On the influential factors in the operation form 

of bottom-set gillnet based on model tests

GUO Shaojian1, XU Zimu1, YE Xuchang1, 2, 3, 

ZHANG Jian1, 2, 3, WANG Yucheng1, ZHOU Cheng1, 2, 3

（1.College of Marine Sciences, Shanghai Ocean University, Shanghai201306, China; 2. National Engineering 

Research Center for Oceanic Fisheries, Shanghai201306, China; 3. Key Laboratory of Sustainable Exploitation 

of Oceanic Fisheries Resources, Ministry of Education, Shanghai201306, China）

Abstract: Bottomset gillnet has the characteristics of harvesting a variety of species, high economic benefits, high sizeselectivity and poor speciesselectivity. The hydrodynamic behavior of bottomset gillnet under fishing operation are dynamically changed in response to combined effects of external forces from wave and current, and catch impact etc., which results in reduced fishing efficiency, fishing gear selectivity and security, and subsequently the remnant netting forms “ghost fishing”. The main internal factors affecting the fishing efficiency and selectivity of gillnet gear include mesh and twine size, working height of gillnet, twine material and hanging ratio, In addition, in the working process, due to the influence of external forces such as waves and streams and the interaction with fish behavior, the form and structure of gillnets change, fishing performance decreases, and selectivity changes. For the intention of further understanding the hydrodynamic behavior and geometry deformation under fishing operation, this study, on the basis of hydrodynamic experiment of polyethylene (PE) and nylon (PA) gillnets in the uniform current, investigated the effect of current velocity, solidity ratio and 90° turned mesh on the working height and shape characteristics of bottomget gillnet, and developed the empirical formulas of deformation degree (k) of PE and PA gillnets. The results showed that the working height of gillnet was negatively correlated with the current velocity, and 96.5%, 69.3%, 43.9% and 29.9% of the initial height under the still water were achieved under current velocity of 0.2, 0.4, 0.6， 0.8 m·s-1, respectively; hanging angle in the lowerhalf part of the gillnet increased with the current velocity.When V= 0 m·s-1, the gillnet was stretched vertically in the water, the gillnet mesh was in the form of a high middle and low on both sides, the headline was inverted catenatelike. With the increase of current velocity, the gillnet gradually shifted backwards and downwards, the projection area of the gillnet in the YZ plane gradually became smaller, the projection area in the XY plane gradually increased, the projection area of the mesh in the YZ plane decreased, the curvature of the left and right siderope got larger and larger, and more folds appeared with it. When the current velocity reached 0.8 m·s-1, the lower part of the gillnet was close to the bottom surface of the tank. The twine material had little influence on the working height of gillnet, but had a significant impact on the expansion form of the bar. The height of the two kinds of twine materials (PE and PA) gillnets was almost the same under the same current velocity, but PA gillnets at different current velocities had a larger projection area in the XY plane, the overall form was more uniform, the lower part of the netting was not wrinkled, and the mesh was always maintained more uniform expansion. Under T0 and T90 assembly, the working height and uniformity of gillnets at different current velocities varied greatly. In still water and when V= 0.2 m·s-1, the working height of T0 gillnet was significantly higher than that of T90 gillnet, and the M4 Zaxis coordinates of T0 gillnet tracer point were 5.1 cm and 9.0 cm higher than those of T90 gillnet, respectively, while the M3 Zaxis coordinates of the center point of T90 gillnet were 8.0 cm and 5.0 cm higher than those of T0 gillnet. When the current velocity increased, the projection area of T90 gillnet in the YZ plane was significantly larger, the working height of the netting was significantly higher than that of T0 gillnet, and the mesh expansion form was closer to the positive diamond, but at this time, the deformation degree of the lower half of the mesh of the two assembly methods was basically the same. The large solidity ratio not only leads to an increase in the current velocity and resistance through the mesh, but also sometimes changed the current state in the net, affecting the selectivity of fishing gear and quality of the catch. In this study, the form changes of PE and PA gillnets with different solidity ratios showed that under the four sets of current velocity conditions, the working height of gillnets of PE and PA materials was negatively correlated with the solidity ratio, the smaller the solidity ratio, the larger the working height of gillnet, the smaller the degree of deformation, the smaller the projection area of the netting in the XY plane, and the closer the mesh form to positive diamond expansion. Based on the influence of twine diameter, bar length, Reynolds number, on the resistance of the netting, the nonlinear fitting of different materials, solidity ratios and current velocities was obtained, and the expressions of deformation degree k of PE and PA materials were obtained, and according to the characteristics of the expression, the response surfaces of the specific current velocity, bar length and deformation degree k were obtained by response surface analysis method, and the calculated results were compared with the test results. The results showed that at a certain velocity, the longer the bar length, the smaller the twine diameter, and the greater the degree of the gillnet deformation. When the bar length was fixed, the degree of gillnet deformation k was negatively correlated with the current velocities and the twine diameter of netting; when the twine diameter was fixed, the smaller the current velocity, the larger the bar length, and the greater the degree of deformation k. In general, the calculated result was in good agreement with the test result, but the calculated result of the degree of deformation of PA gillnet at low current velocity (V=0.2 m·s-1) was quite different from the experimental test. To sum up, in the traditional gillnet operation, PA material twine is mostly selected, which is conducive to the full expansion of the netting in water. The solidity ratio is the key factor of whether the gillnet can maintain the ideal form in the current, the smaller the solidity ratio, the stronger the water crossing ability, the smaller the resistance in the horizontal direction, the higher the height of the gillnet, and the more the mesh tends to be positive diamond expansion. 

Keywords: bottomset gillnet; model test; solidity ratio; hydrodynamic