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.

No abstract

No abstract

SUMMARYThe calculation of net shapes is performed using the ®nite element method. A triangular element is developed for the modelling of the net. The main hypothesis is that the strain in each triangular element is constant, the net has only two directions of twine and the twines are elastic. The forces due to the tension in twines are described and calculated by a direct method. Results of the model based on such triangular elements are given. The comparison with a model where each twine is described as an elastic bar is quite good.

SUMMARYNets made of thick and sti twines are used in ÿshing industry. The meshes of these nets are resistant to opening. To be able to calculate the shape of such nets, a triangular element modelling of the net is modiÿed to introduce the opening mesh sti ness. The basic hypothesis is that the twines remain straight, and that the opening sti ness is located in the knots connecting the twines. The forces exerted on the element due to the opening sti ness, are calculated using the principle of virtual work. The results of calculation are given.