Marine Electronics - Fish-finder basics

Last updated 00:00 01/01/2009
A good sounder can differentiate between a soft or hard bottom, and will indicate the size of fish present. Here, Rick Pollock???s experience and good quality electronics enable Shane Dufaur to enjoy a large kingfish encounter. (Photo: Mark Kitteridge)
Screen Shot A
Screen Shot B
Screen Shot C
Screen Shot D
Screen Shot E

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The following email from Raewyn Manuel is typical of many requests from NZ Fishing News readers when it comes to understanding marine electronics.

‘Like some other keen recreational anglers I know, I was wondering if you plan to print a learners’ article on reading fish-sounders some time in the future. I know when you have one fitted they come with instructional manuals, but they don’t tell you how to read the printout or history on the screen. It is very frustrating, and our sounder often ends up being a colourful decoration on the dash.’

Raewyn’s scenario is no doubt played out many times over each year in hundreds of recreational craft throughout the country.

While the manuals tell you so much, it is hard to decipher what it all means in practical terms. Cameron Burch, in the first of several articles on understanding marine electronics, attempts to provide some of the answers for Raewyn and her fellow boaties…

The most important things we want to know when using a fish-finder (herein referred to as a ‘sounder’) is A) the bottom type and B) whether there are any fish around.

Let’s start with the bottom. The transducer – typically mounted on the transom of a trailer boat or through the hull on larger vessels – sends a soundwave down through the water at up to 50 times per second. When it makes contact with the bottom, the soundwave is reflected back to the transducer. The time it takes for this signal to return is how the unit works out depth.

A good sounder can differentiate between hard and soft bottoms and small/large fish by the strength of the returned signal. Hard objects will return a strong signal, while soft or small objects will return a weak signal.

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Now let’s cover fish arches. Many anglers spend hours sounding around waiting to see arches on their screen, believing that only arches are fish and everything else is rubbish like debris or seaweed etc. This could not be further from the truth.

So let’s look at how and why a fish can appear as an arch on the screen.

Screen A shows a fish entering the sonar beam at D1. At this point you can see that the distance of the fish from the boat is greater than when the fish gets to D3. As the fish moves through to D5 it is further away again. That’s basically how the arch is formed. The area within the sonar beam – D1 through D5 – is commonly referred to as the ‘water column’.

But what happens if the fish moves through the beam more slowly or stays in the beam?

I captured the screen shot on Screen B to explain what we are going to see. It shows a large school of fish holding from five metres to the bottom.

The top right-hand corner is where the boat is currently and the ‘history’ scrolls off to the left of screen.

Let’s start at the left.

I was moving over the school with the boat idling ahead. We can see fish arching up perfectly due to the fish entering one side of the beam and going right through it and out the other side. Then I put the engine into neutral, this is shown in the middle of the screen, and you can see the arches start to elongate to the point where the fish are marking as lines only. If every time the beam goes down it detects a fish, it will produce a mark on the screen. If the fish stays in the beam, all those marks form a line.

The other part of this screen shot that’s worth noting is the ‘colour bar’ on the right of screen. This colour bar represents how the unit differentiates between strong or weak returns. The top of the bar shows a deep-red; the strongest returns will show up in this colour. The bottom of the bar shows blue, then white; the weakest returns will show in blue, and no return or the water itself will show in white. This colour spectrum is used to identify the sea floor, its density, what is on the seafloor, and anything in the water column, including fish.

Now we will look at Screen C. In this case we were looking for snapper and marked some down near the bottom, so changed to ‘zoom’ mode to have a closer look. This produced four to five separate marks across the screen, each representing good snapper.

The reason they are ‘squiggled’ is due to the wave action causing the boat to move up and down and therefore changing the distance between the transducer, the bottom and the fish. As you can see, the bottom has the same curve pattern as the fish marks. These fish didn’t show as arches because they have remained in the beam for a long period of time.

Screen D was taken on a Humminbird 797c working on a split-screen mode, showing standard sonar and side-imaging. All I want to concentrate on at the moment is the standard sonar section of the image. As you can see, I have the sensitivity set at 19, which gives me the opportunity to pick up the maximum detail. However, if the screen was just cluttered with fuzz, I would reduce the sensitivity until the image cleared up.

What could be described as ‘a mess across the screen’ is actually a school of fish; again, because we are holding over this school, they appear as lines. Each line is a fish. When the line appears red, the fish is in the strongest part of the beam directly under the boat; when it appears a lighter blue or green, it has moved to the outer edge of the beam. I can also tell that the fish are active, as the lines are moving up and down.

Now onto Side-Imaging. Basically, Side-Imaging allows you to look out each side of your vessel and see picture-like detail of the bottom up to 220 metres away, from port to starboard. Unlike traditional sonar, where we are looking at the bottom side-on, we are looking down from above. The boat is at the top centre of the screen and the history peels down the screen. The black section in the middle is the water column before the beams make contact with the bottom.

Screen Shot E was taken while operating in split-screen, with the traditional sonar on the left and side-imaging on the right. We were in nine metres of water and looking 45 metres out either side of the boat. We were passing over a large area of reef/boulder structure jutting out of a sandy bottom. As you can see, most of the boulders are to the right of the boat. Looking at the traditional sonar, it also shows the boulders, but we have no idea how they are laid out on the bottom.

Let’s go back to the previous Screen Shot D and look at the side-imaging on the bottom of the screen.

Although there is no terrain to look at, we can see white stripes through the screen. This is the same school of fish we picked up with the traditional sonar above. They are long strips because they have stayed in the beam path longer. If we were moving over them they would appear more like dots.

Side-imaging can show you exactly how the school is formed and whether they are out to the left or right of the boat. In this case they are evenly spread, but quite often we will just pick up fish on the outer edge of one beam up to 100 metres away. Now that’s what I call coverage! We can even put our cursor on that school or structure, and mark the EXACT location on the GPS.

The days of buying a cheap sounder to just show you the depth are gone. If you invest in a high-resolution colour sounder and learn how to read it, your fishing will improve out of sight. There is no point fishing if there are no fish around, so why not find them first, then fish with confidence and accuracy?

Cameron Burch - Fishing News - November 2008

- Fishing News

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