the upper mouth of the net), and weighted "bobbins" attached to the "foot rope" (the rope which runs along the lower mouth of the net). These bobbins vary in their design depending on the roughness of the sea bed which is being fished, varying from small rubber discs for very smooth, sandy ground, to large metal balls, up to in diameter, for very rough ground. These bobbins can also be designed to lift the net off the seabed when they hit an obstacle. These are known as "rock-hopper" gear. Body of the trawl The body of the trawl is funnel-like, wide at its "mouth" and narrowing towards the cod end, and usually is fitted with wings of netting on both sides of the mouth. It is long enough to assure adequate flow of water and prevent fish from escaping the net, after having been caught. It is made of diamond-meshed netting, the size of the meshes decreasing from the front of the net towards the codend. Into the body, fish and turtle escape devices can be fitted. These can be simple structures like "square mesh panels", which are easier for smaller fish to pass through, or more complicated devices, such as bycatch grills. Cod end The cod end is the trailing end of the net where fish are finally "caught". The size of mesh in the cod end is a determinant of the size of fish which the net catches. Consequently, regulation of mesh size is a common way of managing mortality of juvenile fishes in trawl nets. How trawls work The idea that fish are passively "scooped up" is commonly held, and has been since trawling was first developed, but has been revealed to be erroneous. Since the development of scuba diving equipment and cheap video cameras it has been possible to directly observe the processes that occur when a trawl is towed along the seabed. The trawl doors disturb the sea bed, create a cloud of muddy water which hides the oncoming trawl net and generates 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 "cod end", and are caught. The speed that the trawl is towed at depends on the swimming speed of