Effect of mesh size modifications on the sinking performance, geometry and forces acting on model purse seine nets

摘要

To ensure efficient and sustainable purse-seine fisheries, it is necessary to understand well the fishing performance of a purse seine, which can be related to many factors, such as mesh size, twine materials, leadline weight, fishing operations and the marine environment. In this study, we tested two tuna purse seine model nets with different mesh sizes to obtain the lateral and drifting distance, spatial geometry, purse line tension and sinking speed of a purse seine under different operation conditions related to the current direction, three net shooting patterns and two current speeds. It was found that better spatial geometry and sinking behavior of the purse seine occurred by shooting the net normal to the current than shooting it with or against the current. Setting with the current was considered as the optimal strategy of shooting the net in terms of reducing the tension of the purse line. The net drifting distance decreased when the mesh size of the main body was increased, and the minimum lateral and drifting distances occurred when setting with the current. Increasing the mesh size helped reduce the enclosed area in the horizontal direction, especially of the bottom of net, and keep a better net shape. The pursing time and purse line tension reduced by 12% and 9.26% respectively when the mesh size of the lower part of the main body increased from 30 to 45 mm. In general, larger mesh nets can decrease purse line tension and improve sinking performance and net geometry.