If you used bottom and pelagic
trawls or pair trawl, this Simrad catch monitoring sensor system will:
- Measures if the trawl is skewed
- Allow you to correct the trawl geometry by adjusting the warp
If you use a danish seine, the sensor system will:
- Detect variations in the distance from the centre of the headrope
(or footrope) to either wing of the seine
- Detect differences in the rope lengths when the seine is closed
(distances between the wings should then be close to zero)
Why is this?
In order to maximize the performance
and the catch efficiency, the trawl and the warps must be adjusted
properly. This is essential to ensure a square trawl and minimum skew.
This is also important for danish seines.
Any disturbance to
the trawl movements may have a negative effect on the catch. This
is particularly important on a bottom trawl. A skewed bottom trawl
will result in unstable bottom contact, and create escape routes for
fish below the gear and through the side panels. On pelagic trawls
and danish seines a skewed net will make the gear unstable, and the
catch be reduced.
Optimal trawl geometry is therefore vital
for maintaining the catch efficiency.
What causes the gear to be skewed?
A trawl or danish
seine may be skewed for several reasons. Some of these reasons are:
- Incorrect rigging of trawl, sweep lines or trawl doors
- Misadjusted warp lengths
- Side current
- Towing in steep or sloped edges
If the rigging of the trawl is otherwise correct, you can
easily compensate for skew by adjusting either port or starboard wire
The basic facts
This Simrad catch monitoring
sensor system is based on the fact that a correct rigged trawl and
danish seine has exactly the same distance measured from the centre
of the headrope or footrope to either door or trawl wing. These distances
must be identical independent of weather conditions, sea state or
left trawl, distance “A” on the port side is longer than
distance “B” on the starboard side. The trawl is unbalanced.
On the right trawl, the distances "A" and "B" are identical, and the
trawl is symmetric for maximum efficiency.
Measuring the door
spread (“C”) will not detect a distorted trawl geometry.