On arrival, "Sea Dog" reconnoitered the approaches to western Honshū ports to locate minefields; then took up station off the east coast of Sado Island. At 2000 on 9 June, she sank her first target of the patrol, the small cargo ship, "Sagawa Maru". Twenty-three minutes later, she made her second contact; and, at 2044, she fired three torpedoes at another merchant ship, the "Shoyo Maru". One torpedo hit, starting a fire aft. "Sea Dog" pulled away, watched, then moved in for the "coup de grace". Two more torpedoes were fired, one ran erratic; the second hit amidships. The target exploded; her bow broke off; and her stern assumed a 60-degree down angle.