One of the biggest issues we all have when looking at a fish finder is understanding fish arches on the sonar display.
After all, we fishermen are out there trying to find fish so if the arches aren’t appearing we often think, is there no fish to be caught? Or, is our fish finder useless?
Most likely, it’s neither of these issues and by understanding a little more about sonar you’ll be able to flush these concerns out of your mind and use your fish finder more effectively.
Join me as I run through everything you need to know about fishfinder fish arches…
What are fish arches?
Fish arches are arches that represent fish underwater which have been picked up in the sonar beam of your fish finder.
The fish will appear on the screen of your sonar display in the form of an arch which allows you to identify fish under your boat that are worth catching.
How do fish arches help when fishing?
When you’re out fishing, the hardest part of your day is finding the fish in deep water, as they can be anywhere in the lake, river, or ocean your boat is cruising around on.
Learning how fish finders work and being familiar with its function can make this easier.
Fish arches on a fish finder take all the guesswork away and you can go from guessing if there are some fish under your boat to knowing they are 100% there, and then trying to catch them just by seeing a few arches on the screen.
How do you read an arch fish finder?
Reading an arch fish finder is the exact same as reading any other fish finder, the only difference is that everything, except the bottom, will be displayed as a kind of arch on the screen.
This means you have to learn to differentiate one arch which might be a fish, from another which might be some seaweed.
When you look at the display, you’ll clearly see the bottom and a load of arches at different depths depending on how fishy the area is. Arches in the middle of the water column or near to the bottom are likely to be fish, while arches on the bottom are more likely to weed or debris.
What’s the difference with fish ID fishfinders?
Fish finders with Fish ID technology take a fish arch and convert them into fish icons on the screen. By seeing fish icons, you don’t have to work out whether a fish arch is a fish or not, as the fish finder has done the work for you.
But, fish finders don’t always get it right and will often show a bunch of seaweed as a shoal of fish icons. Whereas if you learn to read fish arches, you’ll be able to tell what is a fish and what isn’t more accurately than your unit does.
Why do fish show up as arches on sonar?
The reason a fish shows up as a fish arch in your narrow sonar beam instead of just a flat line is due to the cone angle of your beam.
As the beam is fired to the bottom of the water, the edges of the beam are further away from the transducer than the center of the beam. If a fish swims through the beam, it will reflect each edge of the beam and the center.
Since the center is closer to the transducer, it creates an arch whereby the center of the arch reflects when the fish was in the center of the beam, and the tails of the arch reflect when the fish was at either edge of the beam.
How to Understand and Use Fish Arches
All the fish the transducer sees underwater will be displayed as an arch, so any time you see an arch on the fish finder screen, know that you are probably looking at a fish. Arches might mean debris or weed sometimes, but most of the time it is a fish and with paying attention too.
Not all arches are made equal though and you might end up seeing a big or small arch or a thick or thin one.
What are Half Or Full Fish Arches?
One key point to understanding fish arches is that a fish won’t always create a full arch as to do so it needs to swim through both edges and the center of the beam, as mentioned above.
If a fish is sitting still or swimming in a different direction, then you won’t see full arches and you will see half arches instead.
So, a half arch is a fish arch but the fish has changed direction when in the beam and a half arch should not be ignored, as half arches are still a sign of fish.
Does fish arch width matter?
If an arch is wide (horizontally) on the fishfinder screen, this doesn’t mean anything in terms of the size of the fish.
All this shows you is the gap between one end of the beam to the other end of the beam, not how big a fish is so don’t get excited thinking a wide arch is a big fish, as it only means you have a big beam.
Fish Size & Fish Arch Thickness
The thickness of a fish arch measured in vertical lines across the vertical pixels of the fishfinder screen is a direct correlation to the size of a fish.
Bigger fish will have a much thicker or bigger arch as bigger fish will have a deeper reflection of the sonar.
Smaller fish will show up as a thin fish arch as smaller fish will have a shallow reflection of the sonar when compared to big fish. They could also show up as a shorter arch if the boat speed is fast.
So, when you see a big thick arch on the screen, it’s time to get excited as there is a monster down there you should put every effort into catching.
How Do I Spot A Trophy Fish On My Fish Finder Display?
As I mentioned above small fish have a thin arch and large fish have a thick arch. If you see a large arch or an arch that is super thick (vertically) then you have spotted a trophy specimen and it’s time to get your lines down there and try to catch it.
