Furuno CSH-83, CSH-84 manual How to discriminate fish echoes from the seabed, Points to consider

How to discriminate fish echoes from the seabed

The figure below illustrates how two fish schools aand 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 Fish School a Fish School a

Seabed Seabed b

Case 1 Case 2 Case 3

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 schoolsa 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.

2-4