How To Use Fish Finders and Tips for Choosing Fish Finders

How to Interpret Sonar Fish Finder Signal
How to Interpret Sonar Fish Finder Signal (features an older product EAGLE FishEasy 350C Portable Sonar Fish Finder, replaced by newer product LOWRANCE Elite 4X)

Sophisticated fish finding technologies including sensors, sounders, and sonars greatly increased the efficiency of modern commercial fisheries. For recreational sport fishing, commercially available fish finders are quite commonly used by savvy fisherpeople. With the help of a fish finder, you’ll be able to save tons of time trying to locate the fish and thus to catch more fish faster and easier.

This post tries to cover the following Sections in a broad and easy-to-understand way:
1. Operating theory/principle of fish finders.
2. Type of fish finders.
3. How to read fish finders / how to identify fish on a fish finder screen.
4. Three questions you need to know before buying a fish finder.
5. Frequently asked questions related to fish finders.

First, we want to know that the most recent/newest signal/data is shown on the right most part of the screen, and older signal/data are being pushed gradually to the left side of the screen (see the time bar of the picture above). The signal of a conventional fish finder is just one-dimensional. That is to say, it will only show you the depth of the fish. No information about how far away the fish are to your left, right, front, or back, horizontally. One thing for sure, of course, is that when there’s a signal on the screen, there is a fish in the sonar beam cone down there. Jump to Section 3. for an infographic that I drew to help you understand how fish finders work.

Operating theory/principle of fish finders

A transducer sends out a sound wave into the water. Solid objects such as fish will reflect the sound wave back to the transducer and fish finder for data processing and display. Knowing the speed of sound wave in water and the time lag between sending and receiving the signal, the distance to the object that reflected the sound wave can be determined.
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Types of fish finders

If you want a all-season (open water and ice fishing) bundle/package that comes with a battery pack and works as a stand-alone unit, you definitely want to check the great product below. It’s my favorite pick because it’s so portable and versatile. It works on your kayaks, canoes, and small boats; on open water and on ice!

(1) Broadband Sounder: the most common fish finders nowadays (see the picture on the top). It uses a single-frequency signal. 200 kHz (high frequency, short wavelenghs) for shallow- to mid-range depths (to about 100 meters or 300 feet); 83 kHz (medium frequency) for mid-range depths; and 50 kHz (low frequency, long wavelengths) for very deep waters (up to 1200 meters or 4000 feet). The picture in the beginning of this post is from a single frequency broadband sounder. Below is a cheap and great product with great customer reviews from amazon.

(2) CHIRP (Compressed High Intensity Radar Pulse): it sweeps a spectrum of frequencies, instead of one single frequency as used in Broadband Sounders. Using this cutting edge echo-sounder technology, CHIRP sonar fish finders have the advantages of better target separation and less interference. Below is an example chart and a good pick at amazon. Note that this product may support down imaging mode but it does not come with a down imaging compatible transducer. To get a fish finder with down imaging feature, check number (3) below.

CHIRP (Compressed High Density Radar Pulse) sonar fish finder example chart
CHIRP (Compressed High Density Radar Pulse) sonar fish finder example chart

(3) StructureScan HD or Downscan: this type of fish finder units scan an area with very high frequency signal, allowing you to get picture-like images (charts). With this function, it’s much easier to see where the fish are in the chart. Below is an example downscan image and a good pick with great customer reviews from amazon. This product has all of the three functions/modes: broadband, CHIRP, and down imaging, a truly value product with all the features.

StructureScan HD / Downscan sonar fish finder example chart
StructureScan HD / Downscan sonar fish finder example chart

(4) SpotlightScan: fish finders with SpotlightScan features allow users to scan (horizontally) the surrounding areas to locate fish faster, before you reach there. The transducer can be positioned toward a specific direction desired by the user. This fancy feature empowers you to locate fish accurately and efficiently. Below is an example chart of a fishfinder with SpotlightScan feature. And also, for your convenience, I posted a great product from amazon that has all the wonderful functions to empower you: broadband sounder, CHIRP sonar, DownScan imaging, StructureScan HD, and also Trackback view. In addition, it features a 9-inch touch screen, and built-in wireless and Bluetooth.

