Furuno CH-37 Operator's Manual page 39

Noise and interference
When the fishing ground is crowded with many fishing boats,
the sonar is subject to interference from ultrasonic equipment
(echo sounder or sonar) on other boats as well as those on
own ship.
For instance, interference from the sonar operated on other
boats will show itself on the display as in (A) in the figure be-
low. This interference can be suppressed by changing the Tx
rate on the SONAR menu. Noise from marine life shows itself
on the displays as in (B) in the figure below. This type of noise
can be suppressed with "IR" in the SONAR menu.
(A) Inteference from (B) Interference from
other sonars marine life
Figure 2-22 Noise and interference
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