The influence of twine tenacity, thickness and bending stiffness on codend selectivity

We report on trials which measured the selectivity of haddock (Melanogrammus aeglefinus) and plaice (Pleuronectes platessa) in codends made from netting materials classified by the manufacturers as low, medium and high tenacity. We measured the twine bending stiffness, thickness and tenacity (as defined by Klust, 1982) of the netting materials and investigated which of these most influenced codend selection. For haddock, only twine bending stiffness affected selection, with l50 decreasing as bending stiffness increased. For plaice, none of the twine variables affected l50, but they all influenced selection range. Increasing mesh size increased the l50 of both species, whilst increasing catch size increased haddock l50 but decreased plaice l50. As bending stiffness is difficult to measure, a proxy is required that quantifies the resistance of meshes to opening and that can be reported in future selectivity studies.