down-imaging.

It’s like Fish ID, but it lets you see the structure and the fish in greater clarity and detail.

Adjustable Contrast

Lowrance technology allows you to adjust the contrast in the menu search, which makes the fish stand more visible.

This helps to pinpoint fish hiding in a given structure like ledges, drop-offs and recesses.

The data display sequence is the same for both imaging options, so they work well together.

Both represent data flowing from right to left, which makes finding hidden fish easier.

NOTE

Your Lowrance fish finder should come with a menu how to read section in the instruction booklet

Identifying Underwater Structures

Knowing what the underwater structure is made of gives anglers a big advantage.

Depending on its type and hardness, it could indicate which types of fish are hanging around.

Let’s look at a few soft bottom structures that will pop up on your fishfinder screen.

Vegetation and Weeds

You may want to cast a soft plastic lure in some weeds to stir up a lunker.

These structures usually appear as wispy vertical lines on your fishfinder.

Finding Depressions

These can be easily spotted because they make a small “V shape” dip in the bottom contour. Pinpointing these hotspots is especially helpful when you’re carp fishing because they hide in them during cold months.

Locating a Point

This underwater structure is easy to see using a 2D sonar fishfinder.

That’s because they’re more gradual than drop-offs, so If you’re trolling at a steady pace the incline should be easy to see.

Closing Thoughts

Hopefully, you know a bit more about how fish finders work.

Believe me, how to read a fishfinder is a lot to soak in, but now you have a head start on finding more fish.

Maybe you never heard of fish id technology.

That’s okay, now you know and can use and have success on your next fishing trip.

Should You Get a Fishfinder?

But the real question is this —Is it all worth it? Yes indeed, it is.

A fish finder works great if you apply the basic knowledge and points I broke down in this guide.

They are truly devices sent down from the fish gods!

Good luck and happy fishing my fellow anglers. Be safe, smart and go out there and snag a big one for me!

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Lance Wilkins
Lance Wilkins
Editor @ CallOutdoors. Outdoor gear-head and adventure addict. I fish, camp and enjoy to writing about my adventures.
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