Fluctuation of longline shortening rate and its effect on underwater longline shape

Depth of longline is determined by shortening rate of mainline and elevation due to vertical current shear. Since the shortening rate has not been measured directly, shortening rates used in previous studies have uncertainties. In order to solve this problem, a pair of GPS receiver-installed floats were used for the experimental tuna longline operations in the eastern tropical Pacific during August/September 1997. The shortening rate decreased significantly immediately after setting. which suggests that the shortening rates used in previous studies tend to be overestimated. The shortening rate also changed while the gear was drifting. Fluctuation of the shortening rate is the major cause of depth change of longline when the elevation is small due to weak vertical current shear. Three-dimensional underwater longline shape is estimated by a method proposed by Mizuno et al. (1997) using the observed shortening rate, and appropriate solutions can be obtained in all cases. Obtained shapes of longline are consistent with observed vertical current shear