SplotlightScan sonar fish finder example chart
SplotlightScan sonar fish finder example chart (scanning chart (left), CHIRP chart (top right), and also Downscan chart (bottom right))

 

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How to identify fish on the screen of a fish finder?

How does a fish finder work? Sonar fish finder operating principles
How does a fish finder work? Sonar fish finder operating principles. How to interpret fish finder signal (arches, bars, etc.). Conversion factors for calculating coverage diameter from depth and cone angle.

Signal/marks shown on a fish finder screen can be caused by fish, vegetation, debris, or other objects. So, how to identify fish signal and distinguish it from others?

When your vessel (be it kayak, canoe, or boat) passes over the fish, or the fish swims under your boat, the distance between the fish finder transducer and the fish changes. The closer the fish is to the transducer, the stronger/thicker the signal/mark would be displayed on the fish finder screen. Lets ignore the 60 degree cone first and look at the 20 degree cone only. So, the first moment the fish enters the leading edge of the 20 degree cone area covered by the sonar beam, the signal would be relatively weak or the mark shown would be thin (see Fish Position A and Mark A of Scenario (1) in the picture above). As the fish moves further into the sonar beam cone to Fish Position B, the mark on the screen becomes thicker (see Mark B of Scenario (1)). Similar to when the fish enters the sonar beam cone, the mark would become thinner when the fish is about to leave the trailing edge of the sonar beam cone (Fish Position C and Mark C of Scenario (1)). When the fish goes out of the 20 degree cone, there would be nothing shown on the screen for this fish for the transducer with a cone angle of 20 degree. However, this fish is still detectable with a transducer of 60 degree cone angle before it moves beyond the 60 degree cone to Fish Position D.

If the fish has a chance to be fully covered by the sonar beam, it will form a full arch on the screen of the fish finder (see the full arch in Scenario (1)). However, if the fish headed back at Fish Position A, then we probably would only see Part A of the full arch in Scenario (1). That is to see, we don’t always see full arches of fish. Actually, we could even see flat bars on fish finder screens when the fish stays under your boat/transducer for a while (see Scenario (2)). In this case, you’ll see a steady signal or mark moving to the left side of the screen.

Now, let’s get back to the picture shown in the beginning of this post. The signal of the fish was flat instead of in arch shape because my kayak was anchored and the fish stayed under my kayak and transducer for a while. If I were moving fast passing the fish, the signal would be in shape of arches.

Dropoffs and humps provides ample forage for fish
Fish arches (top left and near the bottom and the hump) shown on a fish finder screen. Dropoffs and humps provides ample forage for fish.

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Tips: What do you need to know before buying a sonar fish finder / How to choose a fish finder for your specific needs?

Here are three questions you need to ask yourself before you decide which fish finder you’ll invest in.

1. Does it come with a sonar transducer?

Most sonar units come with a transducer. For geeks who want to know the technical details of various transducers, visit this webpage of LOWRANCE, a company with a good record of producing great fish finders. Some fish finder modules have expandable features, but you may need to buy an additional transducer to have those extra features.

2. Do you fish deep waters (say, hundreds to thousands meters or feet) or shallow waters (say, less than 30 meters / 100 ft)?

LOWRANCE Fish Finders Elite X series modes
LOWRANCE Fish Finders Elite X series modes. Maximum water depths under different modes.

As discussed above, fish finders typically use three frequencies (50 kHz, 83 kHz, and 200 kHz) for different ranges of water depths. Some fish finders support only one single frequency while others (usually more expensive) ones support dual or multiple frequencies. Ask your sales or read the manual to see if the fish finder satisfies you with enough depth capability.

