SideScan Basics-Part 2: Identifying fish

By Lance Valentine

Over the past few seasons SideScan sonar has become much more popular and for those who understand how to interpret and use SideScan, it has become a tool we could not fish without. The basic idea of SideScan is the ability for the angler to see the bottom, fish, structure and other details up to 300 or more feet to each side of the boat. In an earlier article we discussed the basics of how SideScan works and basic interpretation. Let’s take the next step and talk about identifying fish on SideScan.

Before we start it is important to understand that SideScan shows fish much differently than 2D sonar does. This is due to the VERY narrow cone angle (about 3 degrees) and how the cone is aimed to the side of the boat. Fish on SideScan can best be described as “grains of rice” on the screen. Their brightness indicates how big the fish is and their length indicates how long they were in the cone.

Here is a basic example of some fish on SideScan. Inside the yellow circle are multiple fish that show up a “grains of rice”. Notice the fish are all oriented in the same direction. Unlike 2D sonar, SideScan can give us an indication of how fish are facing and traveling. We know these fish are on the right side of the boat and are sitting at the edge of a piece of structure.

Also notice that “under” the fish marks are black marks the same size. These black “shadows” are extremely important in learning what your SideScan is showing. Since the shadow and the fish are separated, we know that the object (fish) is NOT attached to the bottom and is most likely a fish. The distance between the fish and the shadow indicates how FAR off the bottom the fish is. The further the two are separated, the further off the bottom the fish are. Check out this combined picture showing fish OFF the bottom and fish close to the bottom and how each is represented by SideScan.

Now that we have some basics of identifying fish on SideScan, let’s discuss WHY using SideScan to find fish is important. One of my favorite reasons for using SideScan is to see what side of the boat the fish are on. We’ve all had it happen when trolling….one side of the boat is getting fish and the other side is not. While there are lots of factors that can create that situation, I’m convinced that lots of time fish are only on one side of the boat!! Knowing this allows anglers to adjust how they set a trolling spread. I have often set boards on just one side of the boat because rods on the other side were useless; no sense fishing where there are no fish! Along the same thought, we have all had lures a certain distance from the boat catch fish, while the others on the same side are “dead”. Again, lots of time using SideScan has convinced me that often fish are lined up on some piece of structure and not spread out. The photo to the right is a great example of this!

Another great reason to use SideScan is the ability to actually see fish “behind” structure. Due to the narrow beam and high frequency that SideScan operates at it has the ability to penetrate objects and see what’s hiding behind pieces of structure or drop-offs. The best place for using this is in the weeds, especially taller, thicker weed clumps. SideScan allows you to see fish other sonar systems can’t and lets you catch fish other anglers don’t even know were there!

Hopefully this information will give you a better understanding of how fish will look on your SideScan next time you are on the water. We are just scratching the surface of interpreting SideScan, so stay on the lookout for more blogs, video and workshops to help you catch more fish using SideScan.

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