The effect of grid size on catch efficiency and by-catch in the demersal trawl fishery for Norway pout (Trisopterus esmarkii)

Norway pout (Trisopterus esmarkii) is caught with large trawls with small meshed codends, inevitably causing the fishery to have large by-catch issues. To reduce the amount of by-catch, a rigid sorting grid was made compulsory in 2010. However, there is still a severe by-catch issue, as well as the loss of target species because of the grid. A possible cause might be clogging of the grid, which could be solved by increasing the grid area. Therefore, this study compared the size selectivity of by-catch species and target species in a double-trawl configuration in which one trawl was equipped with a standard grid (6.30 m2), and the other trawl was equipped with a grid that had a 50% larger surface area (9.45 m2). The results demonstrated that the size selectivity and catch efficiency of the target species were unaffected; neither was there any significant difference between the two grids in terms of wanted by-catch species [blue whiting (Micromesistius poutassou), horse mackerel (Trachurus trachurus), and greater argentine (Argentina silus)]. However, the larger grid caught significantly more unwanted by-catch species [haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), and hake (Merluccius merluccius)]. Approximately one-third of the accumulated catches comprised the target species, one-third of wanted by-catch species, and one-third of unwanted by-catch species, demonstrating the by-catch challenges in this fishery. Simultaneously, none of the by-catch limits were exceeded