On the high seas, Yi's successor Won Gyun failed to respond to reports from his scouts and allowed the Japanese to land critical reinforcements at Sosang Harbor for their land offensive unopposed. Without adequate reconnaissance or planning, Won Gyun decided to attack with the entire naval force of Joseon at his disposal; a fleet consisting of 150 warships operated by 30,000 men that had been carefully assembled and trained by Admiral Yi. Won Gyun left anchor at Yeosu with the fleet and sailed into waters marked by treacherous rocks where the Japanese ambushed the Joseon fleet in the Battle of Chilchonryang on August 28, 1597. Ignorant of the strength and disposition of the enemy, Won was stunned to find a Japanese fleet of 500 to 1000 ships which immediately closed for melee combat, denying the Joseon ships the advantages of superior seamanship and cannon fire. The exhausted Joseon sailors were reduced to fighting boarding actions while heavily outnumbered and slaughtered en masse.