Fish OperationA midwater otter trawl is towed by a single boat. Midwater trawling is a well-aimed fishing activity. The fish shoals are located by sonar (but some midwater trawling can also be carried out on slightly scattered fish staying in a given layer of water) and the vessel is towing in the direction of the shoal. The fishing depth level, which is controlled by the net sounder, is regulated by the length of the warps and/or the towing speed.