A sound word on sounders – sensitivity
  |  First Published: May 2003

TAKE any boat show in any state and I would field this question at least six times per day; “I am having problems with my unit. I can see the bottom but nothing in between.” My immediate reply is “What level of sensitivity are you running on the head set?” Their reply is “Aaaaah, pretty high, about 50%.” Poor bugger! I dread to think how many fish these people have been driving over to go and find fish elsewhere!

This function is vital if you want fish in the creel or high numbers on the scoreboard. Sensitivity of a unit is basically its ears. It is its ability to listen to return echo of varying strength.

If sensitivity is reduced, the ability of the unit to listen for weak signals is reduced also, making the unit deaf. If this happens, small fish closer to the surface or even large fish deep down on your favourite bottom bouncing spot, will not show on the screen. Bass fishers will realise the importance of detecting thermoclines, which are habitat in their own right for their target fish and their food source. Some thermoclines are weak and others that are strong may be deep down. Even large fish close to the surface that swim through only the very edge of the beam may be lost. A high sensitivity is often required to detect these.

As with most sounders, one will see a line of return signal showing at the top of the screen. This signal return is referred to, as surface clutter and a widening of this image will result from the sensitivity level being increased. It results from surface noise such as wave action against the boat and interference from electronics and other vessels in the water. This clutter may be annoying to some. It is vital, however, that those weaker signal returns – if you are fishing deep water and looking for bait in the water column or your favourite bottom fish – are not lost. If you’re not interested in what is in the top few metres of the water, use the upper and lower limits function to remove the surface clutter. (I will go into upper and lower limits in another article dedicated to that function.)

This effectively clears the screen and displays the water column over a larger area of the screen giving a clearer view.

The level of sensitivity required in various brands of sounders differs greatly. The screens that you see hereabouts are Lowrance’s LCX series and they prefer a high level of sensitivity, from 84% upwards to produce the best quality screen images. Lowrance’s X series such as the X51 and X 91 will perform at their best from about the mid 70% upwards.

Your unit, whatever the brand name, may differ greatly from these levels so be prepared to experiment. To get the ideal level for the conditions you are in at the time, push the sensitivity level of your unit to the maximum. You should see the screen clutter from top to bottom, especially in water that may be only 20m deep or less. Now reduce the sensitivity bit by bit until the clutter begins to clear near the bottom. The surface clutter will be the last to go and, as I mentioned earlier, we really need to keep that. Once the fishing zone that you are targeting has only just cleared of the last speck, you will be at the prime setting for that depth and quality of water.

It is important to remember that if you now travel into deeper water and retain that same sensitivity level, minute return signals at or near the bottom may not be displayed. You may have to increase your sensitivity again and likewise if returning to more shallow water it may have to be reduced to keep that optimum screen display.

While all this may seem a little labour intensive, it is the only real option for the serious angler. The only other option is AUTO SENSITIVITY. This function takes into account all signal returns and noise in the water and reduces the sensitivity automatically to give the viewer a clearer screen. Unfortunately, many of the weaker signal returns will be lost. They may well be the baitfish you were looking for, the thermocline that may be holding bass tucker or the emperor sitting on the bottom in 100m of water. This particular function is devastating to the quality of information return when fishing in water that is dirty and stirred up from bad weather, causing a lot of suspended material to reflect the signals. Impoundments holding suspended algae blooms or infestations of water fleas will also give a cluttered screen reading, but over time you’ll get used to it and read through it to get the important information.

Remember, reduce the sensitivity and your unit will go deaf. Increase the sensitivity and your unit will hear weaker signals.

1 & 2) The importance of sensitivity is illustrated in these two sounder shots. In the first instance, the sensitivity is up around 90% and the thermocline (1), downrigger bomb (2) and high numbers of fish (3) can be clearly seen. When the sensitivity is dropped down to about 70%, the downrigger bomb (2) is barely recognisable, while the thermocline reading (1) is reduced and you could interpret the fish numbers (3) as being low.

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