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 [[death|mortality]] of [[juvenile (animal)|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 camera]]s 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 the species which is being targeted and the exact gear that is being used, but for most [[Demersal zone|demersal]] species, a speed of around {{ Convert | 4 | kn | km/h mi/h }} is appropriate.
