Experimental method for quantifying resistance to the opening of netting panels

In recent years, the tendency in some sectors of the fishing industry has been to use thicker and stiffer twines in the manufacture of netting material. This increases the mesh's resistance to opening and, consequently, reduces the selective performance of fishing gears. The main characteristic of netting twine contributing to mesh resistance to opening is flexural rigidity (EI). We present a methodology for quantifying mesh resistance to opening through the use of a prototype called the resistance to opening and deflection meter (ROD-m). It incorporates four tension load cells and four stepping motors that are driven by four ministep bipolar chopper drives. Small panels of netting (3 x 3 meshes) can be mounted on ROD-m, which can measure the forces acting on the netting and the position of the knots. Estimates of the flexural rigidity of the netting twine can be made from these data. We present the results of the analysis of Mediterranean polyamide netting, and standard and Brezline polyethylene (PE) netting from the North Sea. In all netting, regardless of the material used, EI increases with increasing linear density. Furthermore, Brezline is much stiffer than standard green PE, although they are equally thick.