How to Read a Fish Finder: The Basics

By Debbie Hanson

Feb 15, 2021

Basics of how to read a fish finder to identify fish, structure, bait. Differences between down-scan imaging, side-scan imaging on a fish depth finder

Learning how to read a fish finder can be a big benefit when you’re fishing from a boat because it gives you a picture of fish activity beneath the surface and provides an indication of the type of structure that fish might be relating to. Most of today’s fish finders are sold as a GPS fish finder combo, which generally includes a fish finder head unit, a GPS receiver, and a transducer.

Fish Depth Finder Basics for Beginners

The best fish finder is subject to opinion because many shallow water anglers prefer fish finders that feature side-scan imaging to view structure or fish to the sides of a boat, while many deep water anglers would rather use a unit with down-scan imaging to see structure or fish beneath the boat. However, there are some fish finders on the market that have both types of technology.

The good news is that whether you are using side-scan imaging or down-scan imaging on a fish depth finder, today’s user-friendly units often mean that the average angler can simply use “auto mode” without having to make many adjustments to the settings.


How Does a Fish Finder Work?

Fish finders use sound to locate objects underwater. They work by sending out sound pulses and waiting for an echo. The frequencies used vary, ranging from very low (infrasonic) to very high (ultrasonic). When learning how to read a fish finder, you’ll notice that fish appear as arches in traditional sonar imaging.


Traditional Sonar, Down-Scan Imaging, Side-Scan Imaging

Traditional sonar is generally used to find fish arches and bait balls, down-scan is often used for picking up deep structure such as submerged trees or ledges beneath the boat, and side-scan allows you to see a wider picture of the underwater environment to both sides of the boat (which is often helpful in places like shallow bays and creeks).

Some fish finder units feature a split-screen mode so that you can identify both fish and bottom features through down-scan or side-scan technology.


Identifying Big Fish

How do you know what size fish you might be seeing on your fish finder? In general, the thicker the fish arch, the bigger the fish is. The length of the fish arch can also give an indication of size, but you have to consider adjustments for water depth. The deeper the water, the smaller the return will be. In other words, if you notice a large arch on your fish finder while fishing in deep water then you know it has to be a large fish.

Since you know the basics of how to read a fish finder to locate the fish, now you can head to the fishing rigs section and learn how to set up a fish finder rig or a two-hook bottom rig to catch the fish.

Debbie Hanson
Debbie Hanson
Debbie Hanson is an award-winning outdoor writer, women’s sport fishing advocate, IGFA world record holder, and freshwater guide living in Southwest Florida. Hanson’s written work has appeared in publications such as Florida Game & Fish Magazine, BoatUS Magazine, and USA Today Hunt & Fish. To learn more about her work, visit or follow her on Instagram @shefishes2.