How to read a fish finder

and use the data to catch more and catch bigger

Just getting started using a fish finder? Know the basics but want to get more out of your fish finder? Not sure you’re reading your fish finder display correctly? Don’t worry, we’re here to help. This short tutorial will teach you how to make sense of your fish finder display so you can tell what’s a bait fish, what’s a trophy catch and what’s just a submerged tree that you’ll snag on. Take just 7 minutes to go through this tutorial and at the end you’ll be able to read your fish finder display for:


    Identifying fish


    Judging fish size


    Identifying different types of underwater structure


    And judging bottom type and hardness

How Sonars Work

how sonars work

In this blog we’re focusing on the actual images you see on your screen, so we won’t be getting too technical about how sonars work – but our page on how sonars work will tell you all you need to know. But for now there are a couple of technical point you need to remember. First, that fish finders scan in cones. Why does this matter?

The size of the area you’re scanning will be affected by the angle of the cone. A wide beam cone scans between 40°-60°, meaning you’ll be covering a large area. A narrow cone will scan between 10°-20°. So make sure you’re aware of whether your fish finder is using a wide or narrow cone when you’re looking at the data on your screen. The Deeper PRO and PRO+ have wide and narrow beam scanning (55° and 15°), the Deeper START has a medium/wide beam (40°). One other point to remember about how you sonar works is that it is constantly sending and receiving data, which means your display will be continually scrolling. The current scanning data will be on the right – the further left on the screen, the older the data.

So just remember these 3 points when you’re looking at your sonar display: 1. Know if you’re scanning with a wide or narrow beam. 2. The screen is constantly scrolling – this doesn’t mean your sonar is moving.

1. Identifying fish

    Fish Icons
    Fish Arches
  • fish icons

    Fish Icons

    Fish icons are a useful tool when you are getting started with a sonar device: your fish finder interprets the sonar data it receives and tries to work out if it as a fish or not. It does this based on factors like the size of the object and the strength of the sonar return. In the case of Deeper sonars, we use an advanced algorithm in the Deeper App to interpret the data. Unfortunately, even the most advanced units aren’t 100% accurate in interpreting correctly.

    Fish swim at different speeds, in different directions through the sonar cone your fish finder sends. This means there is not one uniform reading they always give. What’s more, some underwater objects or vegetation may sometimes give readings similar to that of a fish.

    So for the best accuracy using your fish finder, you need to learn to interpret the data yourself.

    It’s time to take the plunge and switch off fish icons – you’ll be rewarded with more accurate readings of where the fish are. Plus there’s the added bonus or learning and refining a new skill.

  • fish arches

    Fish Arches

    With fish icons switched off, how do you identify a fish on your sonar display? Look for the arches.

    Fish will show up on your screen as an arch (the reason why they are shown as an arch is explained in detail here). But it’s important to remember these arches can vary in size (length and width), and might not be a full arch – look out for those half arches too. The screenshot below gives some nice examples of different arches. They vary in length and width, and some are not full arches, but these are all fish.

2. Judging fish size

Now you have spotted some fish arches, it’s time to make sense of what each arch tells us about how big the fish is. So let’s discuss length, thickness, and half or full arches.

    Fish arch length
    Fish arch width
    Handy tip – use the vertical flasher
    Half or full fish arches
    Finding bait fish on your fish finder
    Spotting a trophy fish on your fish finder display
  • fish arch length

    Fish arch length

    The most common mistake anglers make when reading their fish finder is thinking that a long arch means a big fish. This is not the case. On your sonar display, you should think of length as representing time. For example, imagine you keep your fish finder stationary in the water (in other words you are not reeling or trolling it). If there is a fish underneath that is also stationary, what will you see on your fish finder display? You will see one continuous line. That doesn’t mean there’s a blue whale stranded in the pond you’re fishing. It means there is a stationary fish under your fish finder, and it might be a very small one.

    Now let’s imagine another scenario – again your sonar is stationary, but this time 2 fish swim through your sonar beam, one big and one small. The big fish swims very quickly through the sonar beam, the small one swims slowly. Which one will make the longer fish arch on your screen? The answer is the small one. That’s because a slow moving object will leave a longer mark than a fast moving one, whatever their size.

    Take a look at the screenshot below. This return is from a school of bait fish. Notice how long some of the arches are. This may be because the sonar is stationary or moving very slowly, or because the fish are only moving slowly. Either way, it’s a good illustration of the fact that a long fish arch does not necessarily mean it’s a big fish.

  • fish arch width

    Fish arch width

    So length is not a very good indicator of fish size. It’s much better to look at the width of the fish arch shown.
    Just remember this when you are interpreting fish arches:
    think vertical, not horizontal.
    Even if the arch is short, if it is thick then it means the return has come from a big fish. Take a look at this screenshot again. Of the 4 fish shown, which one is the biggest?

