Comparative study on the full-scale prediction performance of four trawl nets used in the coastal bottom trawl fishery by flume tank experimental investigation

Abstract

Energy efficiency, catchability, and ecological sustainability have become important issues for coastal bottom trawl fisheries. Thus, fishing gear has been modified to increase the swept area and decrease the drag and bycatch. To evaluate the effect of twine thickness, twine material, and mesh size on the drag (bridle tension) and geometrical shape of trawl nets, this study focused on the full-scale prediction performance (geometry and resistance) of bottom trawl nets used in the coastal fishery. Four 1/18-scale bottom trawl models with different twine diameters, different mesh sizes, and even the Geometrical shape of wing-end were designed and tested in a flume tank at Tokyo University of Marine Sciences and Technology (TUMST). The flume tank experimental data were evaluated to predict full-scale performance at sea and to compare the performance of trawl nets built with traditional materials (polyethylene), namely, trawl nets 1 and 4, with those built with innovative materials (Dyneema and nylon monofilament), namely, trawl nets 2 and 3, respectively. The geometrical shape of bottom trawl net was determined from the nonlinear equation constructed and solved using the experimental data with various door spreads and towing speeds. The results show that an increase in mesh size using Dyneema (trawl net 2) systematically leads to a decrease in bridle tension ~ 34.55%, a decrease of twine diameter using nylon monofilament (trawl net 3) lead to a decrease in bridle tension ~ 35.4%, and an increase in mesh size using triangular shape in the wing-end (trawl net 4) lead to a decrease in bridle tension ~ 19.92%. The net mouth area of trawl net 1 is 4.36%, 10.88%, and 6.47% greater than those of trawl nets 2, 3, and 4, respectively. The filtered volume of trawl net 3 is 1.29%, 7.48%, and 4.13% greater than those of trawl nets 1, 2, and 4, respectively, indicating that trawl net 3 has better catch performance than other trawl nets. The shapes of floating rope of all the four trawl nets are very close with a gap of less than 1.5%, indicating that the increase in mesh size and decrease in twine diameter do not affect the geometrical shape significantly as the drag force. The results also show that the ratio between bridle tension and net mouth area of trawl net 1 is 31.57 %, 27.61%, and 14.37% greater than those of trawl nets 2, 3, and 4, respectively.