Using Sidescan

Why use sidescan ?

I'm no fisherman -  I chuck the odd Tazzy in when I'm in the mood, but quickly lose interest unless I'm out in a boat with someone who actually knows what they are doing.

However - because I go places on Lough Corrib in Burlesque  that are seldom traveled, and scan the entire lakebed I tend to spot where the fishes are, where the hatches are, and whats going on.

Something I've learned from the thousands of miles of scans is that the fish seem to prefer a bit of structure. This has not gone unnoticed by the chaps at Navico, Humminbird, Garmin either,all of whom now produce sidescan units for fishermen.

In the image below you can see a standard sonar image (left) from a regular transducer at 83Hz, and a sidescan image (right) containing the same piece of water, taken at exactly the same time. The boat is passing over the edge of an isolated pile of rock in what is otherwise a barren lakebed. The stripes that you can see going generally from left to right in the sidescan image are the effect of fyke nets having been laid and lifted over the years - they leave a permanent scar in the sediment.

In the sidescan image you can clearly see the rocky area, in the standard image you can clearly see two very large fish for whom this pile of rocks is home. In the standard sonar image you have no idea of the nature of the habitat, or its size. In the sidescan image you can see exactly what the attraction is, and that it is isolated with a few large rocks in a pile of smaller stones.

Sidescan is not designed for detecting fish. It is designed to give you a good idea of the bottom structure so that you can determine whether it may be fish holding ground or not. This pile of rocks evidently is, and probably holds a good stock of nymphs or prey that constitute a healthy diet. The other small pile about 100ft off to starboard is likely to be similar, and you can waypoint it and head over to it.

Sidescan for Structure
Sidescan for Structure

How to interpret a sidescan image.
The image on the right is exactly what you see on your Sidescan unit. 

The boat itself can be imagined to be at the top of the screen, in the centre. The light coloured vertical bar is the boats track - so you can see that the rockpile is now behind the boat.

The transducer produces a wafer thin fan of sound which extends from straight down under the boat, to about 5 degrees from horizontal on both sides. Because it looks straight down you can see the water column, just as you can in the standard sonar image, except in the sidescan image it extends right and left instead of down, so the black area in the centre is the water under the boat.
The transducer receives echos from this fan of sound, and simply plots the return, together with an indication of its signal strength (brightness) on the screen in a line extending left and right from the boats position at the top of the screen. This line of returns is persisted and moved down the screen at a predetermined scroll speed,  as the boat moves forwards the image builds up as the thousands of lines of returns move down the screen.

To help get your head around what you are looking at - you are looking straight down at the top of  objects below the boat, and you are looking at the sides of the objects that are 85 degrees.  It naturally follows that you are looking at 45 degrees at something which is half way between the two.

This is no different to flying above a city, where you are looking at the sides of buildings in the distance, and the roofs of buildings straight below.

Sidescan gives you no idea of the depth of water at any position other than directly under the boat. However, by measuring the sonar shadows of objects, or even fish, you can get a good idea of their height and position in the water column off to the sides of the boat. This is easily done in software such as Reefmaster.
By saving your sidescan logs it is very easy to quickly build up a complete picture of areas of the lakebed, enabling you to target exactly the right places.