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 degrees (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 I 10 mm diamond-mesh cod-end the 50% retention length (1-50) 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. (c) 2006 Elsevier B.V. All rights reserved.