Pelagic Single trawling requires vessels with large horsepower as the nets can be more than half a mile long. The trawl is spread horizontally by cambered pelagic doors and vertically with heavy weights on each lower bridle. Pelagic doors also improve lift when towing speed is increased, and are used to raise the net when aiming for a shoal of fish. The net is continuously monitored using a net sounder, and net depth is changed by altering either warp length or towing speed.