    The answer is the fish on the bottom right. All the arches are roughly the same length (and they are all quite short). But the thickness of the bottom right arch tells us it is the biggest fish. This fish is not a monster, but it would be a decent catch.

  • vertical flasher

    Handy tip – use the vertical flasher

    Here’s a useful tip for judging the width / thickness of a sonar return: switch on the vertical flasher (also know as a-scope) on your fish finder if it has one (to do this on the Deeper App, go to Settings > Sonar > Vertical Flasher). The vertical flasher on the right side of your screen will show you exactly what’s under your sonar at this exact moment. You can use it for vertical jigging, and of course forice fishingtoo. For judging fish size, look at both the thickness and width of the returns on your vertical flasher. A larger fish will give both a wider and a thicker return.

  • full arches

    Half or full fish arches

    The final point to remember when you are looking out for fish arches is that it doesn’t need to be a full arch. Half arches (like the ones shown in the screen shot above) also show that there is fish. In ourtutorial on how sonars work, we explain in detail why sometimes you get a full arch and sometimes you get a half arch. The short answer is that you will get a full arch if a fish swims through the whole of your sonar cone, and a half or partial arch if they only swim through part of it.
    But what’s important to remember when reading your display is that a half or partial arch could still be a big fish – the quality of the arch is not an indicator of how big the fish is.
    So be on the look out for both full and partial arches, and remember that the thickness of the arch is the best indicator of fish size.

  • bait fish

    Finding bait fish on your fish finder

    Bait fish will show on your display as dashes, lines or even just dots, so they can sometimes appear similar to vegetation. There are 3 key differences though:

    1. The bait fish will usually be suspended in the water, not on the bottom;
    2. The your fish finder will show bait fish in a different colour to vegetation (in the case of the Deeper App’s standard colour display, they show as yellow rather than green).
    3. The bait fish will often be in balls, which will then be shown like a cloud or cluster, rather than lines.

    The screenshot below shows both individual bait fish and a ball of baitfish. Notice the difference in colour from the green vegetation on the bottom.

  • trophy fish

    Spotting a trophy fish on your fish finder display

    So, let’s bring together what we’ve learned by remembering 2 key points:
    1. Think vertical, not horizontal – the width of the fish arch is the biggest clue to the size of the fish.
    2. Arches can be half or full – a half or incomplete arch doesn’t mean it is a smaller fish. Again, the width is your best clue.
    Your fish finder isn’t going to tell you which species of fish it is, but now you know the size you have a good chance of working out the species based on the lake or river you’re fishing, and the depth at which you find the fish.
    So when you see an arch like the one below on your screen, you’re about to catch BIG!

3. Identifying different types of underwater structure

sonar scanning

Fish finders are ideal tools for reading depth, seeing the bottom contour and finding underwater features. Sometimes this information will be more valuable to you than actually marking fish. So let’s take a look at reading your fish finder display for underwater features.

Before we do this, just remember two things:

  1. Your sonar will scan constantly, which means your sonar display keeps scrolling even if the sonar isn’t moving. If your fish finder is stationary, the bottom will appear flat but it might not be. To get an accurate picture of the bottom contour, make sure you are trolling or reeling in your fish finder at a slow, steady speed.
  2. The depth scale on the right of the screen enables you to identify the depth of any features you find. The depth reading in the top right of the screen shows you the bottom depth under your fish finder right now – bear in mind this is not necessarily the same depth as the features you have just scanned, especially if you are scanning drop offs or points.


These are very valuable features to target for a range of species, and the good news is, they are easy to spot with a fish finder. As you troll or reel in your device, you will see the depth contour change – don’t forget to use the depth reader on your display (on the Deeper App it’s in the top right corner of the screen) so you can track how quickly the depth is rising or falling.

Drop offs and humps will be easy to spot, like the one shown below.

There are 3 tips that can help improve your accuracy when looking for these features:

  1. Once you have identified a drop off, switch from wide to narrow beam scanning, and then scan that area again. This should give you a more exact profile of the contour change, and will also ensure you avoid any ‘dead zones’ in your scanning.
  2. You can use bathymetric mapping rather than scanning functions. With Deeper, you can create bathymetric maps – these are colour-coded underwater contour maps – in Boat Mode (3.0, PRO and PRO+ models) and Offshore GPS Mode (PRO+ only). These maps will also show you drop offs and depth changes, and make it easier to return to the best spots again.
  3. If you are fishing a large area, for example on your kayak, set a depth alarm to alert you when you enter your chosen depth range. This way, you can paddle over shallower water without having to check your fish finder, then slow down and start checking your display when the alarm sounds.

brush piles

These are also very popular features for a range of species, especially predators who will use them as ambush points.

