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

In longline operation, hook depth control is important to make it meet to the swimming depth of target fish species. Basically, hook depth changes in terms of shortening rate of mainline and lift-up effect on mainline and branchline by vertical shear of ocean current, given a certain gear configuration. The shortening rate, which indicates degree of sagging of mainline, is defined by a ratio of horizontal distance between neighboring floats to mainline length between them. Since detection of vertical shear of ocean current is not easy for commercial fishing boats, they adjust the rate so as to place the hooks at favorable depths. However, shortening rate of deployed gear has not been observed directly. It has been mostly estimated from the ratio of ship speed to mainline releasing speed, which provides only the ratio at setting. Mizuno et al. (1997) inferred that the shortening rate changed significantly during the gear was drifting from the results of in-situ experiments. Based on time-depth records of micro-BTs (Okazaki et al. 1997) attached to the mainline, they calculated underwater shape of mainline using numerical optimization method, and found that the assumption of constant shortening rate often yielded inappropriate solution of mainline shape. In order to investigate the change of the shortening rate, we constructed a pair of special floats, which contained GPS (Global Positioning System) receivers, to measure the distance between them. Using these floats, a series of experimental longline operations were conducted in the eastern tropical Pacific Ocean in August-September 1997. The mainline depths were also measured by micro-BTs, and the relationship between the depth of mainline and the shortening rate was investigated. The purpose of this paper is to make clear to what extent the variation of mainline depth is explained by change of shortening rate in actual operation.