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In general terms, fishfinders and depth sounders are basically the same thing. They are both active sonar. They both use 5 basic components; a display, a microprocessor, a transducer, a transmitter and a receiver. This is a general introduction to depth sounders and fishfinders to help you understand what product you really need and how much is worth spending. It’s certainly not a comprehensive write-up on all the possibilities and products available on the market but is a good introduction to the world of depth sounders and fishfinders.

You don’t need to spend a fortune if you are simply using a fishfinder to find fish in shallow water from your kayak on weekends. Low-cost fishfinders will often do the job. More sophisticated and high-cost fishfinders really suit those who are serious about their fishing, including those who operate boat fishing charters.


Sonar starts from the transmitter, which releases a electrical impulses to the transducer, which is much like a radio antenna coverting the electrical impulses into sound waves through the water. This sound wave cannot be heard by humans or fish. The sound waves travel through water and bounce back after hitting objects (like fish and the bottom). The transducer picks up the reflected sound and transfers that input back to the receiver. The receiver then amplifies the signal as it passes the signal along to a microprocessor through to the display.

This process is repeated several times a second. The same principles of operation are the same for both fishfinders and sounders.


So what is the difference between sounders and fishfinders? A basic depth sounder shows the depth where fishfinders have a real-time representation of the environment below. Depth sounders show depth and fishfinders are able to discern fish in the water to give away their location. Modern fishfinders incorporate the display of sounders and fishfinders into one display.


A microprocessor can add a graphic to a display for a sounder, however, modern fishfinders are actually more complex. The major difference between sounders and modern fishfinders is the transducer, which is built to detect fish. Most sonar frequencies actually pass through fish without making a registration.

Fishfinders emit sonar frequencies designed to discern the air inside fish. Many fishfinders operate at more than one frequency. Higher frequencies are used for shallower depths and lower frequencies to read deeper waters. More sophisticated fishfinders have a number of sonars communicating back to the display for even more resolution. Fishfinders with multiple beams use 2 or more transducers and are capable of giving 3D displays. Displays can show the details of fallen trees and wrecks. The next level of sophistication is where even more powerful multi-beam fishfinders scan 360 degrees to give a virtual representation of everything below including the bottom.


Having all that sonar scanning power is useless without a great display. Obviously, higher resolution is what you are looking for. Basic monochrome displays are rapidly becoming out of date except where one is looking for a very low-cost fishfinder. They are hard to read in daylight but you’ll save on power consumption. The real advantage of basic monochrome display fishfinders is the fact they are extremely affordable whereas the more advanced displays will spike the cost of your fishfinder.

As monitors add substantial cost to a fishfinder, more manufacturers are choosing to offer multi-feature electronics, which incorporate fishfinders, navigation, GPS, chart plotter and radar.


Fishfinders normally have features that include a lock on the bottom, the ability to zoom, a grey line and an adjustable sensitivity. Zoom allows us to focus in on your target. The grey line helps to distinguish between stronger and weaker echo strength, which is great for distinguishing between hard and soft bottoms. Sensitivity is normally built in as a default for most fishfinders and adjusts automatically to reduce noise from temperature, rough seas, acoustic noise and dirty water, for example.

Time-varying gain is a radar-like sensitivity control that can be adjusted manually. It gives sensitivity control to reduce noise clutter. This is good for when you are looking for fish close to the bottom.

Additional features often include a depth cursor, which is a horizontal line across the screen so you can see the depth of targets and the list continues. For example, some advanced fishfinders have transducers which are capable of scanning to the side and forward. Forward scanning is excellent for shallow water and side scanning is excellent for peering into caverns and detailed detection of debris.


Fishfinders and sonars are very easy to install, as long as you pay close attention to the instructions. You must be particularly close attention to the transducer location when mounting as it is critical to both function and the integrity of a boat’s hull. Always follow instructions and seek a specialist if you want to install a particularly sophisticated fishfinder.


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Bill Matthews

Bill is as green friendly as they come. He's travelled the world, loves kayak fishing and camping.