Fish School Echoes; Surface Noise/Aeration - Furuno NAVNET 1734C Operator's Manual

4. VIDEO SOUNDER OPERATION

4.15.3 Fish school echoes

Fish school echoes will generally be plotted between the zero line and the bottom. Usually
the fish school/fish echo is weaker than the bottom echo because its reflection property is
much smaller compared to the bottom. The size of the fish school can be ascertained from
the density of the display.

4.15.4 Surface noise/Aeration

When the waters are rough or the boat passes over a wake, surface noise may appear near
the zero line. As surface turbulence is acoustically equivalent to running into a brick wall,
the bottom echo will be displayed intermittently. Similar noise sometimes appears when a
water temperature difference (thermocline) exists. Different species of fish tend to prefer
different temperature zones, so the thermocline may be useful to help identify target fish.
200 kHz tends to show shallow thermoclines better than 50 kHz.
In rough waters the display is occasionally interrupted due to below-the-ship air bubbles
obstructing the sound path. This also occurs when the boat makes a quick turn or reverses
movement. Lowering the picture advance speed may reduce the interruption. However,
reconsideration of the transducer installation may be necessary if the interruption occurs
frequently.
Surface noise
Caused by aerated
water
4-22
Large
Small
school
school
Size of
fish school
Fish school echoes
Thermocline
Surface noise/aeration