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How to discriminate fish echoes from the seabed
The figure below illustrates how two fish schools a and b are
displayed on the screen using three different tilt angles.
Case 1: Tilt angle 30 to 40 degrees. Fish school is obscured by
the seabed.
Case 2: Tilt angle 10 to 20 degrees. Fish school is located above
the seabed (midwater).
Case 3: Tilt angle 0 to 10 degrees. Fish school is located close to
the seabed.
Case 1
Case 2
Case 3
Fish school a
b
Case 3
Case 1 Case 2
Fish school aFish school a
Seabed
Seabed
}
Sea bed
Figure 2-1 Fish echo and tilt angle
Points to consider
• Normally, a vertically distributed fish school is a better sonar
target than the seabed, because it reflects the transmitted pulse
back toward the transducer.
• In case 3, both fish schools a and b are presented. Gener-
ally speaking, however, midwater fish schools tend to be larger
than bottom fish schools and they are often displayed near the
seabed on the display.
• It is difficult to detect bottom fish when they are not distrib-
uted vertically.