To give advance warning of the approach of any Allied naval forces, the Japanese sent submarines , , and to form a scouting line in the ocean about southwest of Guadalcanal. Fletcher's forces had entered the Coral Sea area before the submarines took station, and the Japanese were therefore unaware of their presence. Another submarine, , which was sent to scout around Nouméa, was attacked by "Yorktown" aircraft on 2 May. The submarine took no damage and apparently did not realize that it had been attacked by carrier aircraft. "Ro-33" and "Ro-34" were also deployed in an attempt to blockade Port Moresby, arriving off the town on 5 May. Neither submarine engaged any ships during the battle.