The colour on your display is crucial here for identifying a brush and logs. Because they will send a different frequency of sonar return, your fish finder will show them in a different color to the bottom (otherwise, it will just look like a hump). So make sure you choose a color palette that will highlight this difference. In the Deeper App, choose either the Classic color mode (where brush and logs will show green, like vegetation) or the Day color mode (where they will show purple).

Here’s an example of a brush pile shown in Classic color mode. Notice that the lines of the brush are horizontal, not vertical – this is a good clue for distinguishing brush piles from weeds.

Or have a look at this video for some nice examples of logs in a creek, shown in the Day color mode.

    Vegetation and weeds
    Other underground objects
  • vegetation and weeds

    Vegetation and weeds

    You may want to locate vegetation and weeds as potential areas to cast to (for example, if you’re fishing topwater for bass in the summer), or as areas to avoid (for example, if you’re feeder fishing or want to avoid snagging).

    Just like looking for brush piles and logs, the color will help you a lot. Weeds and vegetation will also feature vertical lines, and perhaps give a spottier return on your display than logs.

    Here is an example of a drop off with lots of vegetation coverage.

  • Depressions


    Especially for carp fishing, small depressions will be an ideal feature to target. Often these depressions are created by the fish themselves. Spotting them with your fish finder is easy – just look out for small, v-shaped dips in the bottom contour as you reel or troll.


  • Points


    Points are more gradual than drop offs, but spotting them is just as easy (see the screen shot below). Make sure you maintain a steady speed when trolling or reeling so you get an accurate reading on how steep the incline is. You should scan with a narrow sonar beam to get the clearest reading, and to ensure you see any shelfs or humps which wider beam scanning might not pick up (our page onhow sonars workexplains why this is).

  • underground objects

    Other underground objects

    Who knows what is hiding under the water? Surprising objects can sometimes pop up on your screen – for example, a Deeper angler in France found a car in his local lake (see the screenshot below).

    While not every object will be as easy to distinguish, you can always get clues from the shape and also the strength of the sonar return. Notice the multiple orange sonar returns underneath the car – these tell you that this is a very hard object. The sonar scans have hit the car, bounced up to the surface of the water, bounced off the surface and hit the car again.
    While knowing this is useful for identifying strange underwater objects, it has a much more practical application – knowing how hard or soft the bottom is.

4. Using your fish finder to judge bottom type and hardness

Whatever species you are hunting, knowing the hardness of the lake bottom and its consistency is another key piece of knowledge when you are trying to crack the code and get the fish biting.There are 3 factors to consider when working out if the bottom displayed on your fish finder is hard or soft: bottom colour, bottom thickness, and the presence of a 2nd bottom return or not.

    “2nd returns”
  • day color mode


    The intensity of the sonar return from a hard bottom will be different to one from a hard bottom. Your fish finder uses colour to show this difference. In the standard colour palette of the Deeper display, the colour varies from dull brown (softest) to intense orange (hardest). In the day mode colour palette (see screen shot), the difference is even easier to see, with the colour ranging from purple (softest) to red to orange to yellow (hardest).

  • second sonar return

    “2nd returns”

    Just like the car we showed earlier, hard bottoms will send back very strong returns which sometimes bounce down off the surface again. These will show on your screen as a 2nd sonar return running parallel underneath the line showing the bottom. If you see one of these, it is another indicator that the bottom is hard.

  • hard bottom


    The thickness of the line representing the bottom is also a great clue to bottom hardness. Here the rule is simple – the thicker the line, the harder the bottom. But, be careful – sonar sensitivity can affect how thick the line is. The Deeper display has adjustable sensitivity. So if you turn up the sensitivity to 100%, the bottom line may appear thicker, if you turn it down to 10% it may appear thinner. So practice adjusting the sensitivity and get used to judging how much the bottom thickness varies based on this.


Hard bottoms will just show as one thick line, so you can’t easily tell different types of hard bottom apart. But for softer bottoms, the line will have dashes and gaps, which can give you a clue as to what the bottom is actually made of.

sand or mud bottom

Sand / silt / mud

Long lines indicate that the bottom is very soft and loose – probably sand, silt or mud.

gravel or stone bottom

Gravel / stone

If the bottom line is flecked and spotty, then the bottom is gravel or stones. Depending on the size, you may even be able to see individual small stones.

Know more, catch more

Fish finders are incredibly valuable tools, and the more you learn about how to read them, the more successful your fishing trips will be. A key part of learning as angler is to go from trial and error to a more knowledge-based approach. Maybe you regularly catch at a certain spot with a certain presentation – but do you know why? A fish finder will help you understand not just your failures, but also your successes. And once you know this, you can recreate this success more easily.

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