Having failed to blockade or bottle up the Russian squadron at Port Arthur by sinking blockships in the harbor's channel, Tōgō formulated a new plan. Ships were to mine the entrance to the harbor and then lure the Russians into the minefield in the hopes of sinking a number of Russian warships. Covered by four detachments of torpedo boat destroyers, the minelayer "Koru-Maru" began to lay a minefield near the entrance to Port Arthur on the night of 31 March. The Japanese were observed by Makarov, who believed that they were Russian destroyers which he had ordered to patrol that area.