Modelling axisymmetric cod-ends made of different mesh types

Cod-ends are the rearmost part of trawl fishing gears. They collect the catch, and for many important species it is where fish selection takes place. Generally speaking they are axisymmetric, and their shape is influenced by the catch volume, the mesh shape, and the material characteristics. The shape of cod-ends is of importance as it determines mesh opening and consequently influences the selectivity of fish from the cod-end. Selectivity is the process whereby a gear retains large fish and releases small ones. In recent years, as many fish stocks have become more threatened, understanding the selectivity process has become more important. This paper presents a model of the deformation of an axisymmetric cod-end. The twine tension and the catch pressure acting on the knots of each mesh along the cod-end profile are calculated, and a Newton-Raphson scheme is used to estimate the equilibrium position of the netting. The software package developed to solve this problem is freely available. Comparisons are carried out with a previous model and experimental data.