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2. SONAR OPERATION
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2.3 Monitoring Trawl Position
The Locked mode can be a valuable tool for monitoring trawl location in order to
prevent damage to the trawl by shoal or other protrusion.
Ground rope
If the ground rope is positioned in the coverage area of the sounding beam it is
displayed as a line almost parallel with the transmission line. If a fish school
appears between the transmission line and the ground rope it can be considered
to be entering the trawl.
The interval between the transmission line and the ground rope is equivalent to
the height of the trawl opening and therefore the interval is kept constant if the
trawl is being properly towed. If it is unsteady, or if the ground rope rises
suddenly, towards the transmission line, the trawl may be caught on some
object.
Image and fishing operations
The illustration and descriptions below show the relation between trawl and the
LOCKED mode image in bottom trawling.
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Ground Rope
Display range changed
Example fishing operation and echo sounder image
1. The trawl is set into water and begins descending. In the image, the distance
between the transducer and the bottom gets shorter and shorter as if the
bottom is rising toward the transmitter unit. On the other hand, the distance
between the transducer and the sea surface increases as if the sea surface
is receding from the transducer. The range has been changed twice for
shorter ranges.
2. The trawl has settled on the bottom.
3. The trawl is being towed along the bottom. Fish entering the trawl are seen
above the ground rope. The ground rope is being kept at a constant distance
from the transducer, indicating that the trawl mouth is opening properly in the
course of towing.