Effect of Water Flow on Bag Net Volume of Large-scale Set Net Monitored by Ultrasonic Telemetry System

Abstract

An ultrasonic telemetry system was employed to monitor the bag net shape and volume of a large-scale set net off Oirase, Fukaura, Aomori Prefecture, during two fishing periods, namely May 15 to July 23, 2019, and May 18 to July 28, 2021. Two-dimensional electromagnetic current meters (logger version) were utilized to record the flow direction and velocity at three depths (10, 20, and 30 m) around the set net during the two fishing periods. The aim was to investigate the effect of water flow on the bag net shape and volume. The water flow direction around the set net was mostly toward the northeast, similar to the direction of the body net with respect to the bag net (46°NE). The flow around the bag net varied. Tidal currents did not significantly affect the flow. The water flow affected both the bag net shape and volume. The acoustic pingers on the bag net sometimes exhibited significant horizontal movement in addition to vertical movement. Generalized linear model analysis was used to express the bag net volume using flow components, with depth as a variable. The northeast flow component at a depth of 10 m increased the volume of the bag net, particularly in the fish-catching section, and that at depths of 20 and 30 m decreased the volume of the bag net.