A midwater trawl net with mouth opening canvas devices for stock assessments

Abstract

The purpose of this work is to develop a midwater trawl net to survey fish stock. A constant net mouth opening is necessary in order to measure fish stock densities accurately. For this purpose, canvas kites were attached to the midwater trawl net, instead of floats and weights. The area of canvas kites attached to the headrope and groundrope was derived from the equilibrium between the sum of vertical components of the towing force and the drag of the net. The model test were made with a scale of one-twentieth in a flume tank, in which the static characteristics of the kite-type net were compared with those of the float-type net. Further, a full-scale kite-type net was examined aboard the RV "Ushio-maru". The kite-type model maintained a more constant form and net-mouth area than the float type net over a range of current speeds. The full-scale kite-type net maintained a constant net mouth height of 7 meters, according to the plan. From these results, we conclude that the kite-type net is to be recommended for a better sampling device for fish stock assessments.