Simulation-based study of the combined effect on cod-end size selection of turning meshes by 90° and reducing the number of meshes in the circumference for round fish

FEMNET, a numerical tool based on the finite element method, was used to estimate the shapes of four different designs of trawl cod-ends during fishing operations. Compared to a traditional diamond-mesh cod-end the design differences were the following: (i) the netting orientation was turned by 90° (T90), (ii) the number of meshes in the circumference was reduced by 50% or (i) and (ii) were combined. The cod-end shape estimates were then entered into the simulation tool PRESEMO, to estimate their influence on the selectivity processes in the cod-end. This enabled us to predict how these design alterations – alone or combined – may act on the selectivity of each cod-end under identical fishing conditions. For instance, we predict that for a 110 mm diamond-mesh cod-end the 50% retention length (L50) is increased by nearly 12 cm by both turning the mesh orientation and reducing the number of meshes in the circumference. Of this combined effect we predict that 24% of it is caused by only turning the mesh orientation whereas 71% of the effect stems from only reducing the number of meshes in circumference. The remaining 5% is due to the interaction between the two factors.