Modelling axisymmetric codends made of hexagonal mesh types

Codends are the rear parts of trawls, which collect the catch and where most of the selectivity process occurs. Selectivity is the process by which the large fish are retained while the small ones are released. The codends applied in many fisheries often consist of only one type of mesh. Therefore it is reasonable to consider these codends as being axisymmetric. Their shapes depend mainly on the volume of catch, on the shape of meshes (diamond, square, hexagonal) and on the number of meshes along and around the codend. The shape of the codends is of prime importance in order to understand the selectivity process. This paper presents a model of deformation of codends made up of hexagonal meshes. Two types of hexagonal meshes have been investigated: the T0 codend where two sides of the hexagons are in axial planes and the T90 codend where two sides are perpendicular to the codend axis. The forces involved in this model are twine tension and catch pressure. A Newton-Raphson scheme has been used to calculate the equilibrium.