If you almost always fish shallow water, there’s no point for you to get a low frequency (50 kHz) fish finder with narrow sonar beam cone angle (e.g., 20 degree). Instead, you want to get a high frequency (200 kHz) one with a wide sonar beam cone angle (e.g., 60 degree) to cover a bigger area at a certain depth instead of deeper water but smaller area at the same depth.

3. Is it portable? do you have a 12 V marine / RV battery on your boat?

Many fish finders assume that you have a 12 V marine / RV battery on your boat. However, if you don’t already have power source on your boat, which is often the case for paddlers on kayaks or canoes. Here’s a great combination of products (about $200) for your vessel. Clicking the images will direct you to amazon. You may find here the LOWRANCE Elite X series Users Manual.


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FAQs

Can I use an ice fishing sonar fish finder in summers? Or can I use an open water sonar fish finder in winters?

Yes. Make sure that you have a portable battery pack for your sonar fish finder. Then you can use your fish finder either on open waters or on ice. To build a portable battery pack by yourself, you’ll need a battery holder (for 8 AA batteries, each AA battery is 1.5 V, so 8 of them used in serial outputs 12 V) and a container to protect the batteries from water. You can use whatever container you have, e.g., a water tight Lock & Lock lunch container would do the trick. I bought a sonar fish finder with a portable power pack, which saves a little bit hassle. However, I think it will worth the effort to make your own portable battery pack as you can save some good bucks. Clicking the image below will direct you to amazon.

LOWRANCE All Season Sonar Fish Finder with portable battery pack and charger is a great value product for open water fishing and ice fishing.

Portable battery pack for sonar fish finder
Portable battery pack/holder for sonar fish finder (8 AA batteries connected in series outputs 12 V)
Water Tight Lock and Lock Lunch Container for Portable Battery Pack
A Water Tight Lock and Lock Lunch Container is great for housing the Portable Battery Pack

How to mount a sonar transducer on a kayak or a canoe or a small boat?

A previous post describes an easy and cheap way to mount a transducer on a kayak using a slotted flat metal bar. In addition to the easy installation, it’s also highly detachable, i.e., very easy to remove the transducer and mounting hardware. Only minimal tools and work are required. Other DIYers have demonstrated innovative ideas on mounting transducers using PVC pipes. While many of the ideas using PVC pipes produce great results, I find that they do take quite a bit of time to design and finish the work.

Does a sonar fish finder tell you the size of a fish?

The closer the fish is to the transducer, the thicker the signal shown on the screen would be. So, when you see a strong signal on the screen, it could mean a big fish further to the transducer or a small fish closer to the transducer. However, the certainty increases when the depth of the fish is shallow, i.e., the vertical distance between the fish and the transducer is short. Since the shallower the depth, the smaller the diameter of the area the transducer covers, we would then expect that the size of the fish, instead of the distance between the fish and the transducer, becomes a dominant factor in forming signal on the screen.

How big an area would my fish finder cover? Or, how far can my fish finder see?

For a 60 degree cone sonar beam: use a conversion factor of 1.15, that is to say, if you are on 10 meters (or say, 30 ft) deep water, your transducer will be able to cover about 10 meters x 1.15 = 11.5 meters or 30 ft x 1.15 = 34.5 ft in diameter on the lake bottom. The area covered decreases as the depth decreases.

For a 20 degree cone sonar beam: use a conversion factor of 0.35, that is to say, if you are on 10 meters (or say, 30 ft) deep water, your transducer will be able to cover about 10 meters x 0.35 = 3.5 meters or 30 ft x 0.35 = 10.5 ft in diameter on the lake bottom. The area covered decreases as the depth decreases.

What’s the difference between sounders and sonars?

Sounders typically only look downward to find the bottom or determine how deep the water is while sonars can often scan around to find fish or other imaging/reflective targets. Sonar stands for SOund NAvigation and Ranging. There’s no fundamental difference between sounders and sonars. Usually sounders are cheaper than sonars because sounders have less features than sonars do.
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