Until the early 1990s, the main control on the design of trawls and seines was the mesh size of the cod-end at the end of the net where the fish collect. Effective fishing gears catch marketable fish over a certain specified size - the minimum landing size - but allow fish below this size to escape.
Research has shown that several design features of fishing gears affect how open the meshes are and hence how easily fish escape. Regulations are now quite complex and include control of mesh size, twine thickness, the use of attachments to cod-ends, the number of meshes round the circumference of the cod-end and the use of areas of square mesh netting, for example. These measures aim to reduce the level of discarding of undersized fish or non-target species.
Lists of UK and EU fisheries legislation can be found at these sites: