An officer on duty on a fishing vessel watches the fishing gear drifting after setting the salmon gill-net or tuna longline. At any time, if he can estimate his ship's position by any method concerning the gear without recourse to an object (flag, light, reflector, radio buoy), he can employ the method to watch the gear effectively. The authors had experimented with the drifting of the Hokuseimaru (273 G/T), and reported that the direction and distance of drifting were estimated by the ship's head and wind velocity. And for the next study, we experimented on the accuracies of estimated ship's positions based on the method, in the North Pacific Ocean. The experiment 1. As the drifting velocity and directions had been calculated by previously reported equations, some drifting positions were estimated. 2. Ship's positions can be determined by the bearings and distances of the corner reflector connected to the gill-net (length 3n.m., depth 7m). 3. Determined and estimated ship's positions are shown in Fig.2 every 30 minutes. The accuracies of estimated positions. 1. We say that the distance error is the length between the determined position and the estimated position, as shown in Fig.3, related to the drifting time every day. Furthermore standard deviations of distance errors are shown in Fig.4. 2. Differences of estimated drifting directions from determined directions are shown in Fig.5 in relation with the drifting time. And standard deviations of direction errors are shown in Fig.6. 3. Under the general condition that the drifting time is 6 to 8 hours, the distance error is 1n.m. (s.d.) and the direction error is 7°(s.d.). According to this method, we could obtain satisfactory results from various points of view for the sake of keeping watch over the fishing gear. We recommend an officer on duty to study the drifting characteristics of his own ship and apply himself to keeping watch over a fishing gear.
