and the physical damage which the trawl does to the seabed. Selectivity Since the practice of trawling started (around the 15th century), there have been concerns over trawling's lack of selectivity. Trawls may be non-selective, sweeping both marketable and undesirable fish and fish of both legal and illegal size. Any part of the catch which cannot be used is considered by-catch, some of which is killed accidentally by the trawling process. By-catch commonly includes valued species such as dolphins, sea turtles, and sharks, and may also include sublegal or immature individuals of the targeted species. Many studies have documented large volumes of by-catch that are discarded. For example, researchers conducting a three-year study in the Clarence River found that an estimated 177 tons of by-catch (including 77 different species) were discarded each year. Size selectivity is controlled by the mesh size of the "cod-end" — the part of the trawl where fish are retained. Fishermen complain that mesh sizes which allow undersized fish to escape also allow some legally catchable fish to escape. There are a number of "fixes", such as tying a rope around the "cod-end" to prevent the mesh from opening fully, which have been developed to work around technical regulation of size selectivity. One problem is when the mesh gets pulled into narrow diamond shapes (rhombuses) instead of squares. The capture of undesirable species is a recognized problem with all fishing methods and unites environmentalists, who do not want to see fish killed needlessly, and fishermen, who do not want to waste their time sorting marketable fish from their catch. A number of methods to minimize this have been developed for use in trawling. By-catch reduction grids (typically made of stainless steel or plastic) or square mesh panels of net can be fitted to parts of the trawl, allowing certain species to escape while retaining others. In fish trawls, the grid is mounted so the smallest organisms (juvenile fish, shrimp) pass through the grid and enter the sea again. In shrimp trawls, the grid pushes the largest organisms (fish) through a hole in the roof of the net, reducing by-catch of fish. The latter type of grid is mandatory in Norway and has been in use for 20 years.Sorting grids – can Norwegian success be replicated in the Mediterranean? – Minouw Project The grids are typically equipped with sensors that measure the angle of the grid, so