A fish aggregating device（FAD） is a floating construction installed at sea to attract and aggregate fish. Fishermen conduct an operation of troll lines to catch tuna and skipjack tuna in areas around FADs in southern Japan. It is very dangerous to catch fish near FADs because FADs are moving objects and can collide with fishing boats. Model experiments using a 1/30 model （length 25.0 cm, width 13.0 cm, and height 11.7 cm） were carried out in a flume tank. The movements of the FAD were recorded in top view using a digital video camera. Video images were converted into static images every 1/30 seconds to measure the periods and the distances of the movement of FAD. The flow speed was changed from 9.4 cm/s （1.0 knot） to 28.8 cm/s（ 3.0 knots）. 8 different size of boards were constructed and installed on the stern side to reduce the movement of the FAD. Amplitude of the movement of the FAD was reduced by installing a rectangular board at the bottom of the stern.
