Fishing and Gear Technology

Unit 15 - Selectivity in fishing gear

15.5.Square Mesh Codend in Trawl
The size and shape of meshes in the codend are main factors which determine the selectivity of trawls. The filtering ability of the trawl can be enhanced either by enlarging the mesh size or by restricting the lumen of the mesh under stow. The size of the mesh cannot be increased beyond a certain limit. Hence, the other alternative is to effect changes in shape of the mesh to keep the meshes open during trawling operation. Square mesh (Fig.3) webbing can be prepared from the ready made webbing and can be attached to either in the throat part or on the upper panel of the codend (Fig.4). The method of maintaining square mesh shape for the codend meshes is to increase the selectivity of the trawl gear and to ensure that juveniles and undersized fish can escape from the net. Moreover that they will not be damaged in the process and thus survive and grow.
Many experiments conducted under commercial condition, showed that the use of square mesh webbing in the codend mesh reduce the catch of small fishes. The main reason for improved selectivity is simply the large area of open meshes, through which fish can escape. The square mesh is also more stable. The diamond mesh on the other hand, tends to distort due to longitudinal and transverse tension, depending on catch size, current direction and other factors.