Finding baitfish on your fishfinder
A key element to catching fish is to find the smaller bait fish they are feeding on.
Since baitfish are so small, they are not going to come up on your finder’s display screen as an arch and will look more like dots, lines, or even dashes that are suspended in the water column.
Baitfish in your display
When you see a dot, line, or dash in mid-water, then it’s a sure sign you have spotted a baitfish. A more likely scenario though is seeing a group of baitfish as a ball of dots, lines, or dashes, as they tend to hold together for security.
When you see a big ball of baitfish, remember to hang around the area as it’s a sure sign that predators will show up to feed.
Identifying Different Types Of Underwater Structures
A fish finder not only shows you fish, it also shows a map of the bottom contours that your boat is driving over plus the depth on the screen. This means you can see underwater structures as the bottom part of the image changes.
Learning to distinguish fish from other objects such as vegetation, weed beds, sunken trees or other underwater objects is an important skill.
Vegetation And Weeds
Vegetation and weeds are useful things to spot on your fishfinder. Species like bass love to hide in vegetation waiting to pounce on unsuspecting baitfish and by seeing it on a fishfinder you can target those areas with different types of tactics.
Vegetation and weeds will come up on your sonar image as vertical lines coming out of the bottom, like plants growing out of the ground.
Once you spot them, you can see what depth they begin underwater and then put your lures down just above them to tempt a fish to take a bite.
Here’s an informative video to help you visualize.
Points are gradual changes in water depth where the bottom is slowly getting shallow or deeper. These are great places to find fish as bait tends to hold in areas with gradual changes as the safety of shallow water is never far away.
To see a point on your fishfinder, just watch for a slow depth change over time on the bottom contours.
Depressions are harder to see than points and you will have to be paying attention to your fishfinder to notice them. They will show up as a tiny water depth change of 1-2 feet over a few seconds.
Here’s an informative video to help you understand more about structures underwater.
Telling Bottom Types and Hardness
Telling bottom types and the hardness of the bottom on a fishfinder is easy and you need to apply the thickness in regards to the fish size we discussed above to the bottom to work it out.
A hard dense material like granite is going to create a much bigger sonar reflection than say a soft bottom like clay or mud that will absorb some of the sonar instead of reflecting it.
When looking at your sonar image, if the bottom is displayed as a thick line then you know you’re over a hard bottom like limestone. If the bottom is displayed with a thin line, then you’ll know you’re over something soft like mud.
Colour and “2nd Returns”
You’ll notice that when you look at a sonar image, different things are in different colors, but why? This is all to do with the returns.
When a fishfinder sends out sonar, the sonar is reflected back by any object it hits which is what creates the image. When the fishfinder detects a strong reflection, aka return, it makes the object it came off a darker color on the screen.
The strongest return you are going to get is off the bottom and the fishfinder by default shows this as the darkest color, usually a deep red. Large fish will often have a red core thanks to a solid return, where as baitfish will usually be light green dots as they provide a weak sonar return.
Why doesn’t my fish finder show arches?
There are multiple reasons why your fishfinder might not show arches, and the most common one is that it can’t and it only has a Fish Icon feature, not an arch one. If this is the case, you’ll need a different fishfinder to see fish arches.
If your fishfinder can show arches but is only showing fish symbols, then you just need to go into the settings and change them to arches instead of Fish ID.
Other reasons for a fishfinder not showing any arches is because the transducer is bad or it isn’t set at the right angle, which is parallel to the water. If it’s not parallel then the beam will be off and the fish won’t create an arch and might create a weird line instead.
How do I get my depth finder to show arches and not fish on the screen?
As I mentioned above, you should first check if the fishfinder can show arches and if it can, then read the manual of how to change it to arches.
Usually, you just have to go into the menu, then settings, and you’ll find something called Fish ID, which you can then turn off.
Thanks very much for reading my article, I hope you enjoyed it and found it useful. The world of fish finders can be very complicated. Buying the right one and installing it is hard enough and then when you have to start learning how to read and understand sonar technology, life truly has changed on the water.
I trust that you now know everything you need to know about fish arches and what you’re seeing on your sonar image.
As soon as you have this mastered, knowing exactly what is under your boat at any time becomes very easy and you’re certain to catch more fish because of it.
Please share this article with your fishing friends, as I’m sure they want to understand what their fishfinder is showing them as much as you do.
Also, you might want to check out some of my other articles as I cover everything you need to know about fish finders from the best out there to how to install them.