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Understanding Sonar and the use of Fish Finders - Part 1 of 3

Viewer Question: “I would like to see a series of information on understanding sonar. I know fish are arches, however actually the arch size is determined by how long the fish is under the sonar beam. i.e.: the faster the boat is going the shorter the arch.

What about when the fish is right or left of center of the beam will the fish look smaller? Should one be looking for just fish or both fish and structure? You could probably have dozens of sonar topics. Thanks for listening” - Bruce

Hello Bruce,

Understanding sonar (fish finders) is a very important part of fishing, so let’s review some important points when it comes to finding fish.

As far as using fish finders (sonar), there are times when it is more beneficial to use it to find fish and other times it is more beneficial to find structure. Let’s key in on finding fish.


When fishing for suspended fish in deep water – like walleye or salmon for example - you will want to actually look for those nice “arches”. The fish finder (Sonar) will easily locate the fish, because they are suspended and they will mark very easy.

Once you have marked your suspended fish, you can now set you down riggers, planner boards or divers to that particular depth (or use lead core line to get you to the right depth).

Most fish finders will give you an option to mark the fish as arches or as fish symbols. The arch setting is actually more accurate than the fish symbol setting, so be sure to use the arch setting.

If you see a full arch that means the fish is directly under the boat. The larger the arch is, the bigger the fish is (assuming you are slow trolling when you mark him). If you see a partial arch that means that you have marked half of the fish and that the fish will be on either side of the boat, just outside of the sonar.

In the case of a partial arch, it is more difficult to judge if it was big or not, but at least you will know that you are marking fish. If you continuously mark partial arches that will indicate that the fish are being spooked by your boat/motor and that they are swimming off to the side of the boat.


This will be your indication to put out some planner boards or side divers (like a dipsey diver) to get you lures off to the side of your boat, to where the fish are. If you do not have anything to take your lures off to the side of the boat, then troll in an “S” pattern in order to achieve the same result.

The other thing to keep in mind is that the deeper the water is, the larger the “cone” of the sonar will be, which will give you more coverage.

In Part 2, we will talk about using fish finders (sonar) to find structure – another very important way to put you on to more fish. Stay tuned……


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