Firstly, lets get to know a couple of aspects on how a sounder Works. This article refers in particular to Furuno and Garmin sounders but aspects of the article apply to most good sounders.

Basic operation:

From the display unit, power is sent to the transducer in the form of a short electrical pulse. This pulse is converted to ancone angles ultrasonic sound signal by the vibrating element of the transducer which in turn results in a cone shaped beam that will spread at a particular angle, (normally between 6 and 45 degrees) through the water at the considerable speeds. In water sound travels at approximately 1,500 Mtrs per Second. The signal travels freely through water but bounces off fishes air bladders, the seabed and its structure. These reflected sounds travel back to the transducer where they are converted back to an electrical pulse which is sent back to the display unit for processing. Most modern fishfinders are digital rather than analog. This means the signal is changed from a voltage into 1's and 0's where they can be examined and manipulated  by software in the sounders computer chip, rather than altering the voltage by applying electrical forces. The resulting picture is then shown on the screen.

The majority of recreational fish finders are dual frequency, and most of these run on 50 and 200kHz.
50 kHz frequency has a wider beam angle, normally between 20 and 45 degrees and is best used in deeper water. Due to the width of the 50 kHz beam, we tend to get a “stretched” picture of both our targets and the seabed, the objects that enter this beam take more time to pass through it, so there is more time for more signals to be reflected back and be included in the vertical lines drawn on the display. 200 kHz is much narrower than 50kHz, normally between 5 and 11 degrees. Objects spend less time within its beam, so we tend to get a “piece” of that object displayed rather than a full “fish arch”. However if an object appears in this picture you can be assured that it is right under your boat and you should have your lines over! It is also better for looking at bottom structure and better fish discrimination as well as confirming individual fish life around the bait school.

Hot tip

Use the 50 kHz to search and the 200 kHz to target. Or you could run the sounder on Dual Mode and then switch to 200 kHz once fish are found.

Using gain

Gain is the most fundamental aspect of a Fish Finder. Many believe that Gain is related to how sensitive the sounder is. This could not be any further from the truth. Gain is a sound amplifier, it takes the return signal received from the transducer and multiplies it.   As sound travels through the water it gets weaker. Much in the same as in air, if someone is yelling at you, they will be quieter the further away you run. Gain is used to amplify the return signal to offset the loss from the fish (running) being further from the transducer (yelling).  Turn it up too much and you’ll get overstated fish sizes and a lot of noise and clutter. Turn it down to low and you’ll miss critical information on certain fish, bottom structure, seabed life etc.
Using Gain on manual is an important skill in finding more fish, not only for seeing fish, but also for analysing what is on the bottom. By leaving the fishfinder on automatic, there is no way of telling if subtle changes in bottom characteristics is just the fishfinder changing itself, or if you are really changing from a sandy to muddy bottom. We all know that Terakihi love manual gainto hang out on paupa rock, but it takes a keen eye sometimes to pick up when you are on their home turf.

Hot Tip:

When in manual mode (Auto Gain set to OFF). Turn all the Clutter, Colour Erase  to 0%. Adjust gain until you see a small amount of light blue clutter on screen, then adjust your clutter slightly to remove this noise. 

Clutter

Clutter is used to eliminate low level return echoes from the sounder picture. Echo Sounders have a Colour Scale that represents each level of echo return. Light Blue/Dark Blue is a low level echo, Green/Yellow is medium level echo and Orange/Red /Maroon is a high level of echo return. Clutter manipulates the scale of echo return. As you turn the clutter up you will make medium and stronger echos appear weaker and blend the low level echoes into the back ground colour, thus eliminating low level echoes (or clutter) from the screen. If used to much you will miss all smaller fish species (Eg Bait,  etc).

Hot Tip:

Use the clutter to a minimum. If set to high the sounder will ignore bait fish and small fish species.

Colour erase

Colour Erase is simply that, it erases colours. It’s a handy tool if you don’t want to
have to adjust your clutter which changes colour scales or muck around with the gain. It simply removes unwanted low-level echoes off the screen by cutting them off, starting with the light blue colours. The higher you turn it up, the more colours it erases. If used too much you will lose undersized or smaller bait fish etc.

