Grid selection in the North Sea industrial trawl fishery for Norway pout: Efficient size selection reduces bycatch

Abstract

Experiments were carried out during three cruises in the period 1997–1999, to develop and test a sorting grid system in the North Sea industrial trawl fishery for Norway pout. The system should separate bycatch species like haddock, whiting and other human consumption species from the main target species Norway pout, and other target species like blue whiting, etc. During the first cruise a prototype of the grid system was developed and tested with different mountings of guiding panel in front of the grid and with different spacing (25, 22 and 19 mm) between bars. The last two surveys tested if the mesh size in the grid section and the thickness of the bars influenced the selectivity of the grid system. Two different mesh sizes and three different thicknesses of bars were tested. Based on the results from the 1997 experiments, only a bar space of 22 mm were used in the later experiments. The 1998 and 1999 experiments were carried out in different seasons (May and September/October) to test the system on different size distributions of target and bycatch species. Hydrodynamic studies of the grid system using the two different mesh sizes and the three different thickness of bars were conducted in a flume tank, and a 25% difference was found in water flow speed behind grids with 22 mm bar spacing but with different thickness of bars (15, 10 and 5 mm). During the 1998 experiment a total of 94.6% (weight) of the bycatch species was sorted out with a 32.8% loss of target species. In the 1999 experiment 62.4% of the bycatch species were sorted out and the loss of target species was 22%. When testing selectivity parameters for haddock, the main bycatch species, the parameters indicated a sharp size selection in the grid system. Size selection differences between different configurations of the grid system are discussed.