A sound word on sounders
  |  First Published: June 2003

Interpreting arches

SECTION: Boating

THROUGHOUT the year I travel the national boat and tackle show circuit, talking to the public about marine electronics. This has been ongoing for the past eight years, and over that period I’ve been amazed at how many boat operators are not getting the best use out of their sounders. While the modern day sounder is more user friendly than those of old, many still battle with the concept and specifically particular functions.

While the photographs with this article are of Lowrance units, the concept applies over almost brands of the recreational-sized sounders. Some of the terms may differ though, such as ‘sensitivity’, which is sometimes referred to as ‘gain’. Another example is ‘grey line’, which is sometimes referred to as ‘white line’ or, in colour units, ‘colour line’.

It is important from the outset to learn to identify what’s on the screen, and to dispel some of the myths instilled into us about how to decipher those images.


Call them ‘inverted boomerangs’ or whatever, these are the images we see at the boat show electronics stand or the marine dealership when the unit is running in simulator. Many anglers wait until they see arches before they stop and fish a particular section of water. Wrong! If you do this you are most likely leaving fish to look for fish elsewhere.

In reality, perfect fish arches are often hard to display for a number of reasons. Firstly, refer to the diagram on this page of the signal cone coming from the transducer. It is a generalised drawing of a single beam transducer. Some manufacturers have various shaped beams with differing frequencies.

At the point where the fish is, draw an imaginary line across the beam parallel to the bottom. The distance between the transducer and the outer edge of the beam is longer at this point where the fish enters it than it is from the transducer to the centre of the beam. As the fish swims through the beam on a flat plane it gets closer to the signal source. When the fish first touched the edge of the beam we received one dot of information. As it swims toward the centre of the beam it is coming closer to the signal source so the marks on the screen scroll upwards until the fish is in the centre of the beam. Note now, the signal is at its strongest and therefore showing as a thicker band. As the fish swims out of the beam it is moving further away from the signal source so the arch tapers back down. This situation applies to a fish that swims through the centre of the beam. Note that on the very outer edges of the beam the fish only shows as one or two dots as the signal return is weak. You will need to recall this soon.

If this fish was to swim through to one side of centre, it would be in the beam for a shorter period. That would make the arch shorter in height and less wide. The closer to the edge of the beam the fish swims, the smaller the resulting image will be on the screen. If that fish was to only just touch the edge of the beam as it travelled past, it may show up as one dot of information – such as we see when the fish in the diagram sticks its nose into the beam before entering it. The small dots on your screen may well be baitfish, but they could also be large fish grazing the edge of the transducer signal beam.

This is a three-part scenario. If you believe you have the right landmarks or your GPS tells you that you are near the spot and there are small dots on the screen indicating baitfish, it may be that some other phenomena such as a change in tide, dawn or dusk or a drop in the barometer may bring the target species right onto the spot and your hook. Those big fish, especially snapper, often lurk on the peripherals of structure until the feed gong is sounded.

Transducer set-up also has a bearing on the end result of fish sonar readings seen on screen. Typically, a transducer must be tilted with the trailing end down so that a run of clean water passes under its signal face to prevent turbulence and the resulting bubbles from creating breaks in the transmission to the head set. This angulation shoots the beam forward with the result that the beam print on the bottom becomes elliptical but importantly, the top of the beam becomes greatly longer than the back edge of the beam. Pick up a torch and shine it vertically to the floor near your feet. Now point the torch beam away from you at an angle to the floor and you will see the result, Your transducer beam is no different. This result has a dramatic effect on the arches in that instead of a perfect fish arch in the perfect situation, it will be a lopsided portion of an arch showing on screen.

Having said all of that, if a fish sits in your beam and doesn’t move it cannot be shown as an arch as it is not swimming in and out of the beam. These fish will be shown as a continually drawing from one side of the screen to the other. This is typically seen when anchored in still water with burley under the boat and fish come in and feed, and also when sitting over schools of bait with bass in attendance with no wind drift.

[Matty - Please put this caption between the sounder screens]

In raw mode (above) the markings of the fish and downrigger bombs are easily discerned. Point 1 shows the first downrigger bomb tracking at about 20 feet. Point 2 is a second downrigger bomb being dropped down to the level of the fish (about 22 feet). Point 3 indicates the first downrigger bomb being raised, possibly after a strike, while Point 4 shows numerous fish rising through the transducer beam to investigate the lures and downrigger bombs. It is easy to see the fish slashes and arches indicating the relative time spent in the transducer beam and the movement through the water column of the fish.

In fish ID mode (below), everything appears as a possible fish – a situation that is not very desirable. Learn to use your sounder in raw data mode and how to decipher the various slashes and marks to increase fishing results.

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