1 Faculty of Fisheries, Kagoshima University, Kagoshima 890-0056, 2Ishigaki Tropical Station, Seikai National Fisheries Research Institute, Ishigaki, Okinawa 907-0451, 3National Research Institute of Fisheries Engineering, Hazaki, Ibaraki 314-0421, Japan
Sinking motions of tuna longlines at the surface layer, 0-30 m depth, were studied at sea by using depth loggers attached to the bases (main lines) and tips (hooks) of branch lines. Characteristic sinking patterns and changes in sinking velocity were compared between main lines and hooks. Initially, main lines and hooks sank at different velocities; the main lines sank at 7.7±1.5 cm/s and hooks at 18.7±3.6 cm/s. There was no significant relationship between the velocities and the duration of the initial sinking. Deceleration for hooks started 36 to 112 s after shooting. Consequently, their sinking velocities became identical to those of the main lines. The sequence of sinking motions of a main line and a hook can be modeled as three stages: independent free fall at their own velocities where a hook sinks faster (initial motion), deceleration of a hook after the branch line is stretched (transitional motion) and simultaneous sinking of a main line and a hook which is suspended from the main line (final motion). The above model suggests that casting of stretched branch lines results in a prolonged sinking time to the critical depth for a hook.