Topic 11 - Echo strength

Through the fish finder it is possible to identify the different sediments of the seabed. The seabed generates a strong echo when hit by ultrasonic waves, which is displayed on the fish finder screen as a red or brown line. When you are out fishing you will notice that the amount of seabed displayed on your fish finder changes depending on its hardness and composition.


The part of the seabed that gets displayed on the fish finder screen can sometimes be called the “bottom trail” or “bottom band”. If the returning echo from the seabed is strong, the amount displayed on the fish finder increases. What this mean is that the bottom is hard and most likely consists of bedrock or reefs.
The opposite is true for a soft seabed. When the seabed is soft the “bottom band” will be very thin, due to the fact that the returning echo is weak. When the returning echo is weak, the seabed likely consists of sandy soil or sand.


The amount of seabed displayed on the fish finder screen also depends on the frequency used by the fish finder. Generally speaking, the “bottom band” displayed will increase when using low frequency ultrasonic waves, and decrease when using high frequency ultrasonic waves. If you wish to get a good view of the seabed on your fish finder, it is recommended to set the fish finder to 50kHz, making it easier to judge the properties of the seabed.


When fishing for rockfish for example, it is very useful to be able to judge the properties of the seabed since these fish prefer rock outcrops.


Bottom band

Rocky areas on the seabed will give of a strong echo, displayed prominently on the fish finder display. The returning ultrasonic waves from the fish finder will be displayed as a thick bottom. On the contrary, when the seabed is soft it will be displayed as very thin line. This is due to the seabed absorbing a big part of the ultrasonic waves emitted from the fish finder.