The Antarctic krill (Euphausia superba) is one of the most abundant resources in the ocean, which provides food for several important species in the Antarctic Ocean, and is targeted commercially by humans for many decades. To sustainably manage and harvest the species, energy-efficient, catch-efficient, and selective fishing gears should be developed for the Antarctic krill trawl fishery. This study investigates the effect of twine area and the liner length on the engineering performance of trawl through flume tank testing of trawl model to predict the performance of the full-scale midwater trawl used in the Antarctic krill fishery. Four 1/35th scale trawl model nets with varying lengths of the liner, based on the traditional trawl used in the Antarctic krill fishery, were designed using modified Tauti’s law and were tested in a flume tank at different towing speed, door spread, heavy bob weight, and the ratio of buoyancy to the fishing line (F/G). The results showed that the reduction in liner length by 25 and 50% from the traditional trawl net led to the decrease in twine area by 11.01 and 19.31% and, consequently, resulted in reductions in the lower bridle tension by 12.44 and 19.78%, and increases in energy efficiency by 17.98 and 25.73%, respectively. In addition, the reduction in liner length by 25 and 50% were found to increase the net mouth opening by 2.63 and 5.38% and the swept area by 6.52 and 8.38%, respectively, both of which are proportional to catch rates. Although the trawl net with 50% liner length is more energy-efficient and large mouth opening than those of the trawl net with the liner length over 75% of the trawl body, the large mesh section without a liner can result in the escape of the krill from the trawl, reducing overall catch efficiency. We, therefore, recommend the trawl with 75% of liner length as a suitable design for Antarctic krill considering energy efficiency and catch efficiency.

This study evaluated the behavior of different codend designs to provide the basic information that is relevant for improving the gear selectivity, energy efficiency, to better understand the fish behavior inside the codend, and prevent the probability of the fish escaping. Three different codends were designed from the standard codend commonly used in the Antarctic krill fisheries based on modified Tauti’s law and evaluated. The first and the third codends were designed with four-panel and two-panel nettings, respectively, both made of diamond meshes. While, the second one was a four-panel diamond mesh design with cutting ratio 4:1(N [NBNBN]16). We measured the drag force, codend shape, fluttering codend motions, and the flow field inside and behind the different codends composed of different catch weights under various flow velocities in flume tank. The power spectra density was undertaken to analyze the time evolution of measured parameters. The results showed that the drag force and the codend motion increased and decreased, respectively, with the number of net panels and the cutting ratio. Due to the catch weight and flow velocity, which caused significant codend motions and deformation, a complex interaction occurred between the fluid and the structure, and there was a strong correlation between the codend drag, the codend motions, and the turbulent flow inside and behind the codend. The study showed that the use of the four-panel codend with cutting ratio and the two-panel codend resulted in drag reductions of 6.07% and 6.41%, respectively, compared to the standard codend. The velocity reduction and turbulent kinetic energy were lower inside and behind the four-panel codend than inside and behind the two-panel codend, indicating that turbulent flow through the two-panel codend is more important than through the four-panel codend. The results of the power spectral density analysis showed that the drag and codend motions were mainly low frequency in all codends, with another component related to turbulent flow street. In addition, the two-panel codend showed more unstable behavior with more pendulum motion compared to the four-panel codends, resulting in a smaller mesh size in this codend that could affect swimming energy and thus influence fish escape, making it the least selective codend. The results of this study provide fundamental insights useful for understanding and improving the hydrodynamic performance and selectivity of trawls in the Antarctic krill fishery, especially to reveal the masking effects of the number of net panels on codend design.

Codends are the posterior components of trawl nets that collect the catch and play a crucial role in the selectivity process. Due to the accumulation of catch and the variety of catch types, the quality of catch and trawl selectivity can be negatively impacted. Therefore, this study aims to investigate the effects of various catch configurations on the hydrodynamic characteristics, geometrical profile, and fluttering motions of the codend in a flume tank. A codend structure was designed and tested using various catch configurations, including grooved-type configurations (canvas, green canvas, basketballs) and spherical configurations (table tennis balls filled with water, balloons filled with water, and balls made of twine) in the flume tank. The sea trial data were compared with the flume tank data. The results indicate that there were no significant differences in the codend profiles between the different catch configurations. The drag of the codend with a grooved-type configuration was 13.63% greater than that obtained using a spherical configuration as the catch. The wavelet coefficient obtained from the codend drag revealed that the oscillations of the codend with a grooved-type catch configuration began at a periodicity of 0.07 s and were more intense than that of the codend with the spherical catch configuration. Moreover, these amplitudes increased as the codend flow velocity increased. The wavelet analysis results showed that the dominant frequency of the periodic high-energy coherent structures for the codend drag and codend displacements was detected at a low-frequency. In terms of displacement oscillation characteristics, the table tennis ball filled with water was an approximate substitute for real catch during the sea trial because the difference in wavelet coefficients for the codend displacements in amplitude and the period between the model codend with the table tennis ball filled with water and the full-scale codend was 91% and 89%, respectively. The findings of this study confirm the feasibility of replacing real catch with simulated catch configurations with similar shapes in model testing. They can provide basic scientific data for improving the hydrodynamic characteristics and selectivity of the codend structure.

The grid is a crucial component in constructing grid-type bycatch reduction devices. The grid’s structural characteristics and orientation significantly impact the hydrodynamic characteristics and efficacy of the separation device. Therefore, it is essential to thoroughly understand the grid device’s hydrodynamic characteristics and flow field to optimize its structure. Thus, this study used CFD numerical simulation and flume tank experiments to investigate the effects of inclination grid angles and grid bar spacing on hydrodynamic forces and flow fields around a circular grid. The results indicated that the hydrodynamic forces acting on the circular grid increased with higher flow velocity and inclination grid angle, decreasing with smaller grid bar spacing. Flow velocity acceleration zones were observed at the upper and lower ends of the grid and between the grid bars. Additionally, upwelling and vortices were present at the back of the grid. It was found that an increase in the inclination grid angle accelerated the vortex and wake effects.