The sea urchin Strongylocentrotus nudus is commercially important but their intensive grazing often leads to the formation of extensive, unproductive barrens along the coast of northern Japan. A simple device that could defend urchin invasion and allow kelp to grow by utilizing waves was developed after repeated field experiments with test devices. The final device for practical use had a swing bed that consisted of a horizontal shaft with frames for kelp settlement and growth on both sides and a plate on the underside. The swing bed was supported with two columns and was swayed by hydrodynamic force on the plate bearing wave-induced oscillatory flows. Invasion by urchins into the bed was prevented primarily in moving from the column to the constantly moving surface of the shaft. To enhance the relative movement of the swing element to the immobile supports, disks were attached to the shaft and to the columns with interstices. The device that was placed in S. nudus-dominated barrens protected from ocean waves successfully prevented urchin invasion into the swing bed leading to the formation of a canopy by the perennial kelp Eisenia bicyclis.
