Oscillation and the resulting bait loss observed in pelagic longline hooks

The oscillation cycle and intensity in each part of the pelagic longline were investigated in Sagami Bay, Japan, to evaluate the effect on bait loss. The results of three longline operations were analyzed. The longline consisted of 25 ordinary and 15 midwater float baskets, with six branch lines per basket. Acceleration loggers were attached to the upper end (main float) and lower end of the float line, and the hook of the first and third branch lines. A depth logger was attached to the upper and lower ends of the branch line (hook). A video logger was attached, facing downward at a position 0.6 m above the hook. The periodicity of the oscillation was clarified by autocorrelation analysis, and the sum of the absolute values of the dynamic acceleration of the three axes (ODBA) was compared. From the video footage and autocorrelation analyses, the oscillation cycle of the hooks was approximately 3 s, which is the same as that of the main float. The ODBA of the first hook was 1.9–2.3 m/s 2 , which was significantly larger than the main float (1.1–1.4 m/s 2 ) and the third hook (0.2–1.8 m/s 2 , p < 0.001). Therefore, the oscillation of the hook was derived from the vertical movement of the main float due to the wave, and it decreased as the distance from the main float increased. The third hook in the midwater float basket had an unclear cycle, and the ODBA was significantly smaller than that of the ordinary baskets.