A new type of depth-controlled gillnet was designed for fish sampling surveys. The net was joined to a vessel with a single warp to control net depth. Flume-tank experiments were performed using two model nets (1/30 scale) to determine the optimum net design. In one model (Direct-type), the warp and net were joined directly, while in the other model (Bridle-type), the joint between the warp and net passed through the bridle. Next, an experiment was conducted using the full-scale net at sea to investigate the effect of operational factors, warp and vessel speed (propeller pitch) on net depth.
The shape of Direct-type was better than that of Bridle-type shape. The former sank faster than the latter at the speed of 25cm/s (0.5kt) speed, and was thus selected as the design for the full scale net. Net depth could be controlled smoothly by paying out of the warp or change of propeller pitch at sea. This change of depth with time fit the first order lag model.
