How to read a fish finder: a simple guide

I am going to let you in on a little secret here … we decided to write this beginners’ guide to reading a fish finder because every other article we found on this topic was poor.

They were either too technical, too general or they only covered one particular element. Admittedly, there’s some great video content on this topic, but not everyone wants to spend time watching lengthy and time-consuming explanations.

So we set out to create a concise but thorough beginners’ guide on how to read a fish finder.

To do this we got in touch with fisherman, wildlife filmmaker and Lowrance pro Romen Dicovski to get him to explain the basics of using a fish finder with the help of five images he took off his Lowrance sonar while fishing recently. Romen has deliberately chosen these images to help us illustrate how easy it is to use a fish finder, and how you can tell what’s a fish vs structure, weed or other underwater objects.

Fish finder image one: a big fish under a school of bait fish

Picture: Lowrance and Romen Dicovski, Roaming Productions ©

Traditional sonar, structure downscan with FishReveal

The hand side is traditional sonar imaging and the right hand side is structure downscan. Left of screen is the front (bow) of the boat and right the rear (stern) in both images.

As a general rule, traditional sonar is better for picking up images of fish and bait balls, while the monochrome structure downscan, as the name suggests, is better for picking up structure.

“With traditional sonar, fish typically appear as arches. You can clearly see the large arch on the bottom left of screen – that’s a big fish,” Romen says.

“The smaller the fish the harder it is to make it out compared to traditional sonar. Structure downscan is design to pick up more detailed structure and the fish returns are more dotted,’’ 

(The ‘return’ is the image that a fish or piece of structure makes on the sonar. A ‘hard’ return is a hard surface such as rock or compacted sand and shows up as bright yellow.)

Most people run their sounder – assuming it has structure downscan – in this split-screen mode so they can pick up both fish and relevant bottom features.

On the right, that same large fish appears as a blob – that’s how fish appear in structure downscan. But the difference here is Lowrance’s FishReveal technology has applied a highlight to that “blob” to ensure it can be recognised as a fish. Now standard across the Lowrance range, FishReveal is designed to help create a best-of-both-worlds image in structure downscan.

The structure downscan view also allows you to confirm that that fish is sitting off a rock. We asked Romen how he could tell it is a rock: “Well, it is connected to the bottom and it’s the same color as the bottom.”

Sizing fish 

So how are we able to determine that the fish hanging off that rock is big (it’s about 40” or one metre long!)?

In general the thicker the arch the bigger the fish and if your boat or kayak is still, and the fish is too, length of the arch can give an indication. But you have to remember to adjust for water depth. “The deeper the water the smaller the return – so if you see a big return in deep water then it is a big fish.” (See the technical section end of this article for more detail on this.)

Fish finder image two: fish hidden in weed and trees

Picture: Lowrance and Romen Dicovski, Roaming Productions ©

Structure downscan and FishReveal

This image shows the importance of structure downscan when it comes to structure, and also how useful Lowrance’s FishReveal technology is.

FishReveal adds a brightly coloured highlight to objects in the structure downscan image that are actually fish. Handy hey?

Take a look at the image on the structure downscan image on the right, and then the traditional sonar image on the left.

You can see the tall weed beds and the spindly tree labelled on the picture in structure downscan, but in traditional sonar this just appears as an ambiguous blob. The FishReveal also shows there is a fish in this cover too – you cannot really see it in the traditional sonar image.

The five fish in the little trench there are all visible in structure downscan thanks to FishReveal. Three of them are clearly visible in the traditional sonar image. If you know what you are looking for you’d probably see the one on the left and the right, but it’s far from guaranteed.

The fish higher up in the water column on the right hand side is easy enough to spot on both images.

Fish finder image three – fish sitting in a weed bed

Picture: Lowrance and Romen Dicovski, Roaming Productions ©

Here we see a very undulating bottom. We can tell it is quite hard or rocky from the yellow color on the traditional sonar image and what we are looking at here is a rocky outcrop in about 9m of water.

“This is a thick weed bed sitting on top,” Romen says.  “You have got a fish on the left-hand side sitting above it and smaller fish sitting in the weed bed.”

The orange/purple return on the crest of the outcrop is the weed bed. Romen says experienced users can tell this from the traditional sonar image, but the structure downscan shows it up clearly anyway.

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Fish finder image four – side-scan imaging showing a tree, rock and several fish

Picture: Lowrance and Romen Dicovski, Roaming Productions ©

Sidescan combined with vertical sonar

There is lots to explain here. For this image, Romen has the Lowrance set up so the screen is divided in half vertically with the top half of the screen showing vertical scanning and the bottom half given over to side scan.

So the top half we should be familiar with now – left-hand side traditional sonar, right-hand side structure downscan. Again, the left-hand side of both images shows what’s in front of the boat and the right-hand side what’s behind.

As for the image that occupies the bottom half, that’s where we need some further explanation.

Sidescan 

This shows the view to the left (port) side and right (starboard) side of the boat with the sonar beam shooting horizontally out from the transducer. The numbers of the bottom axis are metres from the centre line of the boat. As you get five metres (approx 5.5 yards) out on either side it begins to pick up the bottom.

The first thing you see on the left-hand side of the sidescan image is the same group of three fish that we saw in both the vertical scan images sitting off that rock.

The extra information imparted by the sidescan here is that these fish are about 10m (or 11 yards) out to the left-hand side of the the boat. We wouldn’t have had a clue which side of the boat they were sitting on without the side scan here.

What it also shows is there are also some rocks and a fallen tree with some fish sitting off them out to the right, probably beyond the reach of the cone of vertical scanning sonar as they don’t show up in that view.

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Fish finder image five – a lure descending in front of four fish

Picture: Lowrance and Romen Dicovski, Roaming Productions ©

This image was taken while Romen was lure fishing in a river. You can see it is a hard bottom with the yellow there. It is either rock or compacted sand in a river bed.

“You can see the lure dropping towards actively moving fish,” Romen says. “They look quite large, but you have to remember that you aren’t in very deep water.”

“The fish labelled as number four actually came down and took the lure.”

Fish finder basics explained

So there you have it – with these five images from a Lowrance fish finder we have explained the basics of what you need to look for. The second half of this article is given over to a brief and simple explanation of how fish finders work from a technological perspective and what you should look for in a fish finder.

Key features of a good fish finder

These days modern fish finders include the best features – such as structure downscan and sidescan – right throughout their range depending on the transducer chosen.

With Lowrance, Romen says, all this plus the FishReveal technology is offered as standard on all current models in the range: The Hook Reveal series; the Elite Ti2 series and the HDS Live series. The HDS Live also has Lowrance’s LiveSight function, which will be the subject of another article we are preparing. 

Screen sizes vary between 5 inches and 16 inches with touch screens available on the Elite Ti2 and the HDS ranges.

Lowrance’s website breaks down who each series is aimed at with the Hook Reveal pitched at casual weekend anglers, the Elite Ti2 at avid weekend anglers and the HDS LIVE at tournament and serious anglers.

Romens says the Hook Reveal with the Triple Shot transducer (offering sidescan and structure downscan) offers great value for what’s pretty much a fully-featured fish finder for $800 or $900 in the larger-screen models.

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Using a fish finder is easier than you think

Hopefully we have cut through a lot of the jargon and mumbo jumbo on fish finders in the section above. But of course there are still a range of settings we haven’t mentioned. 

We are deliberately not going into these here as in most cases the automatic settings are usually fine.

Romen says with modern fish finders there is often no need for the average fisherman or woman to go beyond the automatic settings. The fish finder will automatically adjust frequency and other settings to display the most and the most accurate information.

You really just have to choose what you are going to show on the screen.


What other key features do fish finders have?

The other main features of a fish finder is chart plotting and GPS. Almost all sounders have built in GPS now giving you the ability to view charts and also to drop pins so you can easily navigate back to your favourite spots.

Higher-end models also have touch screen technology, which is less hassle to use when you are on the water.

Fish finder tech talk

How does sonar work?

A fish finder is a sonar tool which helps locating the fish. Sonar stands for Sound Navigation and Ranging. Depending on the model you choose, a fish finder can be equipped with GPS, marine radar and a compass to help you find the way when you’re on a boat or kayak. 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).

How much of the bottom does a fish finder cover?

The sonar beam from your transducer descends as a cone beneath your boat or kayak so the area being depicted gets wider the deeper you go down. The angle (from the vertical) of the cone depends on the frequency the sonar is operating at (the higher the frequency the more narrow the cone).

How do frequencies impact this?

As a general rule of thumb for traditional sonar, when using the lower frequency (50 kHz or 83 kHz) the width (diameter of the cone) of the area of the bottom you are scanning is roughly equal to the depth. In the higher frequencies (typically 200 Khz) the width is more like one third of the depth. While you can switch frequencies, for most recreational users your fish finder will self select the most appropriate frequency.

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Conclusion

Romen used to fish more traditionally casting sideways into likely structure without relying so much on the fish finder. But that’s changed completely in recent years and his catch rate has drastically improved.

“I wouldn’t go out on the water without one now,’’ he says. “I rely on it 99% of the time I am fishing. I catch more fish vertically fishing relying on the fish finder than I ever caught casting sideways into the wilderness.

“I now spend at least 80% of the time looking at my sonar and sometimes 100% of the time. Once you learn how to read the fish finder you soon learn which fish are active and which fish aren’t.”

Obviously there is much more to getting the most out of your fish finder than we can cover here, but the intention of this guide was simply to given those starting out a quick and easy guide to interpreting the images a fish finder produces so they can begin seeing (and then catching) more fish or to help them decide whether to install a fish finder on their boat and kayak.

Stay tuned for more follow-up fish finder tips from us here.

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