Midwater float for pelagic longline consists of plural additional small fioat, which keeps the depth of each hook to the same layer, and this study presents three-dimensional shape of longline with the midwater float in uniform stream, by model experiment in a flume tank and by numerical simulation based on finite element method Each part of longline was shoaled by the current, and then the depth of the hooks became shallower with the faster water current. In particular, the principal factor affecting the hooks shoaling was the spread of horizontal spacing between the two successive buoys by the faster current. When the longline was cast with a given angle to the stream, the longline gear had threedimensional structure, and then the hooks were set at much shallower depth. The numerical calculation simulated well the shoaling of the longline in the flume tank. The comparison of the numerical simulation in longline shoaling between the midwater fioat setting and the conventional setting suggested that the depth of longline with the midwater float was susceptible to the current. The possible reason was that the in-water weight of the longline gear was lightened by the midwater float, comparison to the conventional setting. For setting the hooks at the target depth in water current by using midwater fioat, the simulation of longline shoaling was useful to determine the specification of the midwater float system, e. g. the number and buoyancy of the midwater float, and fioat line length.
