How do fish finders work

If you are a regular reader of my blog you will have noticed I have been answering some of the questions I get asked via email the last few weeks and this post is no different. I must get asked about this at least once a week so I thought it was time to tackle the subject of how fish finders work.

How do fish finders work
A fishfinder (also called a depth finder or sounder) is made up of two parts that work together seamlessly. The receiving unit processes the signal and interprets the data which it displays on an LCD screen. The sending unit, called the transducer, sends the sonar signal to the bottom and when it receives the signal back it sends this signal to the computer for interpretation and display.

The underwater part
A fish finder sends an acoustic pulse directly downwards to the seabed and records the returned echo. The sound pulse is generated by the transducer that emits an acoustic pulse and then “listens” for the return signal. The time for the signal to return is recorded and converted to a depth measurement by calculating the speed of sound in water. The time interval, measured in milliseconds, between the pulse being transmitted and the echo being received, allows bottom depth and targets to be measured.

Fish finders can be used to detect bottom structure like wrecks, reefs and snags. With practice anglers can even distinguish a hard reefy bottom from mud or sand.

Sound waves travel differently through fish than through water because a fish's air-filled swim bladder has a different density than seawater. This density difference allows the detection of schools of fish by using reflected sound. (wikipedia)

Why do fishfinders show arches

As the fish swim through the beam they are shown as arches. If your boat is stationary and the fish are stationary you will see a straight line instead (as the sound keeps bouncing off the fish).

This is best explained with a picture:


You can set up your fish finder to fish show symbols or arches. Fish symbols are shown when the sophisticated computer detects enough of an echo to be a fish. However the computer can make incorrect detections and most serious anglers use fish arches and choose to interpret the data themselves.











Two soundings - one using fish arches the other using fish symbols.

Do fish finders scare fish? Can fish hear the fishfinder?
The simple answer to this question is no. Fish cannot actually hear the sound emitted by the fish finder as the frequency is out of their hearing range. As for whether they can feel it in their swim bladder, that is still up for debate. For what it is worth I have caught good fish with the sounder on and off so I don't put much reliance in this theory.

I hope you have enjoyed this post. If you have any topics you would like to see me post about or want to know more feel free to email me or leave a comment.