Hot Tip:

As with the clutter setting, use the Colour Erase to a minimum. If set to high the sounder will ignore bait fish and small fish species.

Noise limiting

This adjustment will help remove interference from other equipment on the boat. This interference often shows up more in deep water settings and often as diagonal lines across the screen. Again only use it if it is required as too high a setting will remove fish signals.

TVG

As we stated earlier, as sound travels through water is gets weaker. Therefore, we need Gain (an amplifier) to display the sound on the screen. The deeper you wish to sound the more you will have to turn the gain up. However, as the sound is still loud at the surface, the sounder will get more clutter at the top of the water column while the bottom will be relatively clear. Normally to remove this surface clutter you would either have to use Clutter or Colour Erase. However, of course this affects the whole picture. So by removing the clutter from the surface, you will also be removing it from the bottom where you need all the detail you can get. TVG offsets the signal loss by the sound traveling through the water. It automatically turns the gain up, as the sound gets deeper. Therefore, it keeps the gain relatively low in the shallower depths and turns it up, as it gets deeper on the screen. You should find that as you turn the TVG up, you will need to turn the normal Gain setting down (because the TVG is already turning up the gain in the deeper depths). This will in turn remove the surface clutter. 

Output power

Out Put Power is how loud the sounder is. Some sounders have an adjustable output power. This is done automatically but can also be set manually. This is done to eliminate noise in very shallow depths (E.g Under 5 Mtrs). In shallow depths, the sound can actually bounce twice or three times off the bottom. This is known as the second or third echo. Therefore, sounder can register twice or even three times the actual depth.

Hot Tip:

leave the Output Power (TX Power) in Auto. shift function

Shift System

This is a great feature for deep water. The sounder normally shows whats below the boat from the surface to the range depth, so if your range is 400m, the sounder will show between the surface (0M) to range (400M). With the shift system we can move the start point which the sounder is looking, so if the shift is set to 250M, but range is 200M, what is on the top of the screen will be 200M and the maximum depth shown will be 450M. This is important as the sounder is not trying to cram so much information into the same number of pixels, allowing you to get a better picture of what is below. This will also cut out all the surface clutter and let you concentrate on fish in the depth you are looking at. Don’t forget to reset the shift to zero when going back into shallower water!

Bottom Lock

Bottom lock is fantastic for helping to identify fish which are close to the bottom, and is unbeatable for finding snapper and tarakahi on flat sandy/paupa rock bottoms. Unlike the normal sounder screen which measures depth from the surface, Bottom lock measures depth from the bottom up. This has a couple of very handy outcomes. Firstly the sounder always appears flat, so even if there is a 1m swell running, you won't see the bottom going wavy, allowing you to clearly see if it is a big enough mass to be a fish. It also allows you to always have the last few meters on the screen, allowing you to descriminate between bottom and fish much easier.

bottom lock
More Hot Tips:

• 50kHz is more sensitive to turbulence, so when cruising always have the unit set to 200kHz for best picture. Once off the plane and searching for fish, 50kHz will allow a wider search area and show fish that are to the port and starboard side of the boat but 200kHz will actually show what is below the boat, so use this when deciding when to drop your lines. Optimal performance will always be achieved using the Manual Mode.
• Range scales are adjustable on most units so if the factory defaults are not ideal they can be changed in the menu. So too can the zoom range. Depth can be displayed in Meters, Feet or Fathoms. 
• For all bottom fishing use the bottom lock function to easily identify fish close to and around bottom structure. This is used by the commercial fishing industry extensively and is the best zoom available for exposing fish.
• Always keep the Pic Advance at 1:1 that way when the transducers clicks once, the screen will move across once. 
• Noise Limiter is for electrical interference only and should not be used to clear the screen in Auto or Manual Modes.
• Play with the unit as much as possible to start off with. It’s the only way to learn. Once you figure all the functions out, reset the unit back to default and start fresh.