The trawl doors disturb the sea bed. They create a cloud of muddy water which hides the oncoming trawl net. It also makes a noise which attracts fish. The fish begin to swim in front of the net mouth. As the trawl continues along the seabed, fish begin to tire and slip backwards into the net. Finally, the fish become exhausted and drop back, into the net and are caught. The speed that the trawl is towed at depends on the swimming speed of the species which is being targeted. It also depends on the gear that is being used. For most species, a speed of around 4 knots (7 km/h) is appropriate. Trawling is very easy way of catching large amounts of fish. But it has been compared to "fishing with bulldozers". Like a bulldozer it scrapes up everything on the bottom.