Symmetry
If the trawl does not move symmetrically around the water current, so that the direction of the water flow (the water flow given from the direction the vessel is moving in and side current) will come at a skewed angle at the opening, the whole trawl will be asymmetrical, the mesh on the opposite side of the current will stretch, the water flows through and fish/shrimp escape.
The Symmetry sensor has twice the working time as the combined sensor, and can be used alternately by changing the battery.
Asymmetry leads to open mesh and direct loss of catch.