Last updated on May 15th, 2021 at 12:00 am

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If you know how to read a fish finder, then you know just how powerful these tools are for us anglers. Down imaging fish finders send sonar down from the boat vertically and side imaging sends sonar horizontally away from the boat. Both of these have their place on water but knowing when to use them is key – if we don’t understand what they show or be able to translate that into something useful, it’s going to be much more difficult catching anything! Read on to learn everything you need to know about how to reach a fish finder or sonar. 

How to Read a Fish Finder Sonar

Understanding how to use a fish finder is the key to having success with one. They are made to display information about the bottom of the water column that we cannot see with our eye. These devices are useful for finding fish, identifying underwater structure and understanding what is going on in different depths of water.

The most important thing you need to know how to do is identify fish on the fish finder (duh!). Fish finders display two different types of images – fish icons and fish arches.

Fish Icons

The fish icon is a result of the Fish-ID technology. Fish-ID is a sonar return from the device that displays fish locations. This technology can show small details about different types of fishes based on their size and shape.

This type of fish finder also displays vegetation, rocks, and schools of fish. To determine depths, just look at the screen and pair the icon with the depth on the display. Once you’ve done that, you’ll know the size and depth of the fish. Keep in mind, no fish finder is perfect. There are times when the Fish-ID technology will mistake bunches of plants or other material as a school of fish. This is difficult to avoid so just keep it mind as you’re working your baits.

Fish Arches

Arch fish finders are often said to be the most accurate. Reading these fish finders can take some practice, but after you’re done with this article, you’ll have no problem. These types of systems will show arches in the water, and when you get more practiced with them, it’s easy to tell what an arch is coming from whether it’s a rock, vegetation or fish below. The reason for this accuracy is because these units don’t treat one type of object differently than another. They just try to identify anything under your boat.

Judging Fish Size

So how are we able to judge the size of a fish from our fish finder? There are many factors that go into judging how big a fish is on your screen – and even more if it’s over deep water where there isn’t any reference for ground objects like hills or valleys in shallower waters.

Generally, a bigger arch means a bigger fish. But there’s more to this as this may not be the case in every situation. You need to also consider three main factors – arch width, arch length, and fullness of the arch.

Arch Width

The width of an arch is a direct indication of the size of the fish. The wider it is, the bigger and more heavy that fish probably is.

Arch Length

A longer length means a larger fish – but not always! Arch Length is partly measuring time. In other words, if a fish isn’t moving much, it will likely have a shorter arch than a fish swimming a lot. So, If you see an arc going from one side to another on your sonar screen, remember that this could just be a small fish swimming quickly across your boat. These discrepancies are why arch length is considered one of the less important factors.

Half Arch or Full Arch

Our final factor is half or full arch on the screen. You’ll see a half fish arch and a full fish arc. If a fish swims completely into the cone of the transducer, you’ll see a full arch. If the fish only swims nearby, not completely within its range, you’ll see a half arch. A half arch doesn’t necessarily indicate a small fish, and a full arch doesn’t necessarily indicate a big fish. This reading simply indicates the location of the fish, so when it comes to size, this is another less important factor.

Identifying Different Types of Underwater Structure

We’ll discuss some of the most common readings we’ll see on the fish finder – vegetation and weeds, depressions, points, cliiffs and ledges and other objects.

Weeds and vegetation – If you find spottier readings or vertical lines, this is likely vegetation and weeds. This is a good place to start since big fish often lurk in places like this.

Depressions – Depressions are good if you’re fishing for bottom feeders like catfish. These will show as small V dips on the fish finder.

Points – Points are similar to dropoffs but have a more gradual look.

Judging Bottom Types and Hardness

Something else you want to know when learning how to use a fish finder is identifying different types of bottoms. Where we are fishing can vary between hard rock, soft mud, and grainy sand – the methods used will change slightly based on these factors. The three main factors here are thickness, color and second return.

Thickness – The thickness of the line combined with your transducers chirp will tell you a lot about the bottom of the water beneath you. If it returns a thick line and a loud echo, then you’re probably on a hard surface. If the line is smaller, you’re probably dealing with something softer like mud. Either of these can be good or bad, depending on the type of fish you’re after.

Color – Color is obviously only a factor on a color fish finder. The darker the color, the harder the thickness of the surface.

Second return – 2nd returns can often indicate structure that doesn’t belong, such as a fallen tree. If you receive a 2nd return on the fish finder screen, it is worth narrowing the beam to see if you get better idea of what it is.

What is Down Imaging?

Down imaging is like traditional sonar or chirp sonar on a fish finder. It is a method to generate an image of the seabed or lakebed.

Down imaging fish finders have higher resolution and can easily show smaller objects on the seabed, such as small rocks, baitfish, urchins, lobster traps and sandbars.

To read down imaging data you should:

  • Adjust your depth settings so that you are not looking at too deep or shallow a distance (less than 100 ft for sandy/silt bottoms)
  • Change your beam width from wide angle to narrow angle in order to see what’s directly below your boat. Be careful because if there is structure nearby it may be distorted by side echoes this way. You will need to experiment with different settings.

How to Read a Side Imaging Fish Finder

Side imaging is nice because you can see fish in all directions. Using sonar transducers to send sounds sideways away from a boat allows for better quality images and more of an understanding about where fish are located on underwater terrain. A strong knowledge of how high up off the bottom that different species like to suspend themselves helps anglers understand what they should be fishing at lower or higher depths depending on their current location. If you’re looking for a solid side imaging fish finder, we wrote an article about the best side imaging fish finders on the market. 

What’s the Best Frequency for a Fish Finder?

Finding a dual-frequency fish finder with both 200hHz and 50kHz is a great feature to have. A high frequency of 200Hz is will provide clearer and cleaner imaging but with a smaller cone. The lower 50kHz offers more a larger cone but at the expense of clarity and image quality. This is why it’s so important to have both.

The standard range for fish finders is between 50-200 kHz. However this can vary depending on your needs, such as finding structure below you versus an open area like in offshore fishing where everything may be deep. A higher frequency will work well here because it’ll penetrate deeper into the water column than a lower frequency.

What other key features do fish finders have?

The other main features of a fish finder are chart plotting and GPS.

Chart Plotting

Chart plotting is a way to create your own map of the bottom and points of interest. This can be very useful if you’re fishing in unfamiliar water or want to keep track of depths and locations for where you found fish.


GPS coordinates are particularly helpful when navigating at night with limited visibility because it’ll point out dangers ahead like reefs which may be difficult to see otherwise. It will also automatically mark any structure that falls within its range using sonar, downscan or sidescan depending on what frequency it operates at.

How are fish finders powered?

Fish finders are battery powered and operate on 12V or 24V DC power.

The best way to make sure your fish finder is operating optimally is by keeping it charged with the appropriate voltage power supply (i.e., a 12-volt charger for a 12 volt battery, etc.).

A good rule of thumb: if you’re using your sonar at night time, be sure that both the boat’s electric system and/or engine ignition switch is off before disconnecting from shore or another vessel. This will prevent any accidental discharge spikes which may damage sensitive electronics onboard.

In Conclusion – How to Read a Depth Fish Finder

Hopefully this how to article helps you understand how to read a fish finder and the best ways to use it. If you’re in the market for a fish finder, check out our round up of the best fish finders for the money.

Remember these key takeaways:

  1. The most important factor in gauging fish size is the fish arc width/thickness
  2. Try to find a fish finder that operates with both 50 and 200 Hz
  3. Pay attention to your bottom and look for additional clues with vertical weeds and 2nd returns