Closing force of the flap door in a turtle releasing device (TRD) for set nets with submerged bag nets

Abstract

　A turtle releasing device (TRD) for a submerged bag net of set nets consists of a 1.0-meter-square escape vent on a 2.0-meter-square base net and a flap door covering the vent. In this study, the closing force of the flap door in the TRD was evaluated to investigate the optimum design of the TRD such that the flap door closes automatically to prevent fishes from getting out. The closing force in static or flowing water (10-40 cm/s) was measured with a tension meter. The closing force became larger with a wider width of the overlapping (fixed) part of the flap door and a larger weight of the flap door. Closing moments relating to the overlap increased with a larger opening angle of the flap door, although that relating to the weight of the flap door decreased. The closing force in the flow from the opening side of the flap door was smaller than that in static water because of fluid resistance. A flap door with a frame diameter of 6.0 mm and overlap width of 0.2 m closed automatically in the 0.8-knot flow from the opening side, and the closing force of this flap door in the flow from the fulcrum side was small enough for turtles to open.