Xuefang Wang

The effect of netting solidity ratio and inclined angle on the hydrodynamic characteristics of knotless polyethylene netting

Knotless polyethylene (PE) netting has been widely used in aquaculture cages and fishing gears, especially in Japan. In this study, the hydrodynamic coefficient of six knotless PE netting panels with different solidity ratios were assessed in a flume tank under various attack angles of netting from 0° (parallel to flow) to 90° (perpendicular to flow) and current speeds from 40 cm s−1 to 130 cm s−1. It was found that the drag coefficient was related to Reynolds number, solidity ratio and attack angle of netting. The solidity ratio was positively related with drag coefficient for netting panel perpendicular to flow, whereas when setting the netting panel parallel to the flow the opposite result was obtained. For netting panels placed at an angle to the flow, the lift coefficient reached the maximum at an attack angle of 50° and then decreased as the attack angle further increased. The solidity ratio had a dual influence on drag coefficient of inclined netting panels. Compared to result in the literature, the normal drag coefficient of knotless PE netting measured in this study is larger than that of nylon netting or Dyneema netting.

In-situ dynamics of tuna purse seine deployment in various operations and currents

One of the greatest concerns for a successful catch in tuna fishing operations is the diving behavior of the purse seine. To analyze the integrated effect of fishing methods and sea currents on purse seine sinking depth, a multiple regression model was used. This improved model shows current speeds, shooting duration, the included angle between a two-layer current, and net mouth orientation relative to current direction (shooting angle) have an influence on maximum sinking depth. The current in the mid-layer is considered to significantly resist the seine’s vertical dive by a quadratic relation, but an increased shooting duration and shooting angle can be enhance the sinking. An obvious difference in terms of sinking depth is found between free-swimming school fishing and floating objects fishing, two common methods in tuna purse seine fisheries. It has been suggested that seine sinking dynamics respond to gear manipulation and, therefore, the model provides useful guidance in implementing net casting for an effective capture.