The most common mistake anglers make when reading their fish finder is thinking that a long arch means a big fish. This is not the case. On your sonar display, you should think of length as representing time. For example, imagine you keep your fish finder stationary in the water (in other words you are not reeling or trolling it). If there is a fish underneath that is also stationary, what will you see on your fish finder display? You will see one continuous line. That doesn’t mean there’s a blue whale stranded in the pond you’re fishing. It means there is a stationary fish under your fish finder, and it might be a very small one.

Now let’s imagine another scenario – again your sonar is stationary, but this time 2 fishes swim through your sonar beam, one big and one small. The big fish swims very quickly through the sonar beam, the small one swims slowly. Which one will make the longer fish arch on your screen? The answer is the small one. That’s because a slow moving object will leave a longer mark than a fast moving one, whatever their size.

Take a look at the screenshot below. This return is from a school of bait fish. Notice how long some of the arches are. This may be because the sonar is stationary or moving very slowly, or because the fish are only moving slowly. Either way, it’s a good illustration of the fact that a long fish arch does not necessarily mean it’s a big fish.

So length is not a very good indicator of fish size. It’s much better to look at the width of the fish arch shown.
Just remember this when you are interpreting fish arches:
think vertical, not horizontal.
Even if the arch is short, if it is thick then it means the return has come from a big fish. Take a look at this screenshot again. Of the 4 fishes shown, which one is the biggest?

The answer is the fish on the bottom right. All the arches are roughly the same length (and they are all quite short). But the thickness of the bottom right arch tells us it is the biggest fish. This fish is not a monster, but it would be a decent catch.

Here’s a useful tip for judging the width / thickness of a sonar return: switch on the vertical flasher (also know as a-scope) on your fish finder if it has one (to do this on the Fish Deeper App, go to Settings > Sonar > Vertical Flasher). The vertical flasher on the right side of your screen will show you exactly what’s under your sonar at this exact moment. You can use it for vertical jigging, and of course for ice fishing too. For judging fish size, look at both the thickness and width of the returns on your vertical flasher. A larger fish will give both a wider and a thicker return.

The final point to remember when you are looking out for fish arches is that it doesn’t need to be a full arch. Half arches (like the ones shown in the screen shot above) also show that there is fish. In our  tutorial on how sonars work, we explain in detail why sometimes you get a full arch and sometimes you get a half arch. The short answer is that you will get a full arch if a fish swims through the whole of your sonar cone, and a half or partial arch if they only swim through part of it.
But what’s important to remember when reading your display is that a half or partial arch could still be a big fish – the quality of the arch is not an indicator of how big the fish is.
So be on the look out for both full and partial arches, and remember that the thickness of the arch is the best indicator of fish size.

Bait fish will show on your display as dashes, lines or even just dots, so they can sometimes appear similar to vegetation. There are 3 key differences though:

• The bait fish will usually be suspended in the water, not on the bottom;
• The your fish finder will show bait fish in a different colour to vegetation (in the case of the Fish Deeper App’s standard colour display, they show as yellow rather than green).
• The bait fish will often be in balls, which will then be shown like a cloud or cluster, rather than lines.

The screenshot below shows both individual bait fish and a ball of baitfish. Notice the difference in colour from the green vegetation on the bottom.

So, let’s bring together what we’ve learned by remembering 2 key points:
1. Think vertical, not horizontal – the width of the fish arch is the biggest clue to the size of the fish.
2. Arches can be half or full – a half or incomplete arch doesn’t mean it is a smaller fish. Again, the width is your best clue.
Your fish finder isn’t going to tell you which species of fish it is, but now you know the size you have a good chance of working out the species based on the lake or river you’re fishing, and the depth at which you find the fish.
So when you see an arch like the one below on your screen, you’re about to catch BIG!