Revealing the Fluttering Motions of Mid-Water Trawl Codend Through Sea Trials: Case Study of Antarctic Krill Trawl Codend

The dynamic coupling between the fluttering motions and hydrodynamic characteristics of codend is essential in understanding the trawl selectivity through fish response and the drag force acting on the whole trawl. This study investigated the effect of towing speed, warp length, warp tension, and catch size on the fluttering motions of Antarctic krill trawl codend during net shooting, towing, and hauling by using sea trial data. The time-periodicity of codend oscillation was analyzed by the Morlet wavelet transform method. Results indicated that the period of codend oscillation was between 50 s and 90 s and showed an increasing trend with the warp tension but a decreased value at the towing stage. The coefficient amplitude of codend oscillation was between 0 and 4 at the net shooting and hauling stages, and between 0.2 and 0.6 at the towing stage. The amplitude of codend oscillation increased with the warp tension, towing speed, and catch size, but decreased with the increase of the warp length. In addition, the period of codend oscillation increased with the towing speed at the net shooting and hauling stages, but decreased at the towing stage. These results from codend fluttering motions can improve the understanding of fish behavior and gear shape that modify the hydrodynamic force on the codend